CHAPTER 5:
VARIABLE FREQUENCY OSCILLATOR (VFO)
This chapter includes:
VFO Introduction (You are here)
Parts List for both Oscillator and Buffer
Building the Oscillator
Oscillator Check-Out
Visual
Check
Resistance
Measurements
Current
Measurements
Voltage
Measurements
Buffer; Introduction
Building the Buffer
Buffer Check-Out
Resistance Measurements
Current
Measurements
Voltage
Measurements
VFO Calibration
Transmitter Offset Adjustment
Back to 40 Meter QRP Table of Contents
The VFO module is the heart of the WT-1 40 meter transceiver. This is the module that determines the frequency for both the received signal and the transmitted signal. Figure 5-1 is an initial look at the VFO we (YOU & I) are building.
[]
There are two 12V inputs: 12V comes
from the Power switch and TX 12V comes from the “T” side of the T/R switch.
[] There are two outputs, one going to the receiver section and one going
to the transmitter section.
[] The module provides regulated 9 volts for powering the oscillator and for
tuning purposes.
[] The tuning potentiometer, while a very important part of the VFO circuit, is
not physically located on the circuit board, but will be mounted on the front
panel of the transceiver.
The VFO module is built on TWO Radio Shack # 276-148 General Purpose Dual PC Boards, the Oscillator & voltage regulation on one board and the Buffer & Transmit/Receive switching on the second board. More about this, later.
Figure 5-2 shows a bit more detail. This module contains, not only the oscillator, but a buffer as well as two Zener diode voltage regulation circuits, one for the oscillator and one for the tuning voltage. Notice, also, there is a transmit/receive (T/R) relay, K1, for routing the output to the receiver section or the transmitter section.
A relay?!
Yes, a relay. There
are more clever and elegant methods, but I like relays for T/R switching.
Why? Two reasons.
First, the clever and elegant T/R switching arrangements are (usually)
intended to implement full break-in (QSK) operation and very low power
consumption. For this transceiver, neither low power consumption nor QSK
operation were priorities. Second, and probably more important, on those rare
occasions when things don’t quite work as planned, I find relay switching
easier to troubleshoot.
Alert readers, such as yourself, will probably recall that in Chapter 1: Introduction, the block diagram of this transceiver showed two VFO buffers, one for the receiver section and one for the transmitter section. That scheme has worked very well for me in a number of transceivers. I found via experimentation that using a single buffer and switching it’s output form receiver to transmitter can simplify building and testing of the module, and thus make life easier, especially for the inexperienced or out of practice builder.
It is also important to note that once power is turned on the oscillator runs continuously and the output is routed to either the receiver or transmitter, but never to both at the same time. Voltage distribution and T/R switching will be addressed in another chapter.
Experienced builders can easily place the entire VFO module
on one circuit board. For those who
are not so experienced, or simply like to spread things out a bit, I have
designed this version of the VFO so that the oscillator circuit is on one board
and the buffer & T/R switching is on another.
This wide-open arrangement is easier to build and (if necessary)
troubleshoot.
Figure 5-3 shows the schematic for both the oscillator and the buffer. This circuit is based upon an article by M. A. Chapman, K6SDX, in the November, 1975 issue of Ham Radio magazine (no longer published). Mr. Chapman would hardly recognize the circuit you see here because of the changes I have made.
Any analog VFO circuit will drift. Some drift less than others.
Given no swift changes in the ambient temperature, the drift of this VFO
is well within the tolerable range for Ham radio.
Back to TOP
Let’s step through the schematic before starting construction.
[] “7.xxx MHz” is rather indefinite, the reason being that YOU will determine what part of the band you want the VFO to cover when you do calibration. Calibration is down the road a bit, so don’t worry about that right now.
[] Everything above and to the left of the dotted line goes on the oscillator circuit board (except the 10-turn tuning potentiometer, which goes on the control panel).
[] Everything below and to the right of the dotted line goes on the buffer circuit board.
[] The eleven tie points (TP1 through TP11) provide for voltage distribution and facilitate testing & measurement during check-out on the OSCILLATOR board.
[] Four of the tie points are for power:
oTP1, TX +12V,
powers the transmitter offset circuit.
oTP5, +12V,
powers the tuning circuit.
oTP6, +12V, is
the entry point for power.
oTP8 is the +12V
input for the oscillator circuit
[] TP7 is connected to TP5 and/or TP8 with temporary jumpers for testing and measurements. Once the module has “passes” testing, permanent jumpers are installed.
[] The “ TX +12V” line to TP5 will come from the “T” side of the transmit/receive (T/R) switch in the finished transceiver.
[] The “+12V” line to TP6 will come from the power switch in the finished transceiver.
You’ll see more details about voltage distribution and T/R switching when we get to the control panel.
[] Another thing you have probably noticed in Figure 5-3 is that there are asterisks near some of the capacitors, and there is a note: “* = COG or NPO”. This is to alert you that these are special types of capacitors. Not just any type of capacitor can be used, they should be either type COG or type NPO in order to insure good thermal stability. The obsolescent, and relatively expensive, silvered mica capacitors may be substituted, if you happen to have some on hand.
[] Tie points TP2, TP3, and TP4 provide connection points for the 10k, ten-turn tuning potentiometer.
[] TP10 and TP11 provide convenient connection to the ground buss.
[] The drawing of the MPF102 clearly shows the “legs” for gate (G), source (S), and drain (D). This is one of those components that has a schematic symbol that can be a bit misleading because the symbol shows the gate as the “middle” leg even though the gate is actually one of the outer legs, as shown in the drawing.
[] All radio frequency chokes (RFC) in the VFO are 1 mH (1,000 uH) and should be rated for 50 mA, or more. I recommend 100 mA chokes for RFC2, RFC3 & RFC4 because they have less DC resistance and, thus, cause less voltage drop.
