June 28, 2016
and the Small
Wonder Labs SW30+ Transceiver
Installment #3 ...
Keying & Transmit Mixer
and build-up of the classic 2-watt 30-meter superhet transceiver
designer Dave Benson K1SWL of Small Wonder Labs.
Hope all you new SW30+ Kit builders are making it through the assembly
In this episode, we'll analyze the Keyer and Transmit Mixer circuits,
and discussing its
components, testing and operation.
At the end of this "step 4" in the assembly manual, you'll be seeing the
first evidence of the 10.1 MHz signal that ultimately gets transmitted!
you say you missed the last episode? You can download the podcast and
catch up on things at any time! The whiteboard material and audio
recordings for all shows are listed right there on our home page! (www.cwtd.org)
73, George N2APB and Joe N2CX
Chat window from the night of
the show ...
<20:04:55> "wheelw": when
gathering parts for pcb build, do we need to match crystals
<20:06:35> "Mike WA8BXN": for best results matching helps, but
random crystals on the same frequency will work too
<20:07:16> "wheelw": Thanks Mike- that was my thought, but
wanted to make sure
<20:09:22> "Bruce - N1RX": if you have the ability to match the
crystals, choose the ones closest together for the IF filter,
and the "outlier for the mixer. If you can't match them, don't
<20:11:10> "Joe N2CX": The crystals provided with the kits weree
matched to within 40 Hz to get the best tx/rx offset and best if
<20:17:25> "wheelw": I have an extra one of these little Arduino
DDS kits (featured in QST magazine project a couple of months
ago). Might be perfect for SW+ upgrade. http://www.farrukhzia.com/k2zia/
<20:18:37> "George N2APB": That's a real nice combo! Arduino "Nano"
+ inexpensive offshore DDS board (~$5 or so).
<20:20:10> "wheelw": Do you recommend hot glue or something to
"Fix" toroids, once set? I haven't done this, but have heard
<20:20:41> "Mike WA8BXN": DO NOT attempt to strip the toroid
after putting it in the board, strip first!
<20:22:35> "Mike WA8BXN": the only coil I might "glue" is the
VFO coil, but I probably would not do that
<20:25:25> "wheelw": could you provide URL for the site you
mentioned with toriod tips?
<20:25:28> "Bruce - N1RX": For a VFO coil, I have often dripped
candle way on to secure the windings to prevent freq shifts.
<20:25:42> "Bruce - N1RX": theats wax, not way...
<20:26:42> "Joe N2CX":
<20:31:23> "Clint - KA7OEI": On a kit like the KD1JV ATS I RTV'd
the windings/cores in place as the plug-on LPF modules are not
protected by a chassis. Hot glue will eventually break loose
with temperature cycling.
<20:32:24> "wheelw": RTV'd is??
<20:33:06> "Bruce - N1RX": room temperature vulcanizing rubber,
like pbathroom caulking
<20:33:32> "Bruce - N1RX": look for non-acidic RTV
<20:33:41> "Mike WA8BXN": some types of RTV are VERY bad to use
<20:33:59> "Bruce - N1RX": exactly most give off acetic acid
<20:34:23> "Joe N2CX": Use non-contaminating RTV. The common
type of RTV gives off acetic acid - smells like vineagar and can
cause component corrosion. RTV sold for gluing engine gaskets is
<20:39:00> "wheelw": would this be considered "full Break-in"
<20:39:50> "Mike WA8BXN": pretty much not much delay in
switching, TR switch is electronic
<20:41:22> "Mike WA8BXN": C110 puts in a little delay
<20:42:24> "Joe N2CX": C110 also adds a little keying envelope
<20:47:10> "AC2GL_Dave": In the VFO Transistor Q2 collector
connects to Vr. Is that the 8V supply or am I missing somethine?
<20:48:27> "George N2APB": Not quite. Vr is derived from above
U3 (center-upper part of the schematic.
<20:49:17> "Obe - KC4VZT": it's more like 7.3 to 7.4 because of
the diode drop from 8V
<20:50:32> "Bruce - N1RX": the key is that is is regulated
better than the "raw voltage" that the final power stages may
draw down on TX
<20:53:09> "AC2GL_Dave": won't that track 8V minus a diode drop
<21:01:48> "Bruce - N1RX": AC2GL- Yes, but the goal was to
isolate it from the main supply, and that is accomplished
<21:04:20> "Bruce - N1RX": we learn more from failure than
<21:04:56> "Joe N2CX": BTW Mike said "602". The original design
used NE602 devices, now the SA612 is what is available. For our
purposes they work identically.
