I'm planning to build a 20m SSB transceiver and I want to get the overall architecture straight in my head first. Here's what I'm thinking so far, a single conversion superhet design with a 9MHz IF and a homemade crystal filter. The problem I'm struggling with is in relation to mixers and inversion.
Because it's 20m and conventionally broadcasts are USB, lets say we're tuned to 14.1MHz and there is a USB signal from 14.1 to 14.103 MHz. 14.1 equates to 0Hz audio and 14.103 is the highest audio frequency. My filter has a BW of 3KHz and a centre frequency of 9 MHz. If I tune my VFO to 23.1015 MHz then I end up with an inverted USB signal through my filter. Do I have that correct?
So, 23.1015 - 14.1 = 9.0015 (i.e., the lowest audio frequency in my signal ends up at the highest frequency through my filter) Likewise, 23.1015 - 14.103 = 8.9985 (highest audio is lowest through filter) But, it's often said that a crystal filter favours LSB, is this setup going to cause any issues?
So, if I set my BFO to 9.0015 MHz, I get: 9.0015 - 9.0015 = 0 Hz and 9.0015 - 8.9985 = 3 KHz. So, the highest filtered signal becomes the lowest audio signal and vice-versa.. Do I have that right?
I'm not sure if this is the standard way to build a simple SSB transceiver, are there any pitfalls in this approach?