[] The diodes, D2 and D3, serve as a varactor to provide the “variable” part of the VFO. Most any silicon diode will serve here, but I have found that the physically larger types are less prone to thermal drift than their smaller cousins. If you use diodes that are NOT 1N4001, you may need to change the value of R4. The value shown, 3.3k, was found experimentally. Other diodes may require resistance from 500 Ohms to 15k in order for the low end of the band spread tuning to function properly. The problem is that diodes “forget” that they are acting as variable capacitors as the reverse bias approaches zero. Some only operate OK down to about 4.5 volts and some do just fine all the way down to le3ss than one volt.
[] The 1N4148 diode, D1, switches additional capacitance into the resonant circuit for transmitter offset and sets the transmitter about 750 Hz below the receiver frequency. More about this when we get to calibration.
[] The 1N4148 diode, D5, between the gate of the MPF102 and ground, is for oscillator stabilization.
[] The two Zener diodes, D4 and D6, are used as voltage regulators for the tuning voltage and the oscillator, respectively.
[]
The note regarding the coil, L, tells you it is constructed by winding 21 turns
(about 17 inches) of #24 enameled copper wire on a type T50-7 toroid coil form,
as shown in figure 5-4. Yes, you
will be winding several coils in the process of building this transceiver.
No, this is not just to make life difficult for you.
The reason you will be winding coils is that “off the shelf” coils
with the required specifications are difficult (or impossible) to find.
At first, winding coils may be a bit of a challenge, but by the time you
have your transceiver on the air, winding coils will be a “piece of cake”
for you.
Your ARRL
Handbook gives good instruction about winding coils, so I won’t go into great
detail here. One important ”rule
of thumb” to keep in mind is that windings should be evenly spaced over about
¾ of the core, as shown in Figure 5-4. (Yes,
I know that Figure 5-4 shows only about half the turns – the drawing got a bit
messy when I tried to show all 21 turns.)
This coil is wound on a special toroid core, type T50-7. What this means is shown in Figure 5-5.
The
“T” is a designator for a powdered iron core, as opposed to a ferrite core,
which would be “FT”. Powdered
iron and ferrite cores have different characteristics and you will be using both
types in this transceiver, but only the “T” type in the VFO.
The “50” means the core is
½ inch in diameter. Toroid cores
come in a variety of sizes from about ⅟₈
inch in diameter, such as the T12-6, to 5 inches in diameter, such as the
T500-2.
The type 7 core is made of a temperature stable material which makes it an excellent choice for use in a VFO.
[] You may have noticed that C1 has no value shown. The value of this capacitor is about 1 pF, but it varies from just under one pF to just over one pF. More about C1 when we get to calibration.
[] Nine tie points (TP12 through TP20) provide for voltage distribution and facilitate testing & measurement during check-out on the BUFFER board.
[] TP12, TP13, TP19, and TP20 provide convenient connections to the ground buss.
[] TP14 connects directly to TP9 on the oscillator board when the two boards are joined after being tested individually.
[] TP15, TX +12V, activates the relay and switches the signal to the transmitter section during transmit.
[] TP16, +12V, is the power input to the buffer board.
[] TP17 is for connecting the VFO output to the receiver section.
[] TP18 is for connecting the
VFO output to the transmitter section.
One last note regarding the circuit shown in figure 5-3 before we take a look at the parts list: C5 is a “special” capacitor that is actually two capacitors in parallel. C5a is on the top (component) side of the board and C5b is on the bottom (solder) side of the board. More about this, later, when we get to building and testing the oscillator circuit. The total value of C5 varies from circuit to circuit because of “stray” inductance and “wild” capacitance that occurs in the real world. This means some experimentation may be required in order to find the correct value. By dividing the total capacitance and mounting one part (C5a) permanently on the component side of the board and the other part (C5b) on the solder side of the board, C5b can easily be changed in order to adjust the total value, if required. I recommend C5a be 120 pF and C5b 100 pF as a starting point because that gives the total value of 220 pF, which is the value I have found to be correct most of the time for this circuit. More about this when we get to calibration.
PARTS LIST (for both the Oscillator and the Buffer)
Resistors: All resistors are 1/4 watt.
[] R1: 4.7k
[] R2: 180 Ohms
[] R3: 10k, Ten-Turn Potentiometer
[] R4: 3.3k
[] R5: 33k
[] R6: 100k
[] R7: 220 Ohms
[] R8: 390 Ohms
[] R9: See Text
Capacitors:
[] C1: (See text)
[] C2, C3, C4, C7, C12, C13, C14, C15: 0.047uF disc ceramic or monolithic
ceramic
[] C5: 220 pF, type COG or NPO (See text) Use a 120 pF and a 100 pF in parallel to fabricate C5.
Mount the 120 pF capacitor on top of the board, then solder the 100 pF in
parallel on the under side of the board
[] C6: 6 - 50 pF Trimmer, type COG or NPO
[] C9, C10: 220 pF, type COG or NPO
[] C11, C16: 150 pF
Diodes:
[] D1, D5: 1N4148, or equivalent switching diodes
[] D2, D3: 1N4001, or equivalent 50 volt, 1 Amp diodes
[] D4, D6: 1N4739A, or equivalent 9.1 volt Zener diodes
[] D7: 1N3600, or equivalent (See Text)
Transistors:
[] Q1, Q2: MPF102, or equivalent field effect transistor (FET)
Miscellaneous:
[] K1: Relay (See Text)
[] Two, Circuit Boards: Radio Shack #276-148 General Purpose Dual PC board, or
equivalent
[] About 48 inches #20 or #22 bare copper wire for ground buss and tie points on
circuit board
[] About 17 inches #24 enameled copper wire for winding coil, “L”
[] About 24 inches of hook-up wire for temporary connections and jumpers (#26,
stranded will serve nicely)
[] About 2 square inches of single-sided printed circuit board for making
special washers for attaching the coil to the circuit board (See text)
[] A #8 nylon bolt and nut for attaching the coil to the circuit board (see
text)
[] Hardware for mounting the circuit boards (see text)
[] A variety of small value COG or NPO capacitors for calibration (See Text).
