[time-nuts] Sound Card Spectrum Analyzer

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Feb 19 11:16:52 UTC 2010

Gerhard Hoffmann wrote:
> Bob Camp wrote:
>> Hi
>> Ok, A bit more info:
>> 1) Quadrature PLL using an RPD-1 DBM and a home brew lock box. 2) 
>> Willingness to accept that I'm measuring a pair of oscillators
>> 3) Plenty of sources at the appropriate frequencies
>> 4) First took a shot at this in 1975 (I forget the Fluke app note 
>> number ...) been doing it ever since
>> 5) Appropriate preamp between the RPD and sound card is a work in 
>> progress
> I have built this preamp for this purpose:
> <http://www.hoffmann-hochfrequenz.de/downloads/preliminary_ssm2210.zip>
> Gerhard
Before deciding on a suitable preamp one should first measure the phase 
detector/mixer gain, noise noise and low frequency output impedance.
The phase detector noise noise and gain depend on the phase detector 
output port termination network.

For the common case of a diode ring phase detector  where both inputs 
are saturated the output beat frequency waveform (and the phase detector 
output as a function of the phase difference between its input signals) 
for low beat frequencies will only be triangular if the phase detector 
output port termination impedance is low at the beat frequency and the 
sum frequency.

If the termination network has a high impedance for low frequencies 
(<100kHz) and a low impedance for the sum frequency then the beat 
frequency output waveform will be approximately trapezoidal with rounded 

The range of impedances that can be considered low depends on the turns 
ratio of the transformers used in the mixer/phase detector.
The mixer output impedance also depends on the transformer turns ratio.

The phase detector output noise will be lower if the sum frequency is 
reflected back into the mixer than when the mixer is terminated in the 
specified resistive load.

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.

The only real requirement is that the preamp input noise (including the 
preamp input voltage noise and the noise voltage due to the preamp input 
noise current flowing in the phase detector output impedance ) spectral 
density should be significantly less (preferably at least 3x) than the 
phase detector output noise for all frequencies of interest.


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