Owen,

Thanks for the prompt response. I'll try a step attenuator in line with
the antenna and different values of attenuation to see how these effect
noise level.

Since my post I also tried placing the receiver in a metal enclosure in
case it was picking up and regenerating the LO but this had no effect.

A simple piece of wire soldered to the antenna pads works a treat. I
don't have a variable mismatch available to see how stable the input is
for different VSWR.

Would I get an indication by soldering various values of resistance
across the antenna pads and see if the system becomes unstable again ?

Regards

David

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

David,

There are a number of things that might make the amplifier perform
differently if the source impedance changes, and you seem to be exploring
those.

Another thought:

One of the things that a real antenna introduces to the mix (cf a signal
generator) is the real world RF environment with a host of signals to mix
with each other in your receiver.

A quick check for this is to see if the S/N ratio of a weak signal is
improved by inserting an attenuator after the antenna. Obviously,
inserting a large amount of attenuation will degrade S/N, but try 6, 10,
15dB.

You have described the selectivity of the front end as "and then a tuned
circuit". Front end selectivity is very important in preventing loss of
receiver sensitivity by noise from IMD in the rx front end.

Owen

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

Regards

David