I recommend at least one each: 2.2 pF, 4.7 pF, 10 pF, 12 pF, 15 pF, 18 pF,
22 pF, 33 pF, 47 pF, 68 pF, and 82 pF. If
you are lucky, you will not need any of these capacitors, but don’t count on
it.
Figure 5-6 shows the layout I used for the oscillator.
All
of the components (except the tuning potentiometer) fit on the Radio Shack,
#276-148 General-Purpose Dual PC Board, or equivalent.
These boards measure about 1¾ x 3½ inches. If you choose a different layout, keep in mind that you are
dealing with radio frequency (RF) circuits and the leads must be kept as short
and rigid as possible.
C1 is a special capacitor.
In Figure 5-6, C1 looks like a bug of some sort with long antennas.
What it is, however, is a “gimix” capacitor.
I have found that a gimix about 3/16” long, made with #28 enameled wire
at about 32 twists per inch was perfect for
transmitter offset in this circuit.
In case you are not familiar
with the term “gimix”, it is simply a capacitor of small value made by
twisting insulated wired together to make a tightly twisted pair and using the
capacitance between the two wires. I
have found that twisting #28 enameled copper wire with about 32 twists per inch
is a good way to make a gimix that is temperature stable with characteristics
similar to COG or NPO ceramic capacitors. Simply
cut about 12 inches of #28 enameled wire, fold the wire in half and secure one
end of the six inch pair in the chuck of your variable speed drill, and the
other end in a vice, or other holding device.
Keep modest tension on the wire as your drill slowly twist the wires
together.
Actually, I prefer using two pin
vices with the 6” wire pair secured between them because this gives a better
“feel” for the tension. Whatever
method you use, the excess twisted pair can be stashed away for making more
gimix capacitors, as the need arises.
For a 3/16” gimix, cut ½ “
from the twisted pair and separate about ¼ inch at one end.
Spread the separated wire and strip the enamel and “tin’ the ends
with a light coat of solder. This
end will be used to solder the gimix into the circuit.
At the other end separate about 1/32” of wire and leave the enamel on
the wire for insulation. This end
needs a slight separation to insure the two wires are not “shorted”
together. When calibration is
completed, press the gimix flat against the circuit board and glue it in place
to prevent mechanical movement.
I suggest you try a gimix first,
and if that doesn’t work, you can try a 1.0 to 3.0pF NPO trimmer capacitor,
MOUSER #659GKG3R015, or equivalent.
If you use the gimix, it must be pressed flat and glued to the board when calibration is finished in order to prevent mechanical movement.
The problem is that with this
circuit the magic value for C1 is very close to one pF.
If, per chance, the required value turns out to be less than one pF, you
may have a problem finding a suitable commercially available trimmer.
There are some reasonably priced trimmers available, such as the MOUSER
#81-TZY2Z010A001R00, but these are surface-mount types that are difficult to
work with, especially for an inexperienced builder.
Others, such as the MOUSER Electronics #659-GAA6R012, are physically
larger and relatively easy to work with, but they are expensive, costing well
over $20 each, including shipping.
One alternative for a
less-than-one-pF trimmer is to use a small NPO trimmer, such as the MOUSER #
659-GKG10021 in series with a 1 pF fixed NPO capacitor.
Whatever you use for C1, be
aware that C1 affects the overall calibration of the oscillator, so any changes
you make to C1 will change the overall calibration. This simply means there is a bit of “give and take”
between adjustments of C1 and C6 when calibrating the oscillator.
Notice that capacitor C5a, just
to the right of the coil and below D2 in Figure 5-6, is laying flat on top of
the board. This is to minimize
mechanical “wiggle” which can cause frequency “jiggle” in the
oscillator. Capacitor C5b should be
similarly mounted on the under-side of the board in parallel with C5a. Once calibration is complete, both C5a and C5b must be glued
to the board to prevent mechanical movement.
You may be wondering why in the
world two capacitors are used for C5 instead of one. Flexibility is the answer.
Many times, when building resonant circuits, there is quite a lot of
“cut and try” involved in order to find exactly the right combination of L
and C for a particular circuit. It
is easier to cut and try with a capacitor than with a “caged L”.
With this circuit, I wanted the frequency adjust capacitor, C6, to be at
approximately mid-range when the circuit was oscillating at 7.040 MHz.
In order to do this, I started with a 120 pF capacitor for C5a, and added
capacitance until I got what I wanted. It
turns out that I hit it on third try with a 100 pF capacitor, thus the value of
220 pF for the combined capacitance of C5a and C5b.
You, too, may have to do a bit of experimenting in order to find exactly
the right value because the stray inductance and wild capacitance in your
circuit will probably be different than mine.
More about this when we get to calibration.
Yes, you can accomplish “fine tuning” by changing the inductance of the coil, but this is impractical with a “caged coil” such as the one being used here.
Whether you use the RS #276-148, an “equivalent” perfboard, a printed circuit board, or whatever, it is important to have the coil firmly mounted and “caged” to help minimize frequency drift due to vibration and temperature changes. One wire at each corner of the top & bottom washers makes a fine cage in which to house the coil, as shown below. Use #22, or larger, bare copper wire. The coil mounting is shown in Figures 5-7 and 5-8.
Figure 5-8 shows the wires soldered to the top and bottom washers to form
a shielding “cage”. The top and bottom washers of the mounting are cut from
single-sided printed circuit board, about 3/4” square with a hole in the
middle for a nylon bolt, which holds the coil securely to the circuit board.