<21:07:36> "Obe - KC4VZT": My kit came with 602's in it.
<21:09:31> "Joe N2CX": No problem with either device.
<21:20:45> "Joe N2CX": One difference between hollow state and
small signal semiconductor devices is that with small signal
semiconductors you shouldn't blister your fingers!
<21:20:52> "wheelw": I missed call, but is there a way to verify
VFO operation with a DVM only (I would need to check out an
oscillosope at work)?
<21:21:20> "Pete wb2qll": you can verify the oscillator by
listening wth a receiver tuned to the VFO frequency.
<21:22:02> "wheelw": just push SW+ close to the antenna lead of
<21:22:02> "Obe - KC4VZT": I'm up to step 9 and taking my time,
having fun and learning lots.
<21:22:29> "Armand WA1UQO": Complete through step 6 with no
<21:23:10> "Pete wb2qll": Take the antenna lead from the
receiver and put it very close to the VFO circuit.
<21:24:24> "George N2APB": Wayne: Yep! Tack-solder a wire onto
the output of the U5 mixer and you'll be able to "tune in" the
two frequencies that we've been mentioning (the sum and
difference). One of them (10.1 MHz) will be the one that we're
interested in and will be using going forward.
<21:24:30> "Pete wb2qll": The antenna lead should end with a
short, perhaps an inch long, wire that's beyond the shielded
<21:25:45> "Obe - KC4VZT": I used a short coax cable with an
alligator clip lead to the center pin. Then put it near the VFO
<21:36:02> "Obe - KC4VZT": this is a good place for information
about toroids http://kitsandparts.com/mtoroids.html
CWTD Episode #80:
Elmer 101 and the SW30+ Transceiver
First, a few Notes:
Kits are sold out.
Enclosures are sold out ... but surprise coming
Bare SW30 pc boards are sold out.
You can get the FreqMite Kits now from 4SQRP ...
http://www.4sqrp.com/freq-mite.php ... And AA0ZZ shows
how to connect it to your SW30.
Accessories in progress ... Keyer Kit, Display,
Arduino control, Spectrum output
For the latest & greatest SW30+ Kit information,
see/download the Updated Manual
followed the wisdom of others in using the assembly step ordering of
the various Elmer 101 renditions. They mostly dealt with the 40
meter version of the radio, I've tailored details for our 30 meter
radio. If I had not had that previous foundation to follow, I
probably would have first done the receiver construction and then
work on the transmit side of things.
ordering makes a lot of sense in terms of testing during
construction. The next several parts of the assembly process will
work on the transmitter up to but not including the final amplifier
(which will of course be completed later). This clever approach will
allow us to use the transmitter to provide a test signal for
“Keying” part of this step in assembly, Q3 and associated components
(take a look at the schematic), is part of the rig that handles
changing between transmit and receive. Pressing down on the key
causes Q3 to conduct, supplying 12 volts to most of the transmitter
components (the final amplifier gets power all the time). Pin 8 of
U1 is a convenient place to observe the switching.
up should give little voltage there (I saw a fraction of a volt)
while pressing down the key (a jumper connected between pins 1 and 3
of J3) should give around 7.5 volts there. Its not the full 12 volts
because of the dropping resistor R19 and zener diode D11 which form
a simple voltage regulator to supply proper voltage to U5. This
little regulator circuit is not precise, I got around 7.3 volts when
I did my measurement, that' s plenty close. Seeing 12 volts here
would be a problem to track down as well as not seeing a voltage
change between key up and key down.
Mixer Circuit (reference:
The SW-30+ radio employs three of these chips: one to heterodyne
the incoming small-signal (10.1 MHz.) RF down to the I.F.
frequency of 7.68 Mhz., another to heterodyne that down to
audio, and another to heterodyne the VFO frequency of 2.4 MHz.
up to 10.1 MHz. for the transmitting amplifier.
The term "heterodyne" refers to the mixing function, where two
different frequencies are combined in a non-linear way to
generate an output waveform of a different frequency. Keep in
mind that the process is non- linear: there is no way to
linearly add two frequencies to get a third.
