uSBITX - My build of the sBITX
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Hi Everyone
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This is my version of the sBITX. It uses a Pi Pico 2 rather?than a raspberry Pi. It does not have all the software capabilities of the sBITX but it does have a spectrum display. I designed it to fit a Hammond enclosure of size 160mm x 125mm. I've followed the hardware of the sBITX with a few modifications.
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The SWR circuit uses a pair of MCP302 I2C ADCs
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Decided on relays for band switching because to me it's simpler and proven
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Uses a reed relay for the receiver isolation (I like it because it's silent and operates in less than 2 milliseconds)
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I've added a high-pass section to the 11 pole low-pass filter to keep out strong local AM signals
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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
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I went with an MMG3H21 MMIC for the receiver RF amp (and TX pre-amp). I've used it before and it works great
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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
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The second mixer is a sampling detector using a 74CBT3257 which is also used for TX
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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.
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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.
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Nothing special about the power amp. It's a push-pull configuration using a pair of RD16HHF1 mosfets
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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!
Details are here:?
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I'm still working on the software which I find quite difficult and time consuming!
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Cheers, Ian
VK7IAN
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