The non-foil side on the washers go toward the coil.
This firm mounting helps eliminate mechanical movement and “stray”
capacitance, both of which can cause VFO drift.
Use a ¾” nylon bolt and trim excess length after tightening the nut.
Very little torque is required – tighten just enough to hold the coil
snugly in place.
If you choose to build on the RS 276-148 (or equivalent) board, I suggest the first thing you do is run a bare copper wire around the outside edge on the bottom of the board for a ground buss, as shown in Figure 5-9.
The boards will be attached to the chassis and/or to each other using
spacers, so the spacers should be in place before you put the ground buss wire
on the board. This will assure that
the wire goes around the spacers and leaves plenty of room for mounting
hardware. The loose ends of the
ground bus should ever-lap about ¼” and be soldered. You can see the buss
wire going around the nuts on the top side of the board in Figure 5-10.
Figure 5-10 shows the VFO built on a Radio Shack 276-148 General Purpose
Dual PC Board, with some of the components labeled.
You may find Figure 5-6, and 5-10 to be a useful reference while
assembling your VFO.
The most likely cause of incorrect measurements on a
newly built circuit is a wiring error of one sort or another.
Wiring errors include:
[] Missing or “extra” components
[] Components not connected
[] Components connected to the wrong place
[] Connections not soldered
[] “Cold” solder connections
- insulation
material, such as enamel, not removed before soldering
- not enough
heat applied to “flow” the solder
[] Solder “bridges” causing a “short” circuit
Begin your check out with a thorough VISUAL inspection to make sure that there are no missing (or “extra”) parts and that everything is connected correctly and soldered. Perhaps I am the only person in the whole world who occasionally finds an “extra” resistor or capacitor on my newly assembled circuit, but I doubt it.
If you find a
discrepancy in component count, use the layout diagram in Figure 5-6 and/or the
schematic in Figure 5-3 to find the error.
Component Count
[] The coil, L, should be mounted to the board within its “cage”
[] Count: four 1 mH radio frequency chokes (RFC)
[] Count: 13 ceramic capacitors (Remember, C5 is actually two capacitors.)
[] Count: 2 “trimmer” capacitors, one of which may be a "gimix"
[] Count: 6 diodes
2 Zener diodes
2 Tuning diodes
2 Switching
diode
[] Count: five ¼ watt resistors
(The tuning potentiometer, R3, which is not mounted on the board.)
[] Count: eleven tie points
Missing or extra components will, of course, become obvious during resistance checks and/or current & voltage checks, but a good visual check can save a lot of troubleshooting effort.
Assuming the component count is correct, check all connections on the “under-side” of the board to be sure they are properly soldered. I use a magnifying glass for this check because I have found that magnification sometimes shows problems that I would miss with the naked eye.
Correct any errors
found during the visual check, then proceed to resistance measurements.
Initial resistance measurements are done with NOTHING
except your DMM connected to the circuit. (NO
power connected, NO jumpers installed and NO tuning potentiometer connected.)
First, TP10 and TP11 should measure ZERO Ohms to the ground buss because ALL OTHER RESISTANCE MEASUREMENTS are from TP10 or TP11 to the point indicated.
If any of the resistance readings are “out of the
ballpark” (more than plus or minus about 15%) you have one or more wiring
errors.
[] TP1, TX +12V input: Open, no continuity to ground
[] TP2, Tie Point for R3 wiper: Open, no continuity to ground
[] TP3, Tie Point for “top” of R3: 3.3k
[] TP4, Tie Point for “bottom” of R3: Open, no continuity to ground
[] TP5, +12V input for tuning voltage: Open, no continuity to ground
[] TP6, +12 volt input: Open, no continuity to ground
[] TP7, +12V Tie Point: Open, no continuity to ground
[] TP8, +12V input to oscillator circuit: 356 Ohms
[] TP9, Oscillator Output: Open, no continuity to ground
[] Q1, MPF102
Drain: 140 Ohms
Gate: 33k
Source: 4 Ohms (Assuming RFC3 is a 100 mA choke.
The resistance will be two to five times more for a choke rated at less
than 100 mA.)
Find and correct any wiring errors revealed during resistance measurements, then proceed to current measurements.
It is unlikely that there are wiring errors IF all the
resistance measurements are OK (within ± 10%, or so), but strange things
sometimes happen in newly-built circuits, so current and voltage measurements
must be done before operational testing – just in case.
There are four current measurements that should be checked to insure that excessive current is not being drawn due to a wiring error that, somehow, escaped detection during resistance measurements.
[] TP1: Current being drawn by the transmitter offset
circuit, about, 2.5 mA
[] TP5: Current drawn by the tuning circuit, about 15 mA
[] TP6: NO current should be drawn at the +12V input when NO JUMPERS are
connected to TP7
[] TP8: Current drawn by the oscillator circuit, about 15 mA
In order to do these current measurements, you will need a 12 volt supply
that can be varied from about zero volts to about 12 volts.
If you don’t have a variable 12 volt supply, no problem.
It is easy to fabricate such a supply that will suffice for these tests
using eight AA batteries as shown
in Figure 5-11.
You will need:
[] Batteries: 8 AA batteries
[] Battery Holder: to hold the 8 AA batteries
[] Control: 1000 Ohm, ½ watt potentiometer
[] SPST switch
[] Meter: Your DMM, or other current measuring device
[] Hook-up wire and/or test leads
Notice that the1k potentiometer in Figure 5 -11 is
connected so that the wiper is at the “bottom” (ground) when turned fully counter
clockwise (CCW). Begin each current
test with the potentiometer set fully CCW (zero power out).
As you turn the control clockwise (CW) toward the “top”, both voltage
and current will increase. The
electrons don’t care which way you connect the potentiometer, but the
conventional direction for increase on controls is clockwise.
