I'm not sure what is the proper forum board to ask this question, but as it relates to signal conditioning of the output from a phase noise detector, I imagine Metrology is as good a place as any.
I have run up against a problem analyzing the output of a phase noise detector using a low-frequency spectrum analyzer (PicoScope 4262). The project I am working on is documented in
this EEVblog thread, which in total is quite long. For those unfamiliar with the project, the 3 posts beginning with
this message summarizes the approach using a HP11729C in Frequency Discriminator mode. While I am now trying to use the HP11729C in Phase Detector mode, the problem I am attempting to solve is applicable to both configurations.
Specifically, this problem is the noise floor of the PicoScope 4262 is around -92 dBm, whereas the phase noise information in the output of the HP11729C is below that. For example, I was trying to analyze the phase noise of a Bliley NV47A1282, which has phase noise specs: (10Hz: -120 dBc/Hz ; 100 Hz: -130 dBc/Hz; 1KHz: -140 dBc/Hz). For the given oscillator carrier signal level, all of these values are below -92 dBm. (It is a bit more complicated than this, but the power spectrum information in the HP11729C output, which requires correction to get the phase noise spectrum, is also below the PicoScope 4262 noise floor.)
This suggests using an LNA with suitable bandwidth to amplify the HP11729C signal before processing it with the PicoScope. The problem is I am having trouble finding an LNA suitable for this purpose. Specifically, I need an LNA with bandwidth 1Hz - 100KHz and gain around 30-40 dB. Actually, I need one with 60-80 dB gain. However, I could gang two LNAs with the lower gain together to obtain what I need. Furthermore, the LNA must amplify very low signal levels, as low as 10 nV
RMS.
I have searched extensively for such an amplifier. Most LNAs I have found have bandwidth in the 100 KHz to GHz range. An exception are audio LNAs, but the ones I have found generally have bandwidth 20Hz-20KHz. This would eliminate the low offset frequencies that are of significant interest in phase noise analysis. Furthermore, the audio LNAs I have found are not intended for the low signal levels that the application must handle.
I did find one possible device, the
AlphaLab Oscilloscope Preamplifier LNA10, that might work. It is claimed that the gain of this device is up to 1000X (60dB
- clarification on 8-3-2019: 60 dB voltage gain; 30 dB power gain Nope - dumb error. As long as input and output loads are equal, voltage and power gain are equal), but the device's noise spec is 4 nV/sqrt(Hz), which means it is probably useful in this application at low offset frequencies (say 1Hz-100Hz), but above that range may introduce significant noise that will mask the phase noise in the signal.
If anyone has experience with this device or has suggestions for other devices that might work, I would appreciate their input.