David wrote in news:9SuEh.3761$8U4.3204@news-
server.bigpond.net.au:

I have just completed the design of a 151 MHz ASK transceiver.

On the bench the unit works fine with sensitivity to around -115 dBm
however, when I attach an antenna, the RSSI level increases
dramatically
and it appears something may have become unstable.

I have tried a commercial ground independent antenna and a 1/4 wave
whip
and the results are the same.

If I disconnect the antenna and leave the RF input floating, it behaves
properly. I can also attach a small wire (say 300mm) directly to the RF
input and it also works fine. It just plays up when I have a proper
antenna connected.

I am not sure if this may be some issue with the VSWR of the antenna
causing some instability (I tried a 3dB pad between RF input and
antenna
and this did not fix the problem), or whether the input is so sensitive
that it is picking up the LO from the radio and causing regeneration ?

A Spectrum Analyser was used to determine if anything else is being
transmitted in that region and it appears to be clear to well outside
the bandwidth of my receiver.

Any ideas much appreciated for what might be happening or methods of
debugging my design.
(The circuit consists of an RF switch after the Antenna port (selects
TX/RX) then into a 50 Ohm RF Amp, followed by a tuned circuit into an
SA6505 RF IF. The LO is crystal based harmonic Butler emitter follower.

PS. I also tried bypassing the RF Amp but still no improvement.

Thanks in advance

Have you tried putting a straight 50-ohm dummy load on it? If that
causes the instability, then you need to look at the first device the
signal encounters to see if it's stable in that configuration.

Otherwise, I'd look to strong nearby signals. The step attenuator should
help with that. If introducing 6db of attenuation reduces the problem by
18db or so, then you have intermod problems (3rd order).


--
Dave Oldridge+
ICQ 1800667