The switch is optional, but keep in mind that about 12 mA of current will be drawn continuously whenever the 1k potentiometer is connected, whether or not a circuit is connected for testing.
+12V INPUT
[] Turn the power control to zero output.
[] Connect power source, positive to TP6 and negative to TP10 or TP11.
[] Monitor the current as you slowly advance the power control.
[] NO current (zero, zip, nada) should be drawn at this time with 12 volts
applied to TP6. If you measure any
current here, you have a wiring error.
Correct any errors found during +12V input measurement,
disconnect power from TP6, and proceed to transmitter offset measurement.
TRANSMITTER OFFSET CURRENT
[] Turn the power control to zero output.
[] Connect power source, positive to TP1 and negative to TP10 or TP11.
[] Monitor the current as you slowly advance the power control.
[] The current should rise to about 2.5 mA with 12 volts applied to TP1.
If you measure more than 3 mA, or if you measure no current at all, you
have a wiring error.
Correct any errors found during transmitter offset
measurement, disconnect power from TP1, and proceed to tuning current
measurement.
TUNING CURRENT
[] Turn the power control to zero output.
[] Temporarily connect the ten-turn, 10k tuning potentiometer to TP2, TP3, and
TP4 as shown in Figure 5-3. Turn
the tuning control fully CCW.
[] Connect power source, positive to TP5 and negative to TP10 or TP11.
[] Monitor the current as you slowly advance the power control (not the tuning
control).
[] The current should rise to about 13 mA with 12 volts applied to TP5.
If you measure more than about 16 mA, or if you measure no current at
all, you have a wiring error.
Correct any errors found during tuning current measurement, disconnect power from TP5. Leave the ten-turn, 10k tuning potentiometer connected and set it to five turns from either stop, and proceed to oscillator current measurement.
OSCILLATOR CURRENT
[] Turn power control to zero output.
[] Connect power source, positive to TP8 and negative to TP 10 or TP11.
[] Monitor the current as you slowly advance the power control.
[] The current should rise to about 15 mA with 12 volts applied to TP6.
If you measure more than about 18 mA, or if you measure no current at
all, you have a wiring error.
Correct any errors found during oscillator current
measurement, disconnect power from TP8, and proceed to voltage measurements.
Voltage measurement assumes that YOU HAVE ALREADY DONE
CURRENT MEASUREMENTS, and corrected any problems found during those
measurements. Power is, therefore,
applied directly, full 12 volts applied to the specified tie points.
The voltage measurements shown below were taken with 12.3
volts applied at the specified tie points.
It is unlikely that your “12 volt” supply voltage will be the same as
mine, and your voltage measurements will vary accordingly, except for the tuning
voltage which is a regulated 9 volts. In any event,
your measurements should be within about ±10 percent of the measurements shown
here.
+12V INPUT
[] Apply 12 volts, positive to TP6 and negative to TP10 or TP11.
[] The voltage at TP7 should be the same as TP6.
If they are not the same, you have a wiring error.
Correct any errors found during +12V input measurement,
disconnect power from T
P6, and proceed to tuning voltage measurement.
TUNING VOLTAGE
[] Attach tuning potentiometer; Wiper to TP2, “Top” to TP3, and “Bottom”
to TP4.
[] Apply 12 volts, positive to TP5 and negative to TP10 or TP11.
[] The voltage at TP4, the “top” of the tuning potentiometer, should be 9
volts. If you measure more than 9.2
volts or less than 8.8 volts, you have a wiring error.
[] The voltage TP2, the wiper of the tuning potentiometer, should vary from
about 3.6 volts to about 9 volts as
you advance the control from fully CCW to fully CW.
Correct any problems revealed by the tuning voltage
measurements, disconnect power, then proceed to oscillator voltage measurements.
OSCILLATOR
You can combine operational testing with these voltage
tests. In order to do this, you
need a receiver capable detecting continuous wave (CW) signals in and around the
40 meter band. An oscilloscope is
helpful, but not required.
[] Set the tuning potentiometer to mid-range (five turns
from either stop).
[] Connect a temporary jumper from TP7 to TP5.
[] Connect a temporary jumper from TP7 to TP8.
[] Apply 12 volts, positive to TP6 and negative to TP10 or TP11.
[] Search for a signal by tuning your receiver in the vicinity of the 40 meter
CW band. Be bold in your search for
the signal. You may find it
hundreds (thousands?) of kHz above or below the target frequency of 7.xxx
because you have not yet calibrated your VFO. If you can find no oscillation between about 5 MHz and 10
MHz, you have one or more wiring errors. Find
and correct any problems before continuing with live checks.
[] If you have an oscilloscope, check for a signal of about 2.5 volts, peak to
peak (p-p) at TP9. If you see no
signal here, you have one or more wiring errors because this oscillator will
function with a supply voltage from about 3 volts (with reduced output) to about
15 volts.
Once you have verified that oscillation is taking place,
check voltages on Q1, as follows.
[] The voltage at the drain of Q1 should be about 9 volts.
[] The voltage at the gate of Q1 should be about MINUS 1.5 volt.
[] The voltage at the source of Q1 should be near zero.
You may measure a small positive voltage, 0.01 volt, or so.
Correct any problems discovered during oscillator voltage
checks, then repeat all oscillator measurements to be sure everything is OK.
Congratulations! You
have completed initial check-out of your oscillator circuit.
Remove the temporary jumpers and install permanent
jumpers from TP7 to TP5 and TP8.
Set the oscillator circuit board aside, take a well
deserved break, then proceed to building the VFO buffer circuit.
After the buffer circuit board is built and tested, the
two VFO boards can be combined for calibration and final check-out of the VFO.
