A Look at OSA 8607 BVA Frequency Drift
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.
- Data collected May 2005
- TSC 5110A (upper lab)
- chA = PHM (upper lab)
- chB = Oscilloquartz (OSA) 8607 (upper lab)
- Plots = C:\tvb\tsc036\phm-8607
- Data = C:\tvb\tsc036\phm-8607\log18676.txt
- About one week of continuous 1 Hz phase difference data
- Both standards have been powered up for months, or maybe years
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
Below is a set of TSC 5110A snapshots taken near the end of the 6-day run.
- Normal-looking quartz "bathtub" plot.
- The BVA is known to be better than this plot shows.
- PHM is probably limiting the measured performance, short-term.
- ADEV plot hits 5.7×10-12 at tau 100k seconds.
- The +1 slope near tau one day suggests drift rate is around 5×10-12/day.
- As usual with quartz, the TSC phase difference plot is uninteresting.
- The frequency difference plot is quite clean; 5×10-13/div;
mostly white noise.
- OSA is way off-frequency (4×10-9), but this isn't a
concern for stability.
|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 1×10-12/div,
1×10-11/div, and 1×10-10/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.
Notes: open log file as phase, with tau 1 s, with scale by 2e-7 (= 200 ns = 5 MHz);
- 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 1×10-8 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);
- 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 µs 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
- Time drift is confined to 1 µs 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
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
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;
- 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
- Suggests long-term performance is not solely due to ageing.
- A look at a frequency plot will confirm this.
Stable32 Plots (frequency)
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
- There is a noticeable amount of noise in the graph (probably occasional
- For a better frequency plot, remove some short-term noise with
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
|Average frequency, drift removed
Notes: average again by 10x (now 100 s samples); remove frequency drift;
normalize; center scale to ±10×10-12; 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
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 5×10-12
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
- One week of data clearly shows this OCXO has a frequency drift rate of
about 6×10-12 per day.
- But because the drift rate is so low (compared to the mid-term stability)
a couple of hours, or even a full day is not sufficient to accurately
determine the drift rate.
- The drift rate may vary hour to hour and day to day. Much like frequency,
what is reported is always the average drift rate.
- During data analysis there are multiple ways to determine the drift rate:
- Take the quadratic fit coefficient, c, and multiply by 86 400.
- Take the slope of the best fit line on a multi-day frequency plot.
- Look at drift rate listed at the bottom of the Stable32 ADEV calculation.
- On an ADEV plot, see the ADEV value for tau 86.4k.
- The passive maser reference is not good enough to find the short-term
noise floor of the BVA, but more than sufficient for mid- and long-term
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