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.
Gerhard
Bruce
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