Thursday, October 9, 2008

First sound: receiver project

First sound in a receiver is like first light in a telescope. I finally got enough parts built to hook them together and actually listen to signals on the ham bands. The modules, labeled if you can read them (click to enlarge), are -

HYCAS IF amplifier with AGC, kind of the heart of the receiver
6 pole crystal filter (Butterworth) 500 Hz bandwidth
BFO board, 8 MHz crystal oscillator with 74HCxx buffer, from EMRFD with some changes
Mixer board, just an ADE-1 SMT mixer and three RCA connectors for IF, LO, RF
Product detector board and audio amplifier stages, on R1 receiver board
DDS VFO (NJQRP AD9851 board) on same prototype board with ...
DDS controller using AVR Butterfly demo / development board
Rotary encoder for DDS controller

It sounds great, with some allowance for hum due to the haywired configuration. Still needs some front end filtering and several other additions, plus an enclosure.

I made a table of gains and losses. If you took a 1 uV signal and wanted to raise it to a "line level" of about 2 Vrms audio at the output, the voltage gain would be 126 dB. My table showed my receiver to have 121 dB. (I still plan to add a switchable 20 dB preamp.)

Testing gain, I used my signal to input a -107 dBm (1 uV) signal and found that the output is weak but readable. At -100 dBm it was "OK" and at -90 dB (about S6) it was strong.

What next? Build a SSB bandwidth crystal fillter. A preamp. Front end filtering, and decide on how to switch the filters with band selection. An enclosure. Decide whether to continue with the Butterfly DDS controller or do a more custom one with a PIC and a display with 2 lines and more characters.