CHAPTER 4: PRODUCT DETECTOR
This chapter includes:
[] Introduction (You are Here)
[] Parts List
[] Building the Product Detector
[] Check ? Out
o Visual
Inspection
o Resistance
Measurements
o Current
Measurements
o Voltage
Measurements
o Operational
Test
There are two types of product detectors, active and
passive. Within each type of
product detector, there are a number of circuits that will do the job.
Each type has advantages and disadvantages.
Important differences include:
[] Active product detectors require power whereas the
passive product detectors do not.
[] Active product detectors can provide signal amplification whereas passive
detectors cause loss of signal.
[] Passive product detectors using inexpensive diode ring mixers are, as a rule,
easier and simpler to build, and produce excellent results.
This is the type I have chosen.
Things
to notice in the simplified diagram shown in Figure 4-1:
[] This module uses a passive diode ring mixer, but the module also contains pre
and post mixer amplifier circuits, thus the requirement for a 12 volt power
source. RX 12V is used to power the
product detector module. This means
the module is active only while receiving.
[] In addition to 12 volts for
power, there are two inputs to the 40 meter product detector, a 7.xxx MHz signal
from the antenna and a 7.xxx MHz signal from the variable frequency oscillator (VFO).
[] A VFO, or other signal source, is required in order to test the
product detector.
[] A radio frequency (RF) gain control is provided.
The heart of any product detector is nothing more (and nothing less) than a
MIXER. Mixers that combine two
radio frequency (RF) signals to produce an audio signal are commonly called
product detectors. Mixers
that combine two RF signals to produce an intermediate frequency (IF) are
commonly called converters. Both
types are referred to by the generic term: mixer.
Figure 4-2 shows where the product detector module fits within the
receiver section, and where the mixer (circle around an "X") fits into the
product detector module.
Notice that the product detector module contains a radio
frequency (RF) amplifier and an audio buffer in addition to the mixer.
My original design for this receiver included a VFO amplifier, but during
testing I discovered that performance was virtually identical with, or without
the VFO amplifier, so the amplifier has been omitted.
For communications receivers in general, and for direct
conversion (DC) receivers in particular, an RF amplifier can make a big
difference in the performance of the receiver.
Yes, this receiver will hear signals without the RF amplifier, but,
the RF amp can be the difference between having a good QSO and having no QSO at
all if the signal is really weak.
For those who may not be familiar with the term, direct conversion simply means that the RF signal coming from the antenna is mixed with the VFO signal to produce an audio signal, DIRECTLY, as opposed to going through an intermediate frequency (IF) before being converted to audio frequencies that you can hear. DC receivers have the advantage of being relatively simple and easy to build, and, with careful construction, their performance can be quite good. This particular receiver circuit hears all but the very weakest signals detected by the ICOM transceiver I use as my main rig. Selectivity is a different story because this simple receiver does not have the sophisticated selectivity circuits that my ICOM has. I must point out, however, that, even with less than optimum selectivity, this receiver will perform well in all but the most demanding conditions. I will also add that I have had hundreds of QSOs using receivers with performance much less than the receiver we (YOU and I) are building here.
The product detector circuit, shown in Figure 4-3, uses a
simple and effective diode ring mixer, the SBL-1. The SBL-1 is an eight-pin, inexpensive, physically small,
readily available, device that greatly simplifies construction.
Yes, diode ring mixers can be built from discrete components, but I find
construction of them to be extraordinarily tedious and time consuming, so I have
chosen the SBL-1 in order to make life easier for you and for me.
Browsing through the schematic in Figure 4-3 you see the following:
[] Tie points TP1, TP2, TP3, TP4, TP5 and TP6.
The tie points allow initial resistance and voltage measurements to be
done on individual sub-modules so that wiring errors can be found and corrected
before testing the complete product detector.
TP5 & TP6 provide points for connecting the ?top? and wiper of
R1, the RF gain control. The ?bottom? of R1 can be connected to any convenient
ground point. After initial
resistance and voltage measurements, wire jumpers are installed; W1 between TP1
& TP2, and W2 between TP3 & TP4.
[] Wire jumpers, W1 and W2 are installed AFTER initial measurements are done.
