[time-nuts] Sound Card Spectrum Analyzer
dk4xp at arcor.de
Fri Feb 19 11:42:52 UTC 2010
Bruce Griffiths wrote:
> A preamp like that shown will be best suited to phase detectors with
> low gain and output impedance with a resistive output termination
> matched to the phase detector.
> Phase detectors like the Minicircuits RPD-1, MPD-1 etc have a
> relatively high output impedance and a preamp with a lower input
> current noise and somewhat higher input voltage noise may be better
> suited, particularly if the output is terminated in a capacitive input
> filter and the filter low frequency dc (and low frequency) load
> resistance is high.
> In such cases a version of the Wenzel JFET input amplifier with lower
> noise using fewer JFETs and having a low frequency input voltage noise
> that has a finite spectral density at dc may be more appropriate. An
> ultra low input offset voltage together with low drift is also possible.
I have a version with BF862-FETs, too :-)
Not bad, but close to the carrier, the BJTs win.
BTW I don't like Wenzel's finesse noise remover any more, at least not
for > 5 V. The large input coupling capacitor and fast raw power up/down
can do bad things to the transistors BE diode, including driving it to zener
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