There are tons of free SDR applications, but I'd like to write my own. It isn't my goal to write and distribute a professional SDR application, but to learn the DSP involved in software defined radio.
I absolutely want to implement single side band and AM. These modulations seem easy, and would allow me to listen to 'interesting' ham chatter.
Other than snooping on conspiracy theory chat, I really want to receive the WWV time broadcasts. There are a couple WWV transmitters in the frequencies that I can access, and the timecode modulation seems accessible.
The designer has broken assembly into stages, I hope to more-or-less follow them.
12V Stage
The 12v stage powers the quadrature generator, sampler, and op amps. This was really simple to assemble. |
| 12v power completed |
USB Power supply
The USB supply powers the programmable oscillator and band pass selector circuity. The USB parts of this board are galvanically isolated from the other power supply. This was also simple to construct, with some really tiny components.
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| Top side components installed |
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| Nickel, 3v regulator |
Oscillator & Control
This stage contains the programmable oscillator and the band pass selector circuitry. This oscillator is really cool, being reprogrammable.
I had a number of issues assembling this stage. The oscillator has no-lead smd pads, which proved to be interesting to solder. After some failed attempts with the iron, I ended up using my hot air gun to turn the board into a huge pool of molten metal. This could use some cleanup to wick away extra solder, but I'd rather not mess with a working configuration.
I had a number of issues assembling this stage. The oscillator has no-lead smd pads, which proved to be interesting to solder. After some failed attempts with the iron, I ended up using my hot air gun to turn the board into a huge pool of molten metal. This could use some cleanup to wick away extra solder, but I'd rather not mess with a working configuration.
The most confusing part of this stage was hand wrapping the isolation transformer. The nomenclature was unfamiliar and I'm still not sure if I assembled it correctly.
Doing initial testing of this circuit noting was functioning correctly. I suspected I might've installed protection diodes (D1 and D2 below) on the USB power pins backwards, so I removed them. This didn't solve the issue, and I discovered I installed R10 and R11 sideways. The instructions explicitly mention this sometimes happens. Oops!
I didn't reinstall the protection diodes once I got the circuit working. I probably should, but I don't want to mess with a working circuit.
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| Functional oscillator & control stage. Bottom is the hand wound transformer. |
This is the output after the transformer. I'm not sure how concerned I should be over ringing.
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| 2Mhz center frequency |
I had one interesting thing happen to me while testing this stage. With a 8Mhz center frequency (So 32Mhz at the oscillator) my wifi connection dropped. The frequencies aren't close, and this isn't a high power design. I was able to reproduce this in multiple attempts by simply tuning it to 8Mhz. Weird!
Quadrature Clock Generator
This stage takes the output of the oscillator and generates the two clocks needed to sample I and Q. It uses two D flip flops to generate the phase shift. The assembly is simple, just plop the flip flop down.
A huge mistake.
While investigating something that apparently made no sense, I discovered I was following the directions from the RX1 model, and I have an RX2. Arrgghh!
I need to partially redo the last step; Rewrap the transformer and relocate the flip flop.
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| Clock generator removed and placed correctly |
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| The two clocks |
Band Pass filters
The SoftRock has 4 selectable band pass filters, allowing for greater selectivity of signals. Each band pass filter is composed of numerous inductors, capacitors and resistors. This stage will involve wrapping more transformers, as well and wrapping inductors!
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| 4 turn primary, two 2 turn secondaries. |
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| Wrapping a 30 turn core |
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| Band #0 inductors wound and installed. |
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| Many hours later, all 4 band pass filters are installed. |
Quadrature decoders, op amp circuitry
Last to assemble is the stage that samples the signal into I and Q, as well as the amplifiers.
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| Completed bottom side |
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| Completed top side |
Complete?
Initially powering up my board I didn't see anything useful on the PC. I did some troubleshooting, and I discovered I installed the op-amps backwards. The opamp has a slight 45 degree turn on one edge, and I missed this before.
After fixing that and reverifying every test, I still didn't get anything. I suspect this is because my "random wire" antenna was crap. I unfortunately got stalled at this point for about a month debating how I should proceed.
I could build a cost effective amazing performance antenna, but I would have to buy a tuner and an SWR meter; Neither of these are cheap. I'd additionally have no way to test this antenna to actually know it is working.
Alternatively I could buy a prefab antenna with less (but adequate) performance. If I ever became serious about radio I'd possibly have to replace this, and buy the tools I would be avoiding buying anyway.
After a month of indecision I simply went for the easiest way to make progress. I bought a prefabricated antenna and connecting wires. Reading the numerous reviews on eHam I decided to go with the 10/20/40m MK2 from LNRPrecision. For $100 (Antenna and cables) I should be able to have something I can be reasonably sure about. That puts the cost of this project so far at about $220, not including labor.
Final Build Photos
Cleaned the board up with a combination of windex and ethanol, which works wonders on flux.
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| Top side |
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| Bottom side |
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