[] The RF amplifier circuit (Q1 and Q2) provides for an RF gain control in
addition to boosting weaker signals to a useable level.
The signal is coupled directly into the SBL-1 mixer by feeding the
regulated 6 volts into pin 2, then out pin 1 to the collector of Q2.
[] The SBL-1 diode ring mixer is a passive device.
No external power is required in order for it to mix the signal of
interest with the signal from the VFO.
The regulated 6 volts is simply ?passing through? from pin 2 to pin 1
in order to power the RF amplifier. This
?pass through? also serves to couple the signal into the diode ring mixer.
For those who may be curious as to how this coupling is accomplished,
notice in figure 4-4 that the primary winding of the input transformer in the
SBL-1 is connected to pins 1 & 2. The
RF amplifier will draw varying amounts of current in sync with the incoming
signal, thus causing corresponding variation in the magnetic field surrounding
the primary winding. These
extremely small variations are "transformed" to the secondary windings by
magnetic coupling.
At first glance, it may appear that the diode arrangement in the SBL-1 is the
same as bridge rectifiers used in power supplies, but that is not the case.
Notice that the diodes all ?point? in the same direction, thus the
?diode ring? is formed.
It is amazing that all that circuitry can be placed into such a small package. Its even more amazing that the same function is built into the TUF-1, which is about 1/3 the size of the SBL-1. I have chosen the SBL-1 for a variety of reasons, first of which is that it has a relatively wide-spaced pin arrangement and soldering is easier (the TUF-1 is a 4-pin device, and the pins are much closer together). Secondly, the wide-open pin arrangement of the SBL-1 allows power for the RF amplifier to be passed directly through the winding between pin 1 and pin 2, thus coupling the signal into the mixer with no additional coil(s) or capacitors, thus reducing component count and saving construction time. This is not possible with the TUF-1. Lastly, and least importantly, I happened to have the SBL-1 on hand when I built the prototype for this product detector.
[] Virtually all the RF content is removed by C4, R7 and L1 before the audio signal is passed to the buffer via C7, a 10 uF electrolytic capacitor.
[]
The note above the coil, L1, in Figure 4-3 tells you that the coil is
constructed by winding 20 turns (about eleven inches) of #26 enameled copper
wire on a ferrite toroid coil form. 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 there are no ?off the
shelf? coils readily available with the required specifications.
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 for winding coils, so I won?t go into great detail here. One "rules of thumb" to keep in mind is that windings should cover about 3/4 of the core, as shown in Figure 4-5.
This coil is wound on a
"donut" core, type FT37-43.
What this nomenclature means is shown in Figure 4-6.
What function does L1 perform (you might want to know). L1 serves as a Radio Frequency Choke (RFC). What do RFCs do? They ?choke? radio frequencies and don?t let them pass, which is exactly what we want to happen because at this point in the circuit all we want is the audio frequencies. Specifically, we want only the bursts of a 750 Hz tone that make up the dots and dashes of Morse code. Since 750 Hz is an audio signal as opposed to a radio signal, L1 simply acts as a piece of wire, and has virtually zero effect on the audio signal.
By the way, this product detector will also allow you to hear single sideband (voice) signals, but that's another story.
[] The audio signal coming from the SBL diode ring mixer is
very small, and it is boosted by the buffer/amplifier circuit (Q3 and Q4) before
being passed on to the audio amplifier module.
[] Q4 is a PNP transistor (type 2N3906) as opposed to Q1, Q2, and Q3 which are
NPN transistors. Notice, also, that
Q4 is shown in the schematic "up-side down" with its emitter being more
positive than its collector.
[] Pin arrangement for both the 2N3904 and 2N3906 are shown in the lower
right-hand corner of the schematic. The transistors and the 78L06 voltage regulator used here are
housed in TO-92 cases (?TO? simply means Transistor Outline).
[] I have shown the pin arrangement for the 78L06 voltage regulator by placing a
drawing in the schematic.
The output of the mixer (before filtering) is a real mess;
it contains a variety of signals, the most important being the sum of the two
input frequencies, and the difference between the two input frequencies.