CALIBRATION OF YOUR VFO WILL BE ADDRESSED AFTER YOU HAVE BUILT THE VFO BUFFER.
Now that you have built, and completed the initial tests
and measurements for the Oscillator, it is time to add the VFO Buffer circuit
board in order to complete the VFO module.
The VFO Buffer is relatively easy to build and test.
Figure 5-12 shows the schematic for the buffer circuit.
There are nine tie points on this circuit board, TP12 through TP20.
[] TP12, TP13, TP19 and TP20 are connected to the ground
buss, and provide convenient ground connections for testing and measurement.
TP19 & TP20 also serve as tie points for the shield of coax that
connects to the transmitter and receiver sections of the transceiver.
[] TP14 is the input from the VFO Oscillator circuit board.
[] TP15 is for T/R switching.
[] TP16 is the +12 volt input for the Buffer circuit board.
[] TP17 is the tie point for taking the VFO signal to the receiver
section of the transceiver via miniature coax.
The shield on the coax will be soldered to TP20.
[] TP18 is the tie point for taking the VFO signal to the transmitter section of
the transceiver via miniature coax. The
shield on the coax will be soldered to TP19.
[] Q2 is an MPF102 Transistor.
[] You have probably noticed that R9 has no value specified.
If you use a 12 volt relay, R9 is not required.
If, on the other hand, you use a relay that requires less than 12 volts
for operation (as I did) then a voltage dropping resistor is required.
Use Ohm’s law to calculate the value for R9 for your relay.
[] D7 may also be an optional component. Some
relays come with a built-in diode. If
the relay you use already has a diode, omit D7.
Back to TOP
This mounting scheme is not a requirement – it just happens to be the way I
assembled my VFO module. By doing
it this way, the connections between the two boards are kept short, and no coax
is required for connecting the output from the oscillator to the input of the
buffer. The boards are designed so
that the output from the oscillator board, TP9, is directly above the input for
the buffer board, TP14. These two
tie points and the connection between them are shown in Figure 5-14. As you can see, TP14 is not a “normal” tie point, but
simply a wire connecting the output form the oscillator to the buffer.
In order to keep the drawing simple and uncluttered, I have not attempted
to show the components that are connected to TP9 and TP14.
C11 is soldered to TP9, as shown in Figure 5-3.
The gate of Q2 and R7 are soldered to TP14, as shown in Figure 5-12.
Figure 5-15 shows the layout I used for the buffer board.
This layout is “backwards” compared to the schematic;
that is, the input coming into the right-hand end on the circuit board so that
TP9 on the oscillator board is directly above TP14 on the buffer board.
Figure 5-16 shows a top view of the buffer board with some of the components identified.
The relay, K1, shown here is a 6 volt relay with a built-in diode that I happened to have on hand, so I omitted D7. With a 6 volt relay, the voltage dropping resistor, R9, is required.
An alternate circuit using two SPST reed relays is shown in Figure 5-17.
This dual 12 volt reed relay circuit requires both RX +12V and TX +12V in
order to do the switching. There is
plenty of space on the circuit board for most any switching arrangement you want
to use.
Figure 5-18
shows the small ALTOIDS box containing the oscillator circuitry (about 150% full
size as seen here). The ALTOIDS box
is the tiny one that measures about 1 ½” x 2⅜” x
⅝”. The box provides
excellent shielding, and this is one of the most stable analog VFOs I have ever
built. The two buffer circuits (one
for the receiver and one for the transmitter) are shown on the right-hand end of
the board in this view.
Figure 5-19 shows the VFO with the ALTOIDS box cover removed. The coil (hidden by other components in this photo) takes up almost half of the space inside the box.
I mention this ALTOID box VFO only to show that the VFO can be made very small if the builder (YOU) have small size as a priority.
FYI: The
relatively large disc ceramic capacitors you see in Figure 5-19 near the
right-hand end of the ALTOIDS box are now obsolete.
The much smaller monolithic capacitors, such as the four tiny yellowish
dots seen on the right-hand end of the circuit board in the buffer circuits have
now replaced the older ceramic types. Then,
of course, there is the more modern surface-mount capacitors that are about the
size of this equal sign =, but that’s another story.
This is all well and good, but right now you have a buffer
circuit to check out, so let’s get to it.
The most likely cause of incorrect measurements on a
newly built circuit is a wiring error of one sort or another.
Wiring errors include:
[] Missing or “extra” components
[] Components not connected
[] Components connected to the wrong place
[] Connections not soldered
[] Connections incorrectly soldered
- insulation
material, such as enamel, not removed before soldering
- not enough
heat applied to flow the solder, causing a “cold” solder joint
[] Solder “bridges” causing a “short” circuit
Begin your check out with a thorough VISUAL inspection to make sure that there are no missing (or “extra”) parts and that all connections are properly soldered.
BUFFER RESISTANCE MEASUREMENTS
All resistance measurements are from ground to the point
indicated.
[] TP14: 100k
[] TP15: 440 Ohms
[] TP16: 528 Ohms
[] TP17: Open, no continuity to ground
[] TP18: Open, no continuity to ground
Current measurements are done with NOTHING connected to
TP14 , TP17, or TP18.
[] Set the output of your variable output 12 volt source to zero.
[] Attach the 12 volt source to the buffer board, positive to TP16 and negative
to one of the grounded tie points.
[] Monitor the current as you
slowly increase the output to 12 volts. The
buffer circuit should draw a maximum of about 4.5 mA.
If your buffer draws excessive current, or if it draws no current at all,
you have a wiring error.
Correct any errors found, then proceed to relay current measurement.
RELAY CURRENT MEASUREMENT
Your relay specifications tell you how much current it will
draw. For example, I used a 6 volt
relay which draws about 25 mA.
[] Set the output of your variable output 12 volt source to zero.
