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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