In this case (7.0xx Mhz plus 7.0xx Mhz) the sum is 14.xxx MHz (20 meters)
which is part of the "mess" coming out of the mixer, and is of no interest
to us in this 40 meter receiver. The
difference between the two will be an audio tone that will be amplified and made
available to the headphones so you can hear 40 meter signals instead of 20 meter
signals.
Everything coming out of the mixer, except what we (you and
I) want to hear can be thought of as "noise".
Alert readers, such as yourself, may be thinking:
"Wait a
minute! If both 7.xxx MHz (40
meter) and 14.xxx MHz (20 meter) signals are detected, how does this receiver
know which one I want to hear?!"
Good question.
The answer is: It doesn"t (not yet).
Filtering will provide the selectivity that chooses 40 meter signals, so
don"t worry about it right now. Filtering
will be covered in Chapter 6, which gives the details of the receiver filter
module. Yes, filtering is important
enough to have a whole module devoted to it.
Here is a quick look at how direct conversion works. Both signals coming into the mixer are 7.0xx MHz. Let"s assume the 40 meter signal coming from the antenna is at a frequency of 7.040 MHz (7,040,000 Hz). In order for us (YOU and me) to hear this signal, we need a tone, say about 750 Hz. This is easily accomplished by inserting a signal from the VFO that is different than the incoming signal by 750 Hz. The signal from the VFO can be either 750 Hz above the incoming signal, or 750 Hz below the incoming signal.
Which do we use, 750 above or 750 below?!
The "standard" that has been adopted is to tune the
receiver above the transmitted signal. In
other words, there is a receiver offset of plus 750 Hz. Or, you can think of it as a transmitter offset of minus 750
Hz. The important thing is that
there must be an offset. If the two
frequencies are identical, the difference is, of course, zero, which is exactly
what you would hear.
So, with an incoming signal at 7.040000 MHZ, the VFO
frequency needs to be 7.040750 MHz in order to produce the 750 Hz tone.
Must it be 750 Hz? Must
the receiver be "above" instead of being "below".
No, there is no technical reason, but remember that the
operator on the other end is expecting this offset arrangement (or, at least,
something very close to this arrangement).
There is more flexibility in a super heterodyne type receiver where you
have two oscillators, the VFO and
the beat frequency oscillator (BFO), but that"s another story.
In actual practice, the tone is seldom exactly 750 Hz.
In fact, the tone can be exactly 750 Hz at one, and only one place in the
band spread of the receiver. In a
heterodyne type receiver this is controlled very tightly by using a signal from
a crystal controlled oscillator to mix with a fixed intermediate frequency, but
we must deal with a different set-up here.
I think that's quite enough theory for now. Back to the task at hand, which is a . . .
Resistors (All resistors are 1/4 watt.)
[] R1: 50k potentiometer, RF gain control
[] R2: 22k
[] R3, R4, R9: 10k
[] R5, R8: 56 Ohms
[] R6: 470 Ohms
[] R7: 51 Ohms
[] R10, R11, R12, R14: 3.3k
[] R13: 220 Ohms
Ceramic Capacitors, monolithic or disc ceramic
[] C1, C2, C3, C4, C5, C9: 0.01 uF
Electrolytic Capacitors, radial lead
[] C6, C7, C8, C11: 10 uF
[] C10: 47 uF
Transistors
[] Q1, Q2, Q3: 2N3904, or equivalent
[] Q4: 2N3906, or equivalent
Voltage Regulator
[] 78L06, or equivalent
Diode Ring Mixer
[] SBL-1, or equivalent
Inductor
[] L1: 20 turns (about eleven inches) of #26 enameled copper wire wound on
an FT37-43 core (see text)
Miscellaneous
[] Circuit Board: Radio Shack #276-148 General Purpose Dual PC Board, or
equivalent
[] About 18 inches #20 or #22 bare copper wire for ground buss and tie points on
circuit board.
[] Hook-up wire (#26, stranded will serve nicely)
[] Hardware for mounting the circuit board
Figure 4-7shows a photo of the product detector module with a few of the
components identified so you can get a general idea of the layout.
As you can see, the circuit board shown here is not a Radio Shack 276-148, but an
equivalent board that was cut to
size from a large piece of green perfboard stock.
