[time-nuts] Sound Card Spectrum Analyzer
lists at cq.nu
Fri Feb 19 12:30:16 UTC 2010
Don't forget that the output impedance of the mixer may also be changing with changes in input level.
Lots of small things to check to make sure you have it all set up right.
On Feb 19, 2010, at 6:56 AM, Bruce Griffiths wrote:
> Gerhard Hoffmann wrote:
>> 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.
> Not necessarily, it depends on the phase detector circuit details.
> What phase detector output termination network did you use?
> Aren't BF862's somewhat noisier than 2SK369's at low frequencies?
>> 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
>> for milliseconds.
> Easily fixed with a slight change to the circuit and a few diodes.
> It pays to simulate powering up circuits before building to many as its easier to fix such problems at that stage.
> Some low phase noise RF amplifier designs can even latch in a high current state if one isnt careful to fix such power up transient problems.
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