[] Attach the 12 volt source to the buffer board, positive to TP15 and negative
to one of the grounded tie points.
[] Monitor the current as you
slowly increase the output to 12 volts. If
your relay draws more current than specified by the manufacturer, or if there is
no current draw at all, you have a wiring error.
Correct any errors found during current measurements, then proceed to buffer voltage measurements.
Voltage measurements are from a ground to the point
indicated.
[] Connect 12 volts to the buffer board, positive to TP16 and negative to a
convenient ground point.
[] Voltages on the transistor should be:
Drain: 12 volts
(or whatever the output of your 12V supply happens to be)
Gate: Zero
Source: About 2
volts
One thing that I seldom see mentioned in Ham Radio
literature is the task of getting the transmitter offset adjusted properly in
transceivers that use direct conversion in the receiver section.
The offset is accomplished by switching “extra” capacitance into the
resonant circuit when transmitting. This
method yields the correct offset at one, and only one frequency, call it fx.
As you tune below fx, the offset becomes less and less, and as you tune
above fx, the offset becomes more and more.
In a stand-alone direct conversion receiver, where you need not worry
about transmitter offset, this is no problem, but in a transceiver where the VFO
is shared by both receiver and transmitter it limits the useful spread of
frequencies.
Construction articles usually show a small value variable
capacitor labeled “Offset Adjust” and leave the builder “twisting in the
wind”, so to speak.
Fortunately for you (and for me), this transceiver has a
relatively narrow frequency spread, so the transmitter offset is not so much of
a problem. You will, however,
notice some change in offset as you tune from the bottom of your band spread to
the top. Typically, the offset will be from
about 500 Hz to about 1kHz as you tune "up" the band. I
mention this limitation of simple direct conversion receivers so you will not go
crazy trying to obtain a uniform offset across the entire band spread.
You can eliminate this problem entirely by using a
heterodyne receiver section instead of a direct conversion receiver section, but
that’s another story.
The computer between your ears will learn to compensate for
the variation.
That said, let’s proceed with calibration.
OSCILLATOR CALIBRATION
You will need:
[] A general coverage receiver capable of detecting continuous wave (CW) signals
in and around the 40 meter band
[] Your oscillator module that has “passed” all the tests during check-out
[] 12 volt power supply
[] A tiny screwdriver for adjusting C6 (and C1, if you are using a trimmer
instead of a gimix)
[] About six inches of insulated hook-up wire with insulated alligator clips
soldered to each end
[] Patience and Persistence
NOTE: Simply touching the adjustment slot of a trimmer
capacitor with a metallic screwdriver will change the capacitance and, thus, the
frequency of oscillation. Adjustment
requires a very light touch. Ceramic
adjustment tools are available, but in my opinion, are not worth the cost (about
$50, including shipping from Digi-Key).
During voltage measurements for the oscillator, you
verified that the circuit was working, perhaps a bit off frequency.
By adjusting C6 you can move the frequency more than 500 kHz, so you can
probably obtain the desired frequency with a simple adjustment.
It is unlikely, but possible, that adjusting C6 does not do
the trick. In this case, you must
increase or decrease the value of C5b.
To decrease the frequency, increase the value of C5b.
To increase the frequency, decrease the value of C5b.
This is a simple concept, but doing it may seem daunting, particularly if this is your first time through such a procedure. Experience with “cut and try” is the only guide to knowing just how much to increase or decrease the value of C5b in order to obtain the frequency you want. A large change in capacitance will, of course, create a large change in frequency; and visa-versa. A change in C5b of 5 pF, or less should bring your oscillator into the range of frequencies you want.
OK. Now you
know what is required for calibration, so let’s do it.
First, choose where in the 40 meter CW band you want to
operate. With this VFO circuit, you will have about 50 kHz in which to operate.
For example, if you choose 7.040 MHz at the middle of your band spread
(my recommendation), you will be able to operate from about 7.0150 to about
7.0650 MHz.
Next, find the bottom and top frequencies of your
oscillator, as it exists right now, before calibration.
[] Temporarily connect the ten-turn tuning potentiometer; “Top” to TP4,
“Wiper” to TP2, and “Bottom” to TP3.
[] Set the tuning potentiometer fully CCW (lowest frequency).
[] Be sure nothing is connected to TP1, transmitter offset.
[] Be sure jumpers are installed from TP7 to TP5 and TP8.
[] Apply +12 volts to the circuit, positive to TP6 and negative to TP10 or TP11.
[] Tune your monitoring receiver until you find the signal.
When you hear a tone of about 750 Hz, record the frequency;
this is the bottom frequency of your band spread.
[] Set the tuning potentiometer fully CW (highest
frequency).
[] Tune your monitoring receiver until you find the signal.
When you hear a tone of about 750 Hz, record the frequency;
this is the top frequency of your band spread.
Subtract the bottom frequency from the top frequency.
You should get a value somewhere in the range of about 45 to 60 kHz.
This is your operational band spread within the 40 meter CW band.
If your band spread is where you want it (or close enough),
congratulations, you are finished with oscillator calibration.
Go to transmitter offset
If your oscillator needs adjustment, a suggested method is shown below.
NOTE: By adjusting C6, you can move the frequency about 500
kHz. This means that very little
movement of C6 can make a relatively large change in frequency.
The procedure described below is based on the assumption
that you are using a 3/16” gimix capacitor for C1, or, perhaps, a 1 pF NPO
(non-adjustable) capacitor. If you
are using a trimmer capacitor for C1, the procedure is essentially the same, but
there will be some “give and take” between adjusting C1 and adjusting C6 in
order to complete the calibration.
[] Be sure nothing is connected to TP1, transmitter offset.
[] Set the tuning potentiometer to mid-range (5 turns from either stop).
[] Power-up the receiver you are using to monitor the signal, and tune it to
7.040, set for CW reception.
