July 26, 2016
and the Small
Wonder Labs SW30+ Transceiver
Installment #5 ...
Receive Mixer &
and ... "Anatomy of a Homebrew Station"
and build-up of the classic 2-watt 30-meter superhet transceiver
designer Dave Benson K1SWL of Small Wonder Labs.
This installment is a pretty cool one where we now start working on the
"receive chain" of the SW30 Transceiver.
Also as is now customary, we have an interesting "lead-in" topic to the
main event, and this time it concerns the "Anatomy of a Homebrew
Station". The little Small Wonder rig gives us a terrific start to a
larger homebrew station, and as many of us know, there's all sorts of
exciting "accessories" that can be added to enhance the value,
performance and enjoyment of that station.
So with this episode we'll start by looking at the Big Picture and in
subsequent episodes we can drill down into the many accessories that one
can build just for that purpose. And I do think you'll be impressed by
the prodigious panoply of projects!
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 box during the show ...
<21:18:14> "Wayne W6AHH": This
is the variable bandwidth filter wayne burdick used on sierra (and I
assume K2 ect)- diodes not cap on ladder. Maybe a SW-30 mod?
<21:21:07> "Obe - KC4VZT": it would have to be very slow CW, it's
been a long time since I used CW.
<21:23:21> "Joe N2CX": Mike caps in ladder filter are Voltage
<21:24:14> "Mike WA8BXN": right, controlled by the processor in the
K2 and just a pot in the KX1
<21:29:06> "Wayne W6AHH": RIT kit that used to be available for SW
<21:31:29> "Al ve3gam": on qrp-tech, N8RVE has created a muppet
board for the SW40
<21:32:26> "Al ve3gam": might be usable for the sw30
<21:33:58> "George N2APB": Oh yes, the Muppet board is absolutely
usable for 30-meters!
(Click here if you
want to get straight to the Elmer 101 feature material farther down the
Anatomy of a Homebrew Station
Here is a conceptual block diagram representing the
many varied possible "accessories" that we homebrewers have
available to us in one form or another.
An interesting aspect of this diagram, however, is that all these
accessory functions can be integrated quite nicely with the SW30+
rig ... or just about any other one!
Another interesting perspective, that will be explored in greater
depth in numerous future episodes of CWTD is the "project" nature of
(Envision, if you will, that each of these accessory functions
can_and_will be fully designed and chronicled within the CWTD
Projects section of our website!)
Further, this diagram becomes a "clickable roadmap" that links each
item represented here to the respective project on the CWTD website!
N2CX Bench Tips
1. Power supply reverse
a. Simple schottky diode in
line with power lead from chassis mounted power jack to power lead
on pc board.
b. More sophisticated circuit with series PTC self-resetting fuse
between power jack and board plus 15V zener or transzorb to ground
to protect from reverse polarity or overvoltage.
2. Isolating audio problems ... An old tale of mine centers on
my troubleshooting a non-functioning Warbler transceiver circuit
from years ago ... but still valid! Symptoms were known-good
RF into Rx but no audio out. The solution? ... I found a defective
plated-through hole on pc board under one of the xtal filter
3. Frequency instability of transmitter VFO ... The VFO seemed
to vary erratically and had hum on it. Finally found that it was
caused by light-emitting diode used as voltage variable capacitor
for tuning. Ambient light shining through clear plastic body got to
diode chip inside. Varying light detuned diode and fluorescent
light caused ac hum.
4. Watch for RF Power output Instability ... On one of the early
NN1G designs, he had an 80 meter version. I built it and noted
output power instability. When I cranked the power up to max. (abt
2 watts) the output waveform got fuzzy. Wrote to Dave and he
suggested lowering the value of the Resistor from base to ground of
the final amp transistor, and that others had seen the issue on the
80 meter version. Changing resistance cured the instability. Later
checks with a spectrum analyzer (with the resistor at its original
value) showed that the amp oscillated in the VHF region at high
... AA0ZZ EZKeyer 2i (Internal) ... Perfect for the SW30+ in either the
Primary or Accessory enclosure
CWTD Episode #82:
Elmer 101 and the SW30+ Transceiver
Rx Mixer & Crystal Filter
First, a few Notes:
Kits are sold out.
SW30-specific Enclosures are sold out ... but you
can now purchase the BLANK
ACCESSORY ENCLOSURE KIT from AA0ZZ.
