October 13, 2016
and the Small
Wonder Labs SW30+ Transceiver
Installment #7 (final) ...
Audio Output & Tx Final Amp
and build-up of the classic 2-watt 30-meter superhet transceiver
designer Dave Benson K1SWL of Small Wonder Labs.
This is IT ... the last in our series of Elmer 101 episodes chronicling
the build-up and test of the Small Wonder Labs SW30+ Transceiver!
We'll cover the audio output stage and the Tx final amplifier stage.
73, George N2APB and Joe N2CX
... Click to listen or right-click to save to local computer
Window (during show) ...
"K2LAZ -- laz, FN20": Its a smoking hot product!
<20:21:26> "Obe - KC4VZT": he's in the NJQRP
<20:26:52> "Obe - KC4VZT": I went to the original elmer series and
found that I expanded the range to 48k my changing C8
<20:29:10> "Al - N8WQ": finished
<20:29:17> "Obe - KC4VZT": My is complete
<20:38:34> "Pat W0BM": It all worked the first time
<20:42:07> "Obe - KC4VZT": is that what cause the thump sound when
you shut off the rig?
<20:43:01> "Joe N2CX": Hmmm, could be. I never paid much attention.
<21:18:03> "Armand WA1UQO": Ditto to that Joe!!
<21:29:44> "Obe - KC4VZT": I've really enjoyed this and think I've
learned a bit about RF which was my goal as I'm a digital designer.
<21:35:20> "Armand WA1UQO": Really enjoyed the series. Looking
forward to the next topic!
<21:38:05> "George N2APB": I actually have a couple of left-over
full kits ... contact me offline if interested on one of them.
<21:38:33> "Obe - KC4VZT": Good Night and thanks for show
(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 powerful 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!
Envision, if you will, that each of these accessory functions can 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.
So ... Try clicking on the images below!
... We'll be populating each one of the pages linked
in the diagram to actual, in-progress projects that we all
can build and use!
CWTD Episode #84:
Elmer 101 and the SW30+ Transceiver
Audio Output & Tx Final Amp
Once again we borrow from the nicely-done Elmer 101
materials created for the community circa 2000 by David Ek, NK0E
in part 6 of
The Eks Files ... http://eksfiles.net/elmer-101-kit-building-materials/.
particular value NPO capacitor for C7 that gives the proper frequency
tuning range without C7. If you have a counter, connect to jumper wire
between base of Q6 and top hole for C36, with jumpers still installed
from previous steps. Install a jumper between pins 1 and 2 of J2, record
frequency. Move this jumper to pins 2 and 3 of J2, record that
frequency. These are your lower and upper bounds of tuning range without
follow these excellent instructions from the original SW30+ manual. If
you don't have a tuning range near these, change turns on L1 by first
moving existing turn spacing or adding/removing turns as needed.
This looks complicated, but it is not!
With no C7, my frequency range was 10.149 to 10.184 which would put
me pretty much out of the band. That range is roughly 10.150 to 10.180
which is not in the table. Looks like somewhere between 47 pF and 68 pF
would be needed for C7. I tried 47 pF. That gave me a tuning range
of 10.101 to 10.135. Prefect for my needs. If I wanted to make a finer
adjustment I would just adjust the spacing of the turns on L1.
Don't forget to actually solder the capacitor you pick for C7 to the
For 10.100 – 10.135 MHz operation:
If the frequency
following value for C7
10.100 – 10.120
10.120 – 10.140
10.140 – 10.160
10.160 – 10.180
10.180 – 10.200
10.200 – 10.220
10.220 – 10.240
10.240 – 10.260
table above does not show that more capacitance moves the
frequency higher. It means that more capacitance compensates
for higher frequency. Are we clear on that?
(The approximate formula for
the value of C7 is as follows:)
Δf (KHz) =
1.0 × [C7] (in pF)
Δf is the
desired frequency shift
It’s possible to adjust
the operating frequency as much as 15-20KHz downward by squeezing L1’s
turns more closely together.
values: Without C7, my tuning range was 10.112 to 10.147 Mhz. I tried a
10 pF NPO capacitor I had (I know, it’s not a supplied value) and that
gave me a range of 10.1015 MHz to 10.1358 MHz. I could have adjusted the
spacing of turns on L1 to get right down to 10.100 on the low end but
decided I was close enough to always be in the band.
second half of U4 further amplifies the audio signal after it passes
through Q1. We have been using the transmitter (less the final
amplifier) as a signal source. When we key the transmitter stages at
this point to produce the sidetone Q1 is biased as an open circuit
but R9 still passes some audio that we hear.
you don't have a fair amount of audio coming out of the receiver at
J3 at this point, first check for audio at pin 7 of U4. If nothing
there you can go back to listening for output at J3. You do still
have the transmitter keyed for testing, right? With the transmitter
keyed, Q1 is turned off. Try jumpering R9 to pass more audio from
the preamp to audio output stage. R7 sets the sidetone level should
you want to change it.
you have some audio going into pin 6 of U4 you ought to get more
audio out on pin 7. If not, do the usual verification of proper
components and soldering in this stage.
