October 13, 2016

Elmer 101
and the Small Wonder Labs SW30+ Transceiver

Installment #7 (final) ...
Audio Output & Tx Final Amp

A step-by-step analysis and build-up of the classic 2-watt 30-meter superhet transceiver

 from 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

Podcast ... Click to listen or right-click to save to local computer

Chat Window (during show) ...

<20:12:47> "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. (n2apb@midnightdesignsolutions.com)
<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 page)

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


Block Diagram

Technical Discussion

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/.

Pick particular value NPO capacitor for C7 that gives the proper frequency range.

Check 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 C7. 

Now 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.

 WA8BXN Notes:

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 board!

For 10.100 – 10.135 MHz operation: If the frequency was between

Install the 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


 The 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)

where 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.

Measured 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.


Part 12 – Audio output

The 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.

If 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.

If 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.

WA8BXN Notes:

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.

Part 13 – Transmit final amplifier

What's important in a final amplifier?

Gain:  high gain typically required
Efficiency:  especially important for battery-powered rigs
Linearity:  reproduction of the input signal at higher levels
Load:  impedance of the load must be matched to the amplifier output
Spectral purity:  harmonics and other unwanted components must be filtered

Amplifier Classes ... http://www.electronics-tutorials.ws/amplifier/amplifier-classes.html

Discussion Points:

  1. 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.

  2. Amplifier output is the sum of the transistors in those classes to produce a full sine wave.

  3. In RF amps tuned circuits on output "clean up" signals to eliminate distortion of RF envelope.

  4. Class A, B and AB are linear amps for any type of modulation.

  5. Class C used where RF envelope is constant such as CW, FM, RTTY.

  6. Feedback in Class B and AB amps can reduce modulation distortion.

  7. Special low distortion methods "pre-distort" amplifier input to enhance linearity.

  8. 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.

  9. Can pulse width modulate E and F mode amp inputs to use them as linear amplifiers.

Class A

Class B

Class AB

Class C


And then there's the Final Amplifier in the SW30:

Now 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!

With 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.

With 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.

WA8BXN Notes:

Wire lengths for the coils:

L2         6 turns #24 (4 inches) on FT37-43 core (dark grey)

L3         15 turns #24 (8 inches) on T-37-6 core (yellow)

L4         15 turns #24 (8 inches) on T-37-6 core (yellow)

BE SURE to tin the leads very well!

D12       1N5256 30 V 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 transmitter. 

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. 

Good luck!

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. 

Test procedure:

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 signals.

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

A fair number of signals were heard when connected to antenna.

Note: C1, C30 and C32 locations on the board are not used for 30 meters.



Troubleshooting Schematic