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A Look at OSA 8607 BVA Frequency Drift

09-May-2005 data
01-Sep-2007 analysis

Run Description

Below are plots from a long-term measurement of an Oscilloquartz 8607/008 BVA quartz oscillator. Let's play with Stable32 and see what the frequency drift rate is.

Raw Data

C:\tvb\tsc036\phm-8607> dir/os | tail
LOG186~1 GZ      2,799,341  05-09-05  5:47a log18676.txt.gz
C:\tvb\tsc036\phm-8607> gzip -d < log18676.txt.gz | wc
7.333 MB, 7509 KB, 7689048 chars, 548573 words, 548573 lines
C:\tvb\tsc036\phm-8607> dir/od | more
LOG18~12 GIF        23,900  05-06-05 11:00a log18689v.gif
LOG18~13 GIF        21,739  05-06-05 11:02a log18690v.gif
LOG18~14 GIF        14,245  05-06-05 11:03a log18691v.gif
LOG18~15 GIF        35,136  05-06-05 11:04a log18692v.gif
LOG186~1 GZ      2,799,341  05-09-05  5:47a log18676.txt.gz
LOG18~16 GIF        23,980  05-10-05  1:02a log18698v.gif
LOG18~17 GIF        22,299  05-10-05  1:03a log18699v.gif
LOG187~1 GIF        14,228  05-10-05  1:03a log18700v.gif
LOG187~2 GIF        35,978  05-10-05  1:04a log18701v.gif

TSC Plots

Below is a set of TSC 5110A snapshots taken near the end of the 6-day run.

OSA 8607 vs. PHM out to tau 100k

More TSC Plots

Below a few more 10-minute duration frequency-deviation strip-chart plots for reference. With standard decade scales, these can be used for noise comparison with other frequency standards.

Typical frequency plot re-scaled to 110-12/div, 110-11/div, and 110-10/div, respectively.

Stable32 Plots (phase)

In addition to creating real-time ADEV plots and most-recent--ten-minute phase and frequency strip charts, the TSC 5110A outputs phase data at a 1 Hz rate. This data is logged to a PC and can then be analyzed off-line by tools such as Stable32. Phase differences are in units of period (200 ns in this case) which can be converted into seconds or ns as needed.

Phase, raw

Notes: open log file as phase, with tau 1 s, with scale by 2e-7 (= 200 ns = 5 MHz); plot

  • This is a boring plot, but at least shows no obvious glitches.
  • Note the large accumulating time offset due to OCXO being off-frequency; about 2 ms time drift over the 6+ day run.
  • Note a 110-8 offset in frequency is 10 ns per second offset in time, or almost 1 ms accumulated time error per day.
  • Note also with 500k points these plots take a while to render and it's tempting to average down by 10 or 100. However, ...
  • Averaging hides glitches which may not be a good idea until the data is known good.
  • Averaging also eliminates low tau from stability calculations.

 

Phase, frequency offset removed

Notes: phase slope is frequency offset; calculate and remove frequency offset (linear fit); re-plot

  • Calculated frequency offset is -4.124e-9.
  • With the linear term removed (frequency offset) the data is much clearer.
  • Time drift is limited to about 3 s over the run.
  • The hump shape of the curve indicates frequency drift is present.
  • No glitches visible at this scale either; looks like a clean run.

 

Phase, frequency offset and drift removed

Notes: frequency slope is drift rate; calculate and remove frequency drift (quadratic fit); normalize residual phase; re-plot

  • Calculated frequency drift is -6.984e-17 (change in frequency per second).
  • Multiplying by 86400 makes this -6.034e-12 / day, in conventional drift units.
  • Time drift is confined to 1 s over the run.
  • Note that it makes little difference in calculated coefficient c if a frequency offset is first removed before the quadratic fit.

Stable32 Plots (stability)

Stability, Allan Deviation (tau 1 to 105 s)

Notes: run ADEV from raw phase data (no unfair drift removal at this point); 1-2-4-decade scale; plot

  • Stable32 computes Freq Drift/Day=-6.036e-12

 

Stability, Overlapping & Modified Allan Deviation

Notes: run OADEV & MDEV from raw phase data; re-plot

  • Very little difference between the two and the plain ADEV plot above.

 

Stability, Allan Deviation, drift removed

Notes: run ADEV from raw phase data, selecting remove drift; re-plot

  • Not much difference between this no-drift plot and the normal plot above.
  • This suggests there is some long-term instability that isn't purely linear drift.
  • Suggests long-term performance is not solely due to ageing.
  • A look at a frequency plot will confirm this.

Stable32 Plots (frequency)

Frequency, raw

Notes: conv raw phase to frequency; plot

  • This isn't pretty for a couple of reasons.
  • The y-scale is absolute frequency, hiding relative frequency.
  • But if accuracy were more important than stability this would be a good thing.
  • There is a noticeable amount of noise in the graph (probably occasional maser glitches).
  • For a better frequency plot, remove some short-term noise with averages.

 

Average frequency

Notes: average by 10x (10 s samples); normalize (remove mean frequency); add linear trend line (frequency drift) but do not remove drift; re-plot

  • Much better looking frequency plot.
  • Line fit says drift is -7.226e-16 (per 10 s) = -6.243e-12/day.
  • Short-term frequency noise is small enough that long-term trend is clearly visible.

 

Average frequency, drift removed

Notes: average again by 10x (now 100 s samples); remove frequency drift; normalize; center scale to 1010-12; re-plot

  • This represents best-case potential performance if
  • There were no systematic linear frequency drift over time (ageing).
  • Other random variations are present (it is quartz, after all).

A look into frequency spikes and phase jumps

Phase/Frequency jumps

Notes: open phase, conv phase to freq, use check and stats to look for spikes. Note sample numbers, go back to phase, use stats to zoom in; extract nice 10 minute record using part; [i5500 n600]; use 599 points (not 600); remove slopes and normalize both phase and frequency; plot each

  • This nicely shows the glitches in both frequency and time domain.
  • Frequency spikes are often one of two types.
  • Those with a single spike out of the normal noise, which correspond to a phase jump.
  • And those with a double spike, which corresponds to a bad phase data point or a
  • This example shows one of each.
  • The frequency spikes in this example are about 510-12 in magnitude over a few seconds.
  • The corresponding phase jumps are about 12 ps and 3 ps.
  • Frequency glitches are often one of two types; those with a single spike above or below the line (which corresponds to a one-way phase jump), and those with a double spike, first one direction and then the other, 

Conclusions


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