The two resistors in parallel just above L1 in Figure 4-7 are two 100 Ohm
resistors I used to make R7, the 51 Ohm resistor going from C4 to ground (see
Figure 4-3). If you have a single
resistor with a measured resistance between 50 and 52 Ohms, it will work just
fine.
Figure 4-8 is a drawing of the parts layout on a Radio
Shack 276-148. The drawing shows
the top view of the board to match the photo in Figure 4-7, and I have
superimposed the SBL-1 pin numbers in hopes of avoiding confusion. The parts layout drawing is not to exact scale and is
intended only as a guide for parts placement.
As has been noted elsewhere on this web site, the RS 276-148 board is not
a ?twin? pack ? the two sides, while both carrying the same part number,
are not identical. One side (the
side on the left supporting the SBL-1 in this drawing) is 15 holes across
(horizontally) and the other is 16 across.
At first glance, it may appear that the SBL-1 will fit into a 14-pin dual
in-line pin (DIP) socket, but close inspection reveals that the pin spacing is
not the same as for DIP sockets. Three
mounting methods for the SBL-1 that I have used are listed below:
[] Solder directly into the circuit on the under-side of the board.
This is probably the best choice from a building and performance
standpoint. If you are absolutely
sure you will never want to use the SBL-1 in another project, and are equally
sure you have the pins connected correctly, then soldering directly into the
circuit is the way to go.
[] Occasionally, I fabricate a socket of some sort because that allows easy
removal of the SBL-1 for use in other projects.
As far as I know, there is no off the shelf socket available for the
SBL-1, so this method is probably more trouble than it is worth.
[] "Dead bug" style, with the SBL-1 upside down on the circuit board.
This method is probably as good as any from a
performance standpoint, but when using perfboard with the standard hole
spacing, it is just as easy to push the pins through the holes and solder
connections on the under-side of the board.
Dead Bug component mounting is normally employed when using a bare
printed circuit board with no holes and no printed circuit pattern etched into
the board. This works just fine,
but some folks don't like the looks.
[] The pin numbering scheme on the SBL-1 is shown in Figure 4-9.
[] Figure 4-9 (A) shows pins as they appear on the bottom of the SBL-1. Pin 1 is marked with a blue bead for easy identification.
[] In the top view, Figure 4-9 (B) pin-2 is under the letter "M" in "MCL".
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.
Next, do a component
count as outlined below. You should
find:
[] One SBL-1
[] Four transistors
[] One 78L06 voltage regulator
[] Fourteen resistors on the circuit board, plus the 50k RF gain control
[] Six electrolytic capacitors in the circuit
[] Six 6 ceramic capacitors
If you find a
discrepancy in component count, use the layout diagram in Figure 4-8 and/or the
schematic in Figure 4-3 to find the error.
Missing or extra components will, of course, become obvious during
resistance checks and/or voltage checks, but a good visual check can save a lot
of troubleshooting effort.
Correct any errors found during the visual check, then proceed to resistance measurements.
Resistance measurements are done with nothing connected to
the circuit board except your DMM; NO power connected, NO jumpers in place, R1,
the 50k RF gain potentiometer NOT connected.
Resistance measurements are from the ground buss to the
point indicated.
[] Signal In: Open, no continuity
[] VFO In: Open, no continuity
[] Audio Out: Variable, settling to open, no continuity
[] TP 1, 6 volt input to RF Amp: Open, no continuity
[] TP 2, regulated 6 volts: 5.4k
[] TP 3, 12 volt input to 78L06 regulator: Open, no continuity to ground
[] TP4, RX +12 Volts: Variable, settling to about 13k
[] TP5, "top" of RF Gain Control: Open, no continuity
[] TP6, "Wiper" of RF Gain Control: 42k
Q1
C: Open, no
continuity
B: 450 Ohms
E: 470 Ohms
Q2
C: Open, no
continuity
B: 19.5k
E: Open, no
continuity
Q3
C: Variable,
settling to about 17k
B: 3.2k
E: 3.2k
Q4
C: Variable,
settling to about 3k
B: Variable,
settling to about 17k
E: Variable,
settling to about 16k
SBL-1
Pin 1: Open,
no continuity
Pin 2: Open, no
continuity
Pin 3: 20.4k
(pins 3 & 4 should be connected together.)