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
here 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
one more stage in the transmitter to complete but it will be saved for
the last board assembly step. Once that is done we will have to have a
dummy load or antenna connected or risk damaging the final. Enough of
the transmitter has been completed for the reason it was assembled
before the receiver, namely for use as a test signal for checking the
receiver stages as we complete them. Clever!
Once again we borrow from the nicely-done Elmer 101
materials created for the community circa 2000 by David Ek, NK0E
in part 5 of
The Eks Files ... http://eksfiles.net/elmer-101-kit-building-materials/.
(And by the way, if you are interested in some excellent component and
amplifier theory that directly relates to these specific circuits, we
urge you to look at Dave's full Part 5 installment referenced above.)
First, the incoming signal coming down the
Next, the 1st Receive Mixer ...
special to note about the parts, just make sure you have the right ones!
jumpers: For the J1 jumper I took a cut off component lead and bent into a
U shape and inserted that from the bottom of the board and then tightly
twisted the ends on the top of the board
together. This will be cut out later.
second test jumper, the base of Q6 is hole closest to R29 and D6. From my junk
box I found some wire that just fit in the holes to use for the jumper to the
top hole for C36. Alternately you could trace where the lines go and tack a wire
on the bottom side of the board at already soldered components.
jump pins 1&3 as you have in earlier steps.
seemed to work for me in this step.
the jumpers in place, but do disconnect power.
The various jumpers specified in the test procedure route the
low level transmit signal to the receiver. Another mixer chip
(U1) now combines a 30 meter signal around 10.1 MHz with the VFO
(2.4 MHz), again producing the sum (12.5 MHz that is not of use
to us) and the difference around 7.68 MHz that we want. You can
hear both frequencies on a receiver if you care to listen. The
presence of some signal as indicated by the RF Probe indicates
success, assuming its anywhere near the expected value. A scope
could also be used, but we won't be seeing a nice sine wave
because of the two frequencies present.
T1 is adjusted for maximum signal thus tuning it to around 10.1
MHz. Its not a sharp peak but that's OK, its mostly there to get
rid of the possible image frequency.
If this stage isn't working as expected, make sure U1 is
properly in its socket. That means properly oriented and all the
pins really are seated in the socket. Verify that pin 8 of U1 is
around 7 volts. That voltage comes from D2 Located near U3. It
was installed in Part 3 of the assembly steps but not tested at
that point. If D2 is reversed that will be a problem.
Temporarily connect a jumper between pins 2 and 3 of J1, another a
jumper between base of Q6 and top hole for C36 (neither yet installed),
and a third jumper between pins 1 and 3 of J3. Apply power. Look for RF
signal at pin 5 of U1 using RF volt meter, scope or receiver tuned
around 7.68 MHz. Try to adjust T1 for peak reading (very broad peak).
Disconnect power but leave jumpers in place.
Supply 12 V @ .04 A
U1 Pin 5: 1.25 V
PP using scope (waveform not sine wave), 1.8 V DC using RF probe
To look at signals on the receive side of things I built
a signal generator using and Arduino and an SI5351 DDS. This DDS chip
can produce 3 different frequencies at the same time. I wrote a short
program to produce approximately 10.110, 10.112 and 5.255 MHz for
testing purposes. The first two signals are in the 30 meter band and
near each other to see the selectivity in action. The third frequency is
at the image frequency to see what happens to it.
The outputs from the DDS are square waves, hence
the 3rd harmonic of 5.25
appears around 15.74 MHz. Where does Peak 4 come from?
The previous spectrum display did not show the
two signals near each other in the 30 meter band. Here the
center frequency and span of the display has been adjusted to
show they are both there.
The three output signals from the DDS were combined and
connected to the antenna connection of the SW30+.
With the signal source connected to the input, I
wanted to see what I had at the antenna connector. This is not
quite the same as what was seen coming out the the DDS signal
generator when it was not connected to the radio. Peak 1 at 2.4
MHz must be from the VFO. Peaks 2 and 4 are from the signal
generator. The source of Peak 3 is left as an exercise for the
These signals come out of the receive mixer U1 looking at pin
5. The desired signal is Peak 7.
After all the various signals coming out of the
receive mixer pass through the Crystal Filter we have this
viewed at U3 pin 2 which is the input to the Product Detector.
The desired signal is Peak 1, well above everything else.
Component Layout: Rx 1st Mixer