PFM5 Spectrometer Microphonics Analysis -- Update

OBSID 300122DD, 2007-02-22 (PFM5)

OBSID 300122DE, 2007-02-22 (PFM5)

OBSID 300122DF, 2007-02-22 (PFM5)

OBSID 300122E0, 2007-02-22 (PFM5)

OBSID 300122E1, 2007-02-22 (PFM5)

OBSID 300122E2, 2007-02-22 (PFM5)

OBSID 300122E3, 2007-02-22 (PFM5)

OBSID 300122E4, 2007-02-22 (PFM5)

OBSID 300122E5, 2007-02-22 (PFM5)

OBSID 300122E6, 2007-02-22 (PFM5)

Summary

This is an incremental update to the posting at this site.
Microphonic features repeat rather well as the bias amplitude is changed, with fixed frequency. The microphonic response lowers as the resistance lowers.
As noted before, many microphonic spikes are not confirmed in an alternate bias frequency setting.
A reasonable explanation for this is that the microphonics are located in the neighborhood of higher harmonics of the bias frequency (3rd, 5th, 7th, ...), since the demodulation is square-wave.
The low-numbered SS channels have the most microphonics, followed by the high-numbers SS channels, followed by the SL channels.

1. Data

Now, I'm using noise spectra from Kevin Xu which have been corrected for temperature drift. They are divided up into four bias settings, and I keep them separated in the analysis. The temperature drift correction reduced the 1/f noise in some cases (spot check only).

2. Observational Results

After further consideration, I have added the "first harmonic" assumption in the following statement. In the first set of plots, each noise spectrum for each bias frequency has been plotted twice: one curve as if all of the noise comes from the lower sideband of the first harmonic of the bias frequency (f = f_bias - IF), and the other as if all of the noise comes from the upper sideband of the first harmonic of the bias frequency (f = f_bias + IF).

Here is a directory with all of observed noise spectra.

The individual plots are now four-frame movies, corresponding to the four bias settings.

3. Still Trying to Find the Microphonic Spikes

This next group of plots shows the assumed lower sideband/first harmonic spectra multiplied by the upper sideband/first harmonic spectra. Remember that this multiplication has a goal of maximizing features which repeat between the sidebands.

Here is the directory with those plots.

Here are the 24-channel median plots:

Since the microphonic features appear to be highly repeatable at fixed bias frequency, but they don't overlap in the sideband/first harmonic plots, I'm starting to think that many of the microphonics must be at higher harmonics of the bias frequency. According to the SPIRE Signal Chain Pipeline document, the pre-demodulation bandwidth is large (about 20-4000 Hz), and the demodulation is with a square wave. Therefore, there is signficant sensitivity to noise near third, fifth, seventh, ... harmonics of the demodulation frequency (= bias frequency), with relative response of 1/3, 1/5, 1/7, ....

Unfortunately, the claim that many of the microphonic spikes lie near higher harmonics of the bias frequency cannot be tested with the present data because the sidebands do not have sufficient overlap. For example, here is a plot of the data for one channel assuming the noise is in the lower and upper sideband of the third harmonic of the bias frequency (f = 3 f_bias +- IF): channel SS_A4, 3rd harmonic. More closely spaced bias frequency settings are needed to test this. For the same reason, it is also difficult to predict what the microphonic IF spectra will look like for frequency settings in between the ones already measured.

CDD, 2007 Sep 14