Pin 4: 20.4k
Pin 5: Zero
(pins 5, 6, & 8 connect directly to ground.)
Pin 6: Zero
Pin 7: Zero (May
measure a small amount of resistance, depending upon the sensitivity of your DMM.
This would be the resistance of the coil between pins 7 & 8 inside
the SBL-1.)
Pin 8: Zero
Correct any errors found during resistance checks, then proceed to current and voltage measurements.
If resistor measurements are correct, it is unlikely there
will be any problems found during current and voltage measurements, BUT you never know what strange things may happen in a newly constructed circuit.
To do current measurements, you need a 6 volt supply that
can be varied from about zero volts to about six volts.
If you don?t have a variable 6 volt supply, no problem
(see below).
Why 6 volts? Because
6 volts is the power requirements for the RF amplifier and that is the most
crucial place to be checked for excessive current. Also, six volts is adequate to discover excessive current
anywhere in the module.
It is easy to fabricate a variable 6 volt supply that will serve for these tests using four AA batteries, as shown in Figure 4-10.
You will need:
[] Batteries: 4 AA batteries
[] Battery Holder: to hold the 4 AA batteries
[] Control: 1000 Ohm, ? watt potentiometer
[] SPST switch
[] Meter: Your DMM, or other current measuring device
[] Hook-up wire and/or test leads
The switch, S1, is optional, but be aware that current is
flows continuously whenever the battery is connected to the 1k potentiometer,
whether or not a circuit is being tested. A
switch may prevent an inadvertent drain of your batteries.
Notice that the 1k potentiometer in Figure 4-10 is
installed so that the wiper is at the "bottom" of the resistor when turned
fully counter clockwise (CCW). Begin
each current test with the potentiometer set fully CCW (zero volts out).
As you turn the control clockwise (CW) toward the ?top?, both voltage
and current will increase. Electrons
don't care which way the potentiometer is wired into the circuit, but the
conventional direction for increase on controls is clockwise.
You can use the same variable 6 volt supply powering the RF amplifier when we get to voltage tests a bit later.
Current tests are done with neither the 50k RF gain control
nor the two wire jumpers, W1 and W2 installed.
What we hope NOT to see during these current tests is
excessive current which might destroy one or more components if is full power is
applied.
-- BUT
--
If there is NO current flow, that, too, can indicate a wiring error.
That's the bad news. The
good news is that, if no current flows it simply indicates an "open"
circuit, and no damage will result. Simply
find the bad solder joint or missing connection, correct the error, and re-do
the current check.
Keep in mind that you will be measuring very small amounts
of current, only a few mA.
Set your DMM or other current measuring device accordingly.
If you use a high current setting, you may fry your circuit board before
you notice any reading at all.
78L06 CURRENT
MEASUREMENT
[] Begin this test with your 6 volt supply turned all the
way down to zero volts.
[] Be sure all ground points are connected together.
[] Connect output of your DMM, or other current measuring device, to TP 3, the
input to the 78L06 regulator.
[] Slowly turn the 6 V control to increase the voltage while monitoring the
current.
[] The circuit should draw no more than 1.75 (one point seven five) mA with full
6 V applied to the input. If you get a current measurement of 2 mA, or more at any time
during while increasing voltage toward 6 volts, or if you measure no current at
all, you have a wiring error.
NOTE: With 6 V
input, the regulator output at TP 2 will be only about 4.7 volts with 6 volts
applied to the input, and it will not be regulated. In order for the 78L06 to function properly, it requires a
minimum of 11 volts at the input. That's
OK because, at this time, we are not checking for either voltage or regulation;
we are checking for current.
Correct any errors found during 78L06 current measurements, then proceed to RF Amplifier current measurements.
RF AMPLIFIER AND SBL-1 CURRENT MEASUREMENT
[] Begin this test with your 6 volt supply turned all the
way down to zero.
[] Be sure all ?ground? points are connected together.
[] Connect output from your DMM, or other current measuring device, to TP1, the
power input point for the RF Amplifier circuit.
[] Slowly turn the 6 V control to increase the voltage while monitoring the
current.
[] The circuit should draw no more than 1.2 (one point two) mA with full 6 V
applied to the input. If you get a
current measurement of 2.5 (two point five) mA, or more at any time during while
increasing voltage toward 6 volts, or if you measure no current at all, you have
a wiring error that must be corrected before continuing.
