Let’s continue the transmit chain by adding the parts for the transmit mixer and the amplifiers (up to but not including the power amplifier, or PA). I originally planned to do only the TX mixer but when I was done there wasn’t enough output to drive the digital dial output, so I added the two transistor amplifiers that drive the power amplifier in order to boost the output. There’s really not a whole lot to analyze here, and because I’m on a deadline for finishing this radio in time for QRP To the Field (QRPTTF) in less than two weeks I’m not going to spend much time trying to explain this part of the circuit.
U1 is the other SA612 chip (U2 is the other one) that, on transmit, mixes the 9 MHz signal from the crystal filter with the ~5 MHz signal coming from the VFO, so we call it the transmit mixer. The output of the mixer is actually a bunch of frequencies, made up of the sums and differences of the 9 MHz signal from the crystal filter and the 5 MHz signal from the VFO and their harmonics. That ends up being a bunch of different frequencies, but the strongest signals are going to be the sum of the fundamentals (14 MHz) and the difference (4 MHz).
The mixing product that we’re interested in is the difference between these two, putting us in the range of about 4 MHz. Increasing the frequency of the VFO (by turning the big tuning cap) will reduce the frequency of the output of the transmit mixer. The output from the mixer comes on pins 4 and 5, and that’s fed through a tuned circuit formed by T4 and its neighboring capacitors to filter out the unwanted mixer products. What’s left gets amplified first by Q8 and then by Q4. The output of Q4 is ultimately what drives Q16, the IRF510B power transistor.
So, to build this part of the circuit, install the following components:
U1 (and its socket–make sure to orient correctly)
C12, C17, C18, C20, C21, C22, C24, C25, C26, C38
R7, R8, R9, R10, R12, R15, R16, R19, R20, R22, R24
We’ll test this part of the circuit by powering up the board, putting the rig in tune mode, and checking for a frequency somewhere around 4 MHz at the output of Q4 (I used the bottom left pad of where T2 will be installed, but you could also test at the Q4 leg closest to T2) while transmitting. By this time you should know how to wire everything up to make that test. When I did it, I got a frequency around 3.85 MHz. Here’s a picture of my test setup:
You can see that the digital dial reads 3.84, so I’m at least in the ballpark. I’ll have to adjust the windings on T5 at some point to get the frequency range I want.
Next: The Receive Mixer