On Tuesday 09 October 2012 19:28:33 you 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?
case 1, Oscillation: you are right
case 2, instability: appear as overshoting or ringing response at step or
pulse. See the attached image.
Sometime instability became self-oscillation if load impedance change a
little, e.g. no oscillation if the load is a pure resistance and strong
oscillation if the load have some reactance.
73
Claudio ik2pii
p.s. See also the M0BMU preamplifier schematic output transistor in "Bandpass
Receiving Loop Antennas".
>
> 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
--
Claudio Pozzi - Happy Linux User - http://www.qsl.net/ik2pii
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