An audio "mixer" is an entirely different animal: it simply adds signals
together linearly. If it did output
other frequencies, an audio mixer would be considered faulty,
and in need of repair.
In the case of U5 (a SA612 chip), one sinewave input signal from
the VFO at 2.40 MHz. comes into pin 2, while the chip generates
the other input signal at 7.68 MHz. internally. The output
waveform is available at pin 4 and/or pin 5. An internal
Colpitts crystal oscillator generates the 7.68.0 Mhz sinewave
(the crystal oscillator connections involve pin 6 and pin 7).
The heart of the mixer uses a circuit known as a Gilbert
Cell. A superb feature of the Gilbert-cell mixer is
that neither 2.40 Mhz. nor 7.68 Mhz. signals appear at the
output, provided that switching action is seamless. Ideally,
only two frequency components will appear at the output: one at
the difference frequency ( 7.68 MHz - 2.40 MHz = 5.28 MHz) and
one at the sum (7.68 MHz + 2.40 MHz = 10.1 MHz. That's the best
we can hope for. Usually, these two dominate over a mess of
other mixing products of lower amplitude.
tests for this part of the assembly verify that the mixer (U5 and
associated components) appears to be working. We have a couple of
things to look for. Part of U5 implements an oscillator for one of
the two signals being mixed (the other comes from the VFO). Crystal
Y5 determines the frequency with RFC2 slightly shifting the
frequency from the marked 7.68 MHz on the crystal
pin 6 of U5 we should see the crystal oscillator signal at around
7.68 MHz. Our test equipment will have some impact on circuit
operation. Using a scope, I saw about ¾ of a volt peak to peak.
Using the RF probe and DVM I got about half a volt DC on the meter.
A receiver tuned to around 7.68 MHz should be able to hear the
signal as well. It should go on and off as the key is pressed and
Finally, we should see a signal coming out of the mixer chip U5 on
pin 4. What is useful to us is a signal around 10.1 MHz. Since the
mixer produces both the sum (7.68+2.4 MHz) and the difference
(7.68-2.4 MHz) of its inputs, there will also be a component around
5.28 MHz that we don't need. Viewed on a scope we won't see a nice
sine wave but just seeing a signal there is enough for now. Using
the RF probe I found around 1.5 V DC on my meter.
a receiver you should hear signals around 10.1 and 5.3 MHz with the
Schematic Fragment: Keying & Transmit
check for 0 volts pin 8 of U5. (Measured: < 0.5 V)
jumper between pins 1 and 3 of J3.
7.5 volts on pin 8 of U5. (Measured: 7.29 V)
U5 Pin 6:
Measured: 750 mV PP 7.68 MHz using scope, 0.5 VDC using RF probe
Pin 4 of U5
should have around 10.1 MHz signal. (Measured: 1.5 V DC using RF probe)
Mike's "View from the Bench":
completed this step and applied power, the voltage readings at pin 8of U5
were as expected for pins 1&3 jumpered or not on J3. But I did not hear a
signal around 10.1 MHz as expected. I tuned my receiver to 7.68 MHz to hear
the crystal oscillator and nothing.
is good! I get to troubleshoot the board. I decided to cheat and turned on
my oscilloscope. I connected the probe to the top of RFC 2, there should be
something around 6.68 MHz there. I did see a signal. I disconnected the
jumper from pins 1 to 3 of J3. The signal was still there. Set scope to give
rough measure of frequency and it said 2.4 Mhz or thereabouts. That's just
the VFO showing up there.
should be working, my soldering looks ok. Remember I am now looking for the
7.68 MHz signal. The only components needed for it are C28, C29, RFC2, Y5
and U5. I checked RFC2 as soldered in place with an ohm meter, it was good
at around 1 ohm. I carefully removed U5 from its socket. I really don't
think it’s bad, but I do have a couple others in the kit and it’s easy to
substitute another one there.
Substituting one of the other 602's didn't fix things. I decided to clip the
antenna lead of my test receiver to the case of Y5 and tune the dial around
7.68 Mhz and what do you know I hear something! But that something is not a
nice single carrier (using the receiver in LSB mode). It’s a bunch of
carriers slight separated in frequency. Maybe it’s from something else. I
disconnected the jumper from pins 1& 3 of J3, it should go away. The
carriers were still there. Just for the fun of it, I disconnected power from
the SW30 board. The carrier frequency started drifting and getting until
they were gone. Remember, we have a 220 uF capacitor on the 12 V line that
takes a while to discharge.
this point, things are going downhill a bit. The original test of
the voltage on pin 8 of U5 was OK as pins 1&3 of J3 were jumped or not.