AUDIO BUFFER/PREAMPLIFIER CURRENT MEASUREMENT
[] Begin this test with your 6 volt supply turned all the
way down to zero.
[] Be sure all ground points are connected together.
[] Connect output from your DMM, or other current measuring device, to TP1, the
power input point for the RF Amplifier circuit.
[] Slowly turn the 6 V control to increase the voltage while monitoring the
current.
[] The circuit should draw no more than 1.2 (one point two) mA with full 6 V
applied to the input. If you get a
current measurement of 2.5 (two point five) mA, or more at any time during while
increasing voltage toward 6 volts, you have a wiring error that must be
corrected before continuing.
When all three current tests check out OK, you are ready to proceed to the voltage measurements.
6 VOLT REGULATOR VOLTAGE MEASUREMENT
Voltage measurements are from ground to the point indicated.
Initial voltage measurements are done with the 50K RF gain control NOT connected to the circuit board and No jumpers installed.
[] Connect your 12 volt source to TP 3
The voltage at the output of the regulator circuit, TP 2, should be about 6 volts, plus or minus a tenth of a volt, or so. If you don?t measure six volts, there is a wiring error. Find and correct the error before continuing.
Assuming the regulated six volts checks out OK, continue with voltage checks.
Do NOT install jumpers, yet.
AUDIO BUFFER/AMPLIFIER
VOLTAGE MEASUREMENT
[] Connect your 12 volt source to TP 4 (RX +12 Volts)
Q3
[] C: 9.3 volts
[] B: 3.1 volts
[] E: 2.5 volts
Q4
[] C: 5.9 volts
[] B: 9.3 volts
[] E: 10 volts
Do NOT install jumpers, yet.
NOTE: It is important to have already done current measurements and corrected any errors found in the RF amplifier circuits, Q1 & Q2, before doing voltage tests on this circuit because the regulated six volts that powers the circuit is routed through the SBL-1. The wires inside the SBL-1 are tiny and can not carry much current before they become smoke. In the unlikely (but possible) that preceding measurements failed to detect a wiring error that could cause excessive current to flow and destroy the SBL-1.
Assuming current tests have already been done on the RF
Amplifier circuit, and errors found (if any) corrected, apply 6 volts to TP1.
Q1
[] C: Zero
[] B: Zero
[] E: Zero
Q2
[] C: 6 volts
[] B: Zero
[] E: Zero
[] Disconnect power and install R1, the 50k RF gain
control.
[] Turn your variable six volt supply to zero volts, then re-connect to
TP 1
[]Slowly advance the RF gain control until about 0.6 volts appears at the base
of Q1.
Voltage measurements should now be as shown below.
Q1
[] C: 0.6 volts
[] B: 0.6 volts
[] E: 0.02 volts (a reading, less than .05 volts is OK)
Q2
[] C: 6 volts
[] B: 1.1 volt
[] E: 0.6 volts (a reading slightly above 0.5 volts is OK)
Assuming measurements are good so far, slowly advance the 50k RF gain control until about 1 volt appears at the base of Q1.
Voltage measurements should now be as shown below.
Q1
[] C: 1.4 volt
[] B: 1 volt
[] E: 0.4 volt
Q2
[] C: 6 volts
[] B: 2 volts
[] E: 1.5 volt
Correct any errors found during voltage checks, then proceed to the operational test for your product detector.
PRODUCT DETECTOR
OPERATIONAL TEST
The procedure described below provides the operational test
for both the product detector module and the VFO module.
You will need:
[] Audio module
[] FVO module, both oscillator and buffer circuit boards
[] Antenna, or other weak signal source, such a signal generator.
If you don?t yet have a ?real? 40 meter antenna, you can fabricate
one easily using small wire.
For example, I do initial testing with about thirty feet of #28 enameled
copper wire strung around the ceiling of my work area. Most any type of wire you happen to have on hand can be used.
[] Product Detector module
[] 12 volt power source
This operational test assumes you have already built the
Audio module, the Product Detector module, and the VFO module, and that you have
completed all the initial resistance, current, and voltage measurements
successfully.
