20 METRE RECEIVER

This receiver was designed by Gary Breed, K9AY, and described in the December 1990 issue of QST. The complete article outlines the building of a two Homemade 20m receiverboard QRP 20m transceiver. The January 1991 issue deals with the transmitter part. These QST issues are now available from the ARRL in CDROM format.

I built the receiver a couple of years ago and have been pleasantly surprised with its performance. Despite a fairly simplistic superhet design it delivers ample performance. Being compact allows me to easily take it with me on trips. By adding just a wall-psu and stringing up some wire for an antenna it gets me over any cw withdrawal symptoms.

 

 

    Features include:
    1.    A simple superhet design based around a Motorola MC3362, VHF FM receiver IC
    2.    Audio-derived AGC for listening comfort
    3.    Narrow IF cw fiter - about 400Hz
    4.    Signal meter
    6.    Low receiver noise
    7.    Stable VFO in normal conditions
    8.    Good receiver performance
    9.    Availability of components
    10.   Easily replicated
    11 .   Low cost

Although there was a kit of parts available at the time the article was printed in QST, its availabilty is now dubious. After building it from scratch it worked first time - no problems! The hardest parts were designing a pcb, winding coils and finding a stable vfo capacitor. This is worth careful cosideration as it will determine the finished receiver's frequency stability. Although Breed recommends  a silver-mica or NPO capacitor here, I used an smd capacitor since these also exhibit a low temperature coefficient - in normal indoor use (ie: no sharp temperature changes) drift is negligible. The only other minor change made was to use a 10-turn potentiometer for tuning instead of two normal pots (coarse and fine) as suggested. A dearer choice but the resulting silky smooth tuning makes it worth it.

Receiver performance is promising - the article reports good sensitivity, with a minimum discernible signal of about -123 dBm. A third-order IMD dynamic range of 70 dB is not exceptional but good for the circuit's simplicity and the audio-derived AGC is reasonably smooth, and better than no AGC at all.

Adjustment is straightforward. Tune-up the frequency range you want - mine is set up for the bottom end of the band (until about 14.050MHz), peak the input coils for maximum signal at band centre, adjust the BFO trimmer to get a 750Hz tone and adjust a resistor to obtain the deflection you want on the S-meter. For more info ...read the article.


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