• The SWR circuit uses a pair of MCP302 I2C ADCs
• Decided on relays for band switching because to me it's simpler and proven
• Uses a reed relay for the receiver isolation (I like it because it's silent and operates in less than 2 milliseconds)
• I've added a high-pass section to the 11 pole low-pass filter to keep out strong local AM signals
• I found that the 40MHz crystal filter had an impedance of 50 Ohms when the band-pass was about 15Khz - so went with that - no matching circuit needed
• I went with an MMG3H21 MMIC for the receiver RF amp (and TX pre-amp). I've used it before and it works great
• Used a T-622-KK81 trifilar transformer in the first mixer in conjunction with an 74LVC1G3157 analog mux which means that the Si5351 can drive the mixer directly
• The second mixer is a sampling detector using a 74CBT3257 which is also used for TX
• The receive (and mic) signal is sampled using the Pico ADC which is 12 bits but 16x oversampling brings that up to 14 bits. It seems to work pretty well.
• Audio and TX signals use PWM with a range of 12 and 10 bits respectively. I was surprised at how good the audio sounds with just 12 bits.
• Nothing special about the power amp. It's a push-pull configuration using a pair of RD16HHF1 mosfets
• On the software side. I went with FIR filters and digital mixing. The spectrum is processed separately from the audio signal processing. I'm no expert and I found this way easier to implement!

1. I usually start with a block diagram. Not really an issue with?the sBITX.
2. Once I have the block diagram I'll do some software validation (since there is always a uC invloved, right). This is the first project I've done using DSP for both receive and transmit (as well as control). So I'll work out the DSP details and write a C program in Visual?Studio to test them out. For example, I know (from other projects) that the FIR filter code needs about 10 cycles per tap to run on a Pico 1. On a Pico 2 that reduces to about 5 because of it's fused multiply-add DSP instruction. I know I'm going to run the clock at 225Mhz in order to get the PWM at a reasonable resolution (12 bits) for audio and 10 bits for the TX signal. That tells me there is capacity for about 1400 taps running on one core with a sample rate of 31250 Hz. And 31250 Hz is based on the ADC being clocked at 500 KHz and 16 times oversampling. For SSB I'm using a low-pass/high-pass FIR filter with 200 taps to reduce the opposite sideband. Then another FIR filter (300-2400) of 200 taps for the audio pass band. Add in other bits and?pieces and that's well within the limits of the Pico 2. I can test the functionality of the filters in VS.
3. I'll develop the schematic since I need to know which pins of the uC are needed.
4. Validate what circuit functions I can using LTspice. It's great for verifying component values for filters and amp gain, etc.
5. Develop some minimal code for the uC. I like to do this since once the hardware is built you can then start testing immediately rather?than building something and then spending hours writing code before the hardware can be fully tested.
6. For some things (like filters) I'll build something on a breadboard and test it?out. I did that with the 40MHz crystal filter. This is also where previous projects?are immensely important because they tell me what works. I know the SM filters work because I tested them out on a DC receiver. I know the LCD display works because I have one tacked to a Pico board. I know the audio amp circuit works. I know the kiss mixer circuit works, I know the sampling detector circuit works and so on. All have been tested on other projects.
7. Developing a PCB is probably the most enjoyable part for me. I learned early on that a 4-layer board is best. I follow some strict rules. The 2 inner layers must be solid ground plane with no breaks. Every pin that connects to ground must have its own via to the inner ground plane. Every top-to-bottom connection needs a nearby ground via. I route all the analog signals on the top layer and all the digital signals on the bottom layer. I was initially worried that digital noise would get into sensitive receiver circuits but earlier projects (like the DC receiver and the 4020 transceiver) revealed that this isn't a?problem if you follow the basic layout rules.
8. The PCBs are so low cost now that I'm not fussed about needing to do another board. For the last few projects I haven't needed to. I learned a great lessen with a dummy load project early on. I thought if the top layer was signal and the bottom layer ground then that would make a great heatsink. Well it also makes a great capacitor which is not idea for a dummy load!
9. I'll build the prototype in stages and test them as I go. Starting with the power supply, then the Pico and the Si5351, then the LCD, then the audio, then the BFO mixer and so on. The software can be tweaked as I go to test things.

Do you have a complete writeup or hints for the homebrewer on this homebuilt SBITX? like which outlets to source the parts,crystals,etc?