As you know, a product detector requires TWO RF signals in
order to produce an audio signal. One
of the signals will come from your VFO module.
Ideally, the ?other? RF signal would come from a signal generator
with a variable output, but a signal from you antenna will do the trick.
Assuming all the resistance and voltage measurements on your Produce Detector module are correct, be sure the two jumpers are installed and the 50 k RF gain control is installed before beginning operational checks. A suitable set-up for operational tests is shown in Figure 4-11.
Notice that you will be using headphones as the output device for these tests. An oscilloscope could be used, but your ear will be the final arbiter of performance, so you might as well start there.
Figure 4-12 shows the modules assembled and ready for testing. On the left side of the photo, you see the audio module sitting atop the product detector. I used 1/2 inch spacers between the circuit boards and 1/4 inch spacers for "feet" under the bottom board.
The RF Gain control is shown in front of these two modules.
The ten-turn VFO tuning potentiometer is in the middle of the photo.
On the right side of the photo, you see the VFO module assembled with the oscillator sitting atop the VFO buffer. The VFO signal is sent to the product detector via about 6 inches of RG-174 coaxial cable.
Hook-up wire from the audio board to the audio gain control pass in front of the VFO tuning potentiometer to the Audio Gain control which is laying down in front of the VFO and VFO Buffer modules.
Earphones, which are not shown in the picture, attach via the yellow/green/red connector you see just in front of the VFO Buffer module.
The signal generator in the background behind the rat's nest of wires was not used for testing. Instead, signals captured by my ceiling-mounted wire antenna were used, and they served nicely.
I know this photo is a bit messy, but it will give you an idea of one way to set-up for testing.
[] Be sure all ground points are connected.
[] Connect your signal source (antenna, or signal generator set to low output)
to the Signal In port on your product detector module.
[] Connect the Audio Out from the product detector module to the Audio In port
on your audio amplifier module.
[] Set the Audio Gain control on your audio amp module to about mid-range.
[] Set the RF Gain control on the product detector module to about mid range.
[] Plug in your headphones and use them for your output indicator.
[] Power-up the VFO, the product detector, and the audio module.
Assuming you are using an antenna rather than a signal
generator as your signal source, "Tune around" by adjusting the frequency
control on your VFO. At this time,
you should hear something in your headphones, probably a lot of noise because a
properly operating product detector module is sensitive to a wide spectrum of
signals that may be coming into the Signal In port, including AM broadcasts
signals, Ham Radio signals, and any other electromagnetic signals that may be
striking your antenna at the moment, such as the spark plugs in a car passing
by. It is possible, but highly
unlikely that you will hear any recognizable Ham Radio signals in all this
uproar.
Why? (you might want to know) Because the receiver 40 meter filter module is not yet hooked
into the signal path ahead of your product detector. Without the filter, your product detector hears every signal
that happens to come along. More
about filtering in the upcoming RX Filter module.
Turn the Audio gain control "up" and
"down".
The signal you hear in your headphones should go "up" and
"down"
in loudness.
Do the same with the RF Gain control.
If the sound in your headphones goes "up" and
"down" as you turn the RF gain control, your product detector is working.
If you live fairly close to an AM broadcast antenna, you
may hear a broadcast signal that overpowers most, if not all, other signals.
Try tuning as far away from an interfering broadcast station as possible.
You may be unlucky and hear a strong AM broadcast station from one end of
your tuning range to the other. You
should be able to hear other (weaker) stations in the background as you tune
across your band spread. This is
normal at this stage of development; undesirable, but normal.
We will fix this problem with the receiver 40 meter filter.
If you hear nothing, there is a loose or missing connection
somewhere. Two of the most common
problems at this stage of testing are missing ground connections between modules
and missing power connections. Carefully
inspect the inter-module signal connections and power connections, correct any
errors found, and try the operational test again.
Now that the operational test has been successfully completed, you almost have a functioning 40 meter receiver. I say "almost" because one very important function is missing. That function is the SELECTIVITY to limit the signal you hear to those on the 40 meter band. That selectivity will be available when you have completed the receiver 40 meter filter module.
- END OF CHAPTER 4: PRODUCT DETECTOR -
Back to 40 Meter QRP Table of contents
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