[] Be sure jumpers are installed from TP7 to TP5 and TP8.
[] Apply +12 volts to the circuit, positive to TP6 and negative to TP10 or TP11.
[] Adjust C6 until you hear a tone in the monitoring receiver.
Congratulations! You
know you are “in the ballpark”. Proceed
to transmitter offset adjustment.
In the unlikely event that adjusting C6 does not get your
oscillator to where you want it, you must do some “trial and error” type
adjustment by changing the value of C5b.
If the existing band of frequencies is above where you want
it, increase the value of C5b.
If the band of frequencies is below where you want it, decrease the value of C5b.
As you add or subtract capacitance, swing C6 through its
limits after each change in order to determine where the upper and lower
frequencies are. This will give you
a feel for how much more (or less) capacitance you need in order to be on
target.
Its as simple as that, but making it happen can be time
consuming, particularly if you must go through several iterations of adjusting
the oscillator so it is where you want to be in the 40 meter CW band.
Patience and persistence will win the day!
If you have used the gimix capacitor for C5 (other things
being equal and with any luck at all) the transmitter offset will be very close
without making any changes or adjustments.
First, find out what, exactly, the transmitter offset is as
it exists right now:
[] Apply power to the oscillator.
[] Find the signal on your monitoring receiver and tune for a tone of about 750
Hz.
[] Record the frequency displayed on your monitoring receiver.
This is Freq 1.
[] With the oscillator running, apply +12V to TP1 by connecting a temporary
jumper from TP6 (+12V) to TP1 (TX +12V). I
recommend a wire with alligator clips at each end.
The frequency will now be BELOW wherever it was before the
jumper was connected from TP6 to TP1.
NOTE: If there is no change in the tone, you have a wiring
error in the transmitter offset circuit. Find
and correct any errors before proceeding.
[] Tune your monitoring receiver until the tone is once
again about 750 Hz .
[] Record the frequency displayed on your monitoring receiver.
This is Freq 2.
[] Calculate your transmitter offset by subtracting Freq 2 from Freq 1.
The result should be in the range of 500 to 1000 Hz.
If your offset is more than about 1000 Hz, decrease the
value of C1.
If your offset is less than about 500 Hz, increase the
value of C1.
Repeat the Transmitter offset adjustment until the offset is somewhere in the 500 Hz to 1000 Hz range.
This is where using a "gimix" for C1 can be problematic because the length of the gimix must be changed in order to change its capacitance. If your offset is more than 1kHz, that is relatively easy to ajust - simply cut s bit (about 1/32" per try) off the free end, separate the wires at the cut end to prevent a short, re-apply TX Offset voltage, and see where you are. If your offset is less than 500 Hz, that is more of a problem because you can't "stretch" the gimix to increase the capacitance; you must replace the gimix with a new (longer) one.
NOTE:
If you have made changes in C1 in order to have the correct transmitter
offset, you may need to repeat the oscillator calibration procedure because
there is interaction between the circuits even without TX offset being
activated. That’s the bad news.
The good news is that the procedure goes much quicker the second time through. Patience and persistence will win the day.
VFO & BUFFER (combined) CURRENT MEASUREMENT
[] Mount the oscillator board atop the buffer board using
½” spacers, as shown in Figure 5-13. Route
the wire from TP14 through a hole adjacent to TP9 on the oscillator board and
solder it to TP14.
[] Connect TP10 to TP12 using bare copper wire.
Solder both ends.
[] Connect TP19 to the ground buss on the oscillator board using bare copper
wire. Solder both ends.
[] Connect TP20 to the ground buss on the oscillator board using bare copper
wire. Solder both ends.
[] Connect TP6 to TP16 with a short piece of insulated hook-up wire. (I use red
for 12V power leads.)
[] Connect TP20 to the ground buss on the oscillator board using bare copper
wire. Solder both ends.
[] If not already in place, connect the tuning potentiometer; “Top” to TP4,
“Wiper” to TP2, and “Bottom” to TP3.
[] Be sure your variable 12 volt supply is set to zero output.
[] Connect to the combined VFO
& Buffer cards, positive to TP6 and negative to a convenient ground point.
[] Monitor the current as you slowly increase the output to 12 volts.
The combined VFO & Buffer should draw about 29 mA.
If excessive current flows,
or if no current flows, you have a wiring error.
Correct any error found during the combined current
measurement, the set the VFO and buffer cards aside.
You are now ready for an operational check of your VFO
and Buffer cards. In order to do
this, you need ALL the receiver modules because the operational test is a live
“on the air” test. You have
already built and tested the Product Detector module and the Audio module, so
only the receiver 40 meter filter must be completed before doing the operational
test. The filter is detailed in
Chapter 6.
Now that you have completed your VFO module, Product
Detector module, & Audio module, and (almost) have a usable receiver.
I say ‘almost” because, while the modules are functional at this
point, the receiver section of your transceiver is NOT ready for use until you
add the 40 meter filter. Without the filter, the receiver is marginal, at best; and nothing more
than an interesting paper weight, at worse.
Yes, you can hear lots of signals without the filter, probably AM
broadcast signals. If you are very
lucky, you may be able to hear a 40 meter Ham Radio signal, or two.
With the filter, you will hear no AM broadcast signals.
With the filter, and with favorable propagation, you will hear 40 meter signals from hundreds of miles away, and with any luck at all, signals from many other countries. For example, here on the west coast of the US, I regularly hear signals from Japanese, Korean, and Siberian Ham Radio operators. Occasionally, I hear European stations, but not nearly as
often as far eastern stations. If
you happen to be on the east coast of the US, you will probably hear European
stations more often than far eastern stations.
Meanwhile, there is more work to do before the receiver is
finished.
END OF CHAPTER 5: VFO
Back to 40 Meter QRP Table of Contents
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