On Sun, 13 Mar 2005 16:42:18 -0500, "Netgeek"
wrote:

I'd like to build some homebrew VHF-AM receivers - specifically a receiver
for the VHF 108-118 Mhz band.

Note that the FM broadcast band is just below that band with lot of
strong transmitters, so any configuration with low side local
oscillator injection and the more or less standard 10.7 MHz first IF
is most likely going to give image frequency problems from FM
broadcast stations in the 86.6 .. 96.6 MHz range. To avoid this, a
good narrow tunable filter is required in front of the first mixer,
which can be a problem if good frequency agility is required.

One approach would be to use a much larger first IF or put the local
oscillator above the desired band (which also swaps the sidebands)
with image responses in the rest of the aviation band.

One idea would be to use a fixed downconverter e.g. with a 98 MHz
crystal frequency, mixing the VOR band down to 10-20 MHz, filter out
the strong mixing products from the FM broadcast band that is on
frequencies below 10 MHz and use a DDS with I/Q outputs to get I/Q
demodulation of the signal.

Paul OH3LWR