Thanks to everyone who comented on this topic. The contributions contained useful and germane information. Several commentors noted that oscillator phase noise increases significantly as offset frequency approaches 1 Hz and for this reason a good low noise preamp probably would suffice:
(1) Low noise oscillators use low flicker noise transistors for this very reason which suggests medium frequency amplifiers using bipolar transistors will always be sufficient for this application.
Oscillators tend to have more noise in the low frequency band - so some 1/f noise may be acceptable.
The AlphaLab Preamp should provide low noise and enough gain for either situation.
I decided to explore this line of reasoning using the
AlphaLab Oscilloscope Preamplifier LNA10 as an example. I welcome comments on the following argument.
When using the HP11729C in Phase Detector mode, the lowest phase noise device should be the reference oscillator. Its phase noise should be less than either the DUT or the HP11729C itself. The device I am using as a reference oscillator is the
Wenzel HF-ONYX-IV (part 501-11578-04), which has typical phase noise of -135 dBm at 10 Hz (which is less than that of the HP11729C and likely that of the DUT). Since oscillator specs generally quote phase noise as power relative to a 50 ohm load, this equates to 3.976e-8 V
RMS/sqrt(Hz) = 40 nV/sqrt(Hz). The input-referenced noise of the AlphaLab LNA10 is 6 nV/sqrt(Hz).
To obtain the gain I need requires cascading two AlphaLab LNA10s. Since noise adds like power, the noise resulting from two cascaded LNA10s is sqrt(2*(6^2)) = 8.5 nV/sqrt(Hz). So the noise contributed by the cascaded amplifiers is about 20% of the noise contributed by the reference oscillator.
It is desirable that the noise added by the preamp is no more that 10% that of the reference oscillator. Nevertheless, the reference oscillator phase noise should be much less than either the DUT or HP11729C phase noise, so the noise added by the preamp is likely less than 10% of the total system phase noise, which should suffice for my purposes.
From 10-100 Hz, the phase noise of the AlphaLab LNA10 is 4.1nV/sqrt(Hz), meaning in a cascaded configuration the added phase noise would be approximately 5.7nV/sqrt(Hz). This is about 15% of the noise contributed by the reference oscillator. This is still not at the 10% desirable level, but again probably sufficient for my purposes.
Given this analysis, I am leaning toward purchasing two AlphaLab LNA10s and using them as the preamp for the PicoScope 4262. If for some reason this approach doesn't work out, I may then take a closer look at the
EM Electronics A29.