July 26, 2016
Elmer 101
and the Small Wonder Labs SW30+ Transceiver
Installment #5 ...
Receive Mixer & Crystal Filter
and ... "Anatomy of a Homebrew Station"
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.
Overview
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!BTW, did 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)
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? http://www.elecraft.com/TechNotes/ABX.html
<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 variable caps.
<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 http://smallwonderlabs.qrpradio.org/docs/SW+RIT.pdf
<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 page)
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 these components.
(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 polarity protection
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 crystals.
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 drive levels.
Product Preview ... 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 shows 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
Block Diagram
Technical Discussion
There is 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 antenna ...
Next, the 1st Receive Mixer ...
Assembly:
Nothing special to note about the parts, just make sure you have the right ones!
Test 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.
For the 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.
On J3 jump pins 1&3 as you have in earlier steps.
Things seemed to work for me in this step.
Leave the jumpers in place, but do disconnect power.
Test:
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.
Measured values: 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
Spectrum View
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 reader!
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
Troubleshooting Schematic
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