Hello Stefan,
Did you try feeding with battery ?
Also, an isolating-transformer between the power-feeder and RX ?
I experienced 1-2dB drop of noise level with the battery,
and small improvement with the transformer.
(no IM experience though)
Perhaps these may help you further finding the cause of your problem.
73s,
Hideho YAMAMURA, JF1DMQ
Stefan Schäfer wrote:
Thank you Claudio,
I will consider that when the probe is down from the roof next. There
are a few dB in reserve, i'm still far away from the RX noise.
But now it looks much better! Still some IM but only a little. Taking
into account that AFN is arround, that's a positive result.
If there would be oscillations on the output of the probe, then i would
see them across the 50 Ohm resistor in the shack, at full amplitude, right?
73, Stefan/DK7FC
Am 09.10.2012 15:26, schrieb Claudio Pozzi:
The emitter follower transistor have a very low output impedance, no
problems
for receiving purpose but... if you use a long coaxial cable you
connect a
capacitance of about 100 pF/meter between output and ground. The
circuit can
become unstable and oscillate, I found that a 27 or 33 ohm resistor in
series
to the output help to kill the oscillation and the signal loss is a little
less than 6 dB (voltage, 3dB power).
I's a good practice to put a few ohm resistor in series at the low
impedance
output devices (OP-AMP, emitter follower, 74HCxxx buffers etc) driving a
capacitive load to prevent oscillation.
73
Claudio, ik2pii
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