Now I get the 7.5 volts all the time on pin 8 of U5. Are we having fun yet?
can solve this. I think I can at least! I'm glad I am doing step by step
testing rather than having built up the whole board before doing any tests.
First to make my work a bit easier maybe I removed the 602 from U5 socket
and put it back in the foam it came in. It’s good to simplify things as much
as possible. I disconnected the scope leads, all that I have connected to
the board now is power. Measure pin 8 of U5 again to make sure that problem
is still there. I see 7.29 volts. It should be close to zero.
to look at the schematic. My current problem centers on Q3, the 2N3906 that
is supposed to be a switch to turn on and off power for U5 (and other stages
when we get to them) as the key is pressed. Our jumpering pins 1&3 of J3
simulates pressing the key. I measure the voltage at pin 3 of J3 and see 13
V, so pin 3 didn't get accidentally shorted to ground somewhere.
Q3 is sort of upside down in the schematic. It’s a PNP transistor and the
emitter and base voltages are the same, so it should not be
conducting. Either the transistor is now shorted, or I may have done some
bad soldering. Disconnect power and inspect the board under the magnifier
(again). It’s the right transistor there, 2N3906, flat side where it should
be. R20 and R21 are the right values. I don't see any soldering problems.
Check from E to C with an ohm meter. I get 9.8 ohms in both directions. Sure
looks shorted. Center lead on the transistor is the base. What to do next. I
could get out the solder wick and remove the transistor. I don't think it’s
a bad transistor though. What else could be causing that short? Let me look
some more. I just don't see anything on the board.
me carefully remove Q3. That wasn't too bad. Checking the transistor with my
ohm meter, it does look shorted. I am surprised! From my junk box I found a
new 2N3906 and it does not appeared shorted. I solder in the Q3 location.
U5 still removed from the socket, I apply power. Pin 8 of U5 without the J3
jumper is .395 volts, not quite zero. My replacement transistor has some
leakage. With pins 1&3 of J3 jumpered, I see 7.29 volts on pin 8 of U5, as
it should be. With the jumper removed, its 5 volts???? Wait, that may be
OK. C110 got charged up and with no IC in U5 socket there isn't much to
discharge it. The voltage is slowly dropping.
put a 602 in U5 and now hear the 7.68 MHz signal (and nearby birdies, that
could possibly be my receiver not liking the out of ham band frequency). I
will worry about that later. The signal keys on and off properly with the jumpering
of J3 pins 1&3. I then tried using the other two 602 ICs, and it looks like
one is actually bad.
I looked for a signal around 10.1 MHz on pin 4 of U5 using my receiver. Yay,
I found it at 10.146 MHz, close enough at this point. And it sounds clean
and does key on and off with the jumpering of pins 1&3 of J3. So at this
point my board seems to pass the tests. I have no idea why I had problems.
Maybe I should have taken static discharge precautions and turned off the
Van de Graaff generator while doing my building. But I do enjoy those sparks
jumping around the workbench!
Now we are looking at the transmit mixer pin 6 which is
the crystal oscillator at about 7.68 MHz, indicated as Peak 2. What is
that other stuff? Well Peak 1 at 2.4 MHz is some bleed through of the
VFO. Peak 3 is somewhat near what we expect for the mixed output, but
maybe not close enough to be that. Although the frequencies are
displayed to many decimal places their accuracy is not know. Peak 4 is
probably the 3rd harmonic of the oscillator frequency.
Here (below) are the signals present at
the output of the transmit mixer U5 pin 4. Who would have expected all
that? Peak 5 at around 10.06 MHz is the desired output signal. Also
present are the two input frequencies noted as Peak 1 and Peak 4. Peak 3
is the difference between the input frequencies, an expected but
undesired output signal. Peak 2 looks like the second harmonic of the
VFO. Peak 6 is about the second harmonic of one of difference of the
inputs. The other peaks are various mixing results of the signals
present (and their harmonics). Now it is very clear why the Transmit
Bandpass filter is needed!
Component Layout: Keying & Transmit Mixer