Thursday, July 8, 2010

Success! Transmitting extremely low power with my 20M DIY SSB radio

Ok, so this project has been tough recently. I found out that my home-brew crystal filters using 4 Mc CPU xtals were no good (their response was far from ideal). So, i ordered a 2.5 KHz SSB filter from Inrad which was very inexpensive and provide excellent magnitude response. I should have ordered one of these in the first place considering the price.

The Inrad filter has a center frequency of 9 Mc, therefore i had to re-design the VFO, BFO, LPF's on each of these and the IF bandpass filters before and after the high-gain IF amplifier module. Alot of work!

With all of this complete i was able to tune in local amateur radio stations with ease.

Next step the AF and TX audio. I used a dual audio power amplifier chip, one half for RX to speaker audio amplification, the other half for TX audio. I purchased a surplus Motorola microphone from the MIT flea market for $5, found the schematic on repeater builder, and with advice from Mike Volz i biased it up.

I fed the output of one channel of the audio amp through a 100 ohm resistor to the IF port of the product detector. Wired the PTT switch from the surplus Motorola mic to the TX line of the bi-directional IF amp. Using my PARTS radio, i was able to listen to myself transmit using the DIY 40M radio! Success!

Next step: design the AGC system, then build it.

Next step: design the TX driver/power amplifier assembly

Next step: design the front-end LNA and TX/RX relay assembly

Stay tuned....