02 December 2009

Which RX for my 50 MHz QRPp FM TX

I received this question from Pedro, a LU HAM: "What are you planning to use for the 50 MHz FM project receiver?" Here's my answer to him.

Originally I planned to use the TDA7000 chip but configured at NBFM it is not good according to SM0VPO.

Then I switched my mind to TBA120, an FM IF chip from TV sets that works better but it works up to 10 MHz (or somewhere there). Since my TX is single-frequency, as the original SixBox, I thought of using a 50.0 MHz canned oscillator to feed a mixer and tune my IF at 800 kHz (my TX XTAL is 16.934 MHz or so).

Last chance is to use a direct conversion receiver and agree with the other station will reply in SSB or AM, so working a mode-split QSO. I am not planning to build two (R)TX to talk together!

I don't have a diagram of the TX yet. It is based on 74HC240 chip:
- one gate oscillator
- one gate buffer with LC output tuned on 3rd harmonic
- one gate buffer
- 4 parallel gates as PA

The second buffer could be removed but then the 4 parallel gates give about 14pF or capacitance, which is too much for the LC.

I have measured about 28mW at the output of a single 74HC86 gate, so hopefully 4 HC240 in parallel can deliver more, even if at 50 MHz (operating limits).

Let me know if you have some ideas for PA and/or RX!