A Prototype for a Si5351-Based SSB Rig


Now that I think I've fairly well determined that the Si5351 is suitable for use in a ham radio transceiver, it seems like time to put thought into action and actually try to build one. Ever since discovering that the Si5351 can output multiple independent clocks from one IC, I thought it would be neat to use one output as a VFO and a second as a BFO. As I showed with my Grabber RX prototype, this is certainly a viable thing to do.

One type of SSB transceiver architecture that I've been experimenting with in the NT7S shack is one using an unidirectional IF for both the receive and transmit signal paths, as opposed to the bidirectional designs seen in radios such as the BITX. The Lichen transceiver seen in Chapter 6 of Experimental Methods in RF Design is a nice example of such a radio. In past experiments, I have switched the VFO and BFO signal paths using analog switch ICs. But I realized that when using the Si5351, all you would need to do to implement this type of architecture is to connect, for example, the CLK0 output to the first mixer and the CLK1 output to the second mixer, then swap the frequencies on each CLK output when switching to transmit.

With that in mind, let me present the block diagram of my implementation of this below:


The mixers are the ubiquitous 602/612 loved and hated by QRP homebrewers around the world. I'm not a huge fan of the 602, but it has a couple of things going for it in this application. First is that there are essentially two inputs and two outputs on the IC, which makes it very handy for this type of design. And while it has fairly atrocious intercept figures, it does reduce component count quite a bit. So you could consider this more of a cheap & cheerful radio for fun, not a design for work in seriously crowded conditions. The rest of the elements in the design are pretty much your standard circuits. Nothing too groundbreaking there. One thing I neglected to put on the diagram above is 10 dB attenuator pads on Si5351 outputs in order to get the ~3 Vpp output down to around the 300 mVpp that the 602 likes to see for oscillator drive.


So here's the beautiful ugly mess on a piece of copper clad. This was originally a CC1 prototype board, but I decided to cannibalize it for this SSB rig since it already had the microcontroller and Si5351A, and because I was feeling too lazy to start from scratch. The radio build only took a couple of half-day sessions in the shack, and worked mostly as expected right off the bat. The T/R VFO and BFO swapping scheme worked perfectly, needing only a few extra lines of code to implement in the already-existing code. I ended up making my first QSO with the rig (5 watts transmitter output) checking into the Noontime Net and getting a S7-S9 report from net control. The second QSO was last night with fellow Oregonian, Joel KB6QVI, who was kind enough to give me a sked in order to check out how the radio was working on the air. Finally, I had a very brief QSO with Dave AA7EE, who gave me an inciteful audio report although we had a poor propagation path between us. Right now, I've got it back off the air to tweak a few thing, such as the audio response in the mic amp, but expect to get it back in working order for use at Field Day.

Overall, I'm pretty happy with the direction this radio is proceeding. If I can get all of the bugs worked out, this could be a pretty potent design. Not in the performance category, but in the cost and component count sense. I'm seriously considering whether it may be feasible to do crowdfunding for a run of kits if I can nail down the design well enough. I have come to believe that the Si5351 could be a game changer for ham radio HF and VHF radio designs.

17 thoughts on “A Prototype for a Si5351-Based SSB Rig”

  1. Hi - looks like an interesting project - and with All QRP-related projects the power consumption can be a consideration (if used portable)

    Have you done any power consumption measurements of the Components for just the frequency synthesizer parts (micro controller,display and the Si5351)

    Seriously considering prototyping this to replace an AD 9850-based DDS VFO if it uses less than the ~ 140ma. Always on the look-out for any possible options of low power components to swap out VXO's in mono-band QRP Rigs.

  2. Bob,

    I haven't done current measurement yet, but that's an excellent idea. I'll try to get to that soon and report back the results.


  3. Here's a thought. Replace your receiver front end '602 with the FCT3253 as in Ashhar's latest Minima, his schematics attached to this recent post:
    Note that his S1 input on pin 2 is grounded, could drive that pin high on transmit to select transmit audio from currently unused pins 3,4,12,13. You get a great mixer with two inputs, and the tx audio port should not interfere at all on rx..
    Then make the IF amp a VGA, controlled from the uC. On transmit in CW mode, center the carrier in the middle of the crystal passband and shape the keying envelope with that VGA, so does away with his audio osc (sidetone is the usual uC square wave). If the uC has an ADC listening to recovered audio, that VGA can also serve for rx AGC, and of course as a volume control. Back off to supporting only 80/40/20 in the basic version, and this could be a very simple high performance XCVR. If your Si5351's prove to be of low enough phase noise. I browsed briefly through your code, looks like you mostly update the output MultiSynth rather than muck with the VCO freq to avoid glitches. Maybe continue with that scheme, except after a 1/2 second timeout of no tuning activity it reconfigures for an integer divide on that output multisynth, Silabs claims this gives less jitter.
    Jerry, KE7ER

  4. I assume that FCT3253 mixer suppresses the carrier rather severely, and possibly a bit inconsistently from build to build.. So for CW transmit, might be better to drive the PA more or less directly from the Si5351, using an analog switch to select either that or SSB voice from the second IF mixer.
    There's a long history of of using one IF chain for rx and tx, though the Si5351 adds a lot by drastically improving stability, flexibility, and parts count. The Epiphyte3 SSB transceiver from the 90's had a clever way of adjusting gain around a '602 used as the second mixer to add rx agc.

  5. Thanks Jerry,

    I will definitely check out the FCT3253 mixer idea. I've been wanting to build one for quite a while, so this may give me an excuse to try one out. I'm not the biggest fan of the 602/612, but the goal of this project was the build something simply and cheaply with parts I had on hand, and it fit the bill quite nicely. I wouldn't want to use it on a contest day, but it works great for a fun little radio to play with.

  6. There's a tradeoff to be made between rx dynamic range and battery longevity. For a small trail friendly radio, the 602/612 might be the best choice. The KD1JV ATS/MTR rigs get lots of complements on receiver performance, In part because of careful attention to the tuned rx filter up front. Works well for 40m CW, for example, but screens out the SW BC stations in the upper part of that band. Perhaps use an analog switch that works over a range of power supply voltages, raise the voltage when you need the dynamic range.

  7. Did you have to filter the output of the Si5351? But then again I've heard that mixers don't have a problem with the square wave output of the 5351.

  8. Michael,
    Since this was a quick and dirty homebrew project, I didn't filter the Si5351 VFO/BFO output to the mixers. It does work, however, you're not going to get the best performance from your radio going this way. As long as you have a decently-filtered front end, I believe the scheme works well enough for a popcorn-style radio.

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