100 Mhz reference unlocks

    On 25nov02 peter backus noticed that the seti test tone was drifting. Since the drift rate did not match that of the earth's rotation (and it was monday) he decided it was probably a problem with the LO system rather than ET. On 28nov02 we found that the AO 100 MHz reference was drifting. We temporarily replaced it with a synthesizer locked to the 10 Mhz.

    The 100 MHz reference is generated using a PLL driven from the 5 Mhz. Ganesh pointed out that most VCO's drift with time and all that was needed was to tune the oscillator.  The loop bandwidth  was about 60 Hz. We adjusted the  VCO putting the 5 Mhz at the center of the loop bandwidth.  After this, the system locked up  ok.

    The question is when did the 100 Mhz unlock? A time line of the events are:

20nov02: sband radar ranging to titan with echo --> it's locked
22nov02: vlba holography run. Need to see if they get fringes.
23nov02: sband radar run with narrow echo but 50 Hhz from the ephemeris prediction.
25nov02: peter finds the system unlocked
28nov02: 17:00 replace pll with synthesizer.
02dec02: take data to characterize drift, then retune vco.

        The 23nov02 echo probably means that it was locked on the 23nov02.

    I took some data on 02dec02 before we adjusted the vco. I let the system warm up for about 1 hour before running. A test tone was transmitted from the birdie maker (in the bowl) at 2380.005 Mhz. This was mixed to base band and sampled at 100 Khz bandwidth for 60 seconds. The plots show the wandering of the tone at sband:

• Fig 1 top. Transforming the entire 60 second period at 1 shot. The average offset is about -126.5 hz with a motion of about 10 Hz in 60 seconds.
• Fig 1 bottom. I broke the data set up into 60 1 second records and transformed each second separately. I then fit a gaussian to find the center frequency of each second. The offsets from the expected frequency are plotted. There is a cycle that lasts for about 3 seconds.
• Fig 2 top. This is the voltage time series for 12 seconds. In software i mixed the average frequency of the tone to base band. You can see that the signal is being chirped (swept in frequency)  during this 3 second period.
• Fig 2 middle. To resolve the frequency chirp, i broke the data set up into .2 second records, transformed each record, fit to the peaks, and the plotted the time versus peak frequency offset from the true frequency.
• Fig 3 bottom. To look at the fast chirp region (red line in top plot) I  broke the data into 5 millisecond records, transformed each record, and the fit for the peak. The plot covers .2 seconds of time and shows the frequency offset (in Khz) from the expected frequency.  The VCO is sweeping across 15 Khz in about 60 milliseconds. The middle plot is reversed in frequency because of under sampling in the time domain.

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