Resistors - This step finishes off the resistor supply, if you have
any left over , well something is wrong!
The silkscreen label for R12 is a bit confusing, refer to illustration
in assembly instructions.
Audio level in headphones should be moderate, no doubt it’s there.
What's important in a final amplifier?
gain typically required
especially important for battery-powered rigs
reproduction of the input signal at higher levels
impedance of the load must be matched to the amplifier output
harmonics and other unwanted components must be filtered
Amplifier Classes ...
Curves to the right of the
schematics show collector voltage in amplifier. I-V for
only one of the transistors in class B and AB.
Amplifier output is the sum of the
transistors in those classes to produce a full sine
In RF amps tuned circuits on output
"clean up" signals to eliminate distortion of RF
Class A, B and AB are linear amps
for any type of modulation.
Class C used where RF envelope is
constant such as CW, FM, RTTY.
Feedback in Class B and AB amps can
reduce modulation distortion.
Special low distortion methods
"pre-distort" amplifier input to enhance linearity.
Other modulation types such as E
and F drive the ouput amp with square waves to use them
as on/off switches for high efficiency.
pulse width modulate E and F mode amp inputs to use them
as linear amplifiers.
And then there's the Final Amplifier in
we are looking for real power output from the transmitter. R24 is
also adjusted to provide the proper drive level for the final
amplifier. I found 1 watt output to give a clean signal and 2 watts
to be a dirty signal. Err on the side of caution!
the final amplifier components in place its probably not a good idea
to continuously key the transmitter. Also, be very sure you have a
good dummy load connected to the rig.
the rig unkeyed, you should see 12 volts on the collector/banded end
of D12. If not, check soldering of L2. With the key closed there
should be output at the antenna connector as you carefully advance
R24 to increase drive level. If that doesn't happen suspect
soldering of L3 and/or L4.
Wire lengths for the coils:
6 turns #24 (4 inches) on
FT37-43 core (dark grey)
15 turns #24 (8 inches)
on T-37-6 core (yellow)
15 turns #24 (8 inches)
on T-37-6 core (yellow)
BE SURE to tin the leads very well!
1N5256 30 V ½ W Zener
diode, note banded end up, body of diode in circled hole.
Note: Drawing below shows 1N5257B (33V instead of 30V).
When I wrote the assembly instructions I used connectors on the board
for the various J connections. Now I am trying to not be doing that as
they are not included in the CWTD kit. I have the beautiful red PCB case
to put together but I first want to test this last stage assembly before
putting the board in the case and cutting all the various wires going
off board to final length.
Adequate testing can be done with only power, antenna and J3 pins 1&3.
Actually we just need to be able to ground J3 pin 3 to key the
So using the provided RG-174 I temporarily wired up the antenna
connector for testing. I don't have to connect a pot to J1 for example,
because I know the extremes of the tuning range are within the band. I
will also be using a dummy load.
Drum roll .... I connected power, grounded pin 3 of J3 and got 2.1
watts output. I think it works! Next I need to build up the case and
wire up everything.
You should have 2 RFCs each 22 uH still taped to the capacitor sheet and
various alternate values of capacitors that could have been used for C7
left over to add to your junk box.
Connect 50 ohm load to “RF OUT” (coax center goes to RF, braid
goes to OUT).
Connect key J3 pins 1&3, headphones J3 pins 1&2. Connect 100K
pot to J2.
Apply power. Key rig and adjust R24 for no more than 2 watts out
(higher power makes an ugly output signal). Peak T3 and T2 for
max power output, reduce R24 as needed. Replace dummy load with
antenna. Peak T1 for max noise or signals. Tune VFO, hear many
Measured: 12.0 V supply, 0.01 A keyup, 0.19 A keydown
R24 adjusted for 1.2 W output using watt meter, 22 V DC using RF
probe. At higher drive adjustment on R24, signal started looking
bad on scope.
Tuning range: 10.102 to 10.136 MHz
fair number of signals were heard when connected to antenna.
Note: C1, C30 and C32 locations on the board are not used for 30