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bg7tbl gpsdo master reference

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MIS42N:

--- Quote from: Electro Fan on June 21, 2024, 02:07:42 am ---Update - as I'm writing this the Amazon OCXO is now remaining stable to 100 150 200 1215 10 second gates, so maybe it just needed longer to warm up and/or I need to keep the AC off. :)

--- End quote ---
If you are relying on a local crystal oscillator it needs plenty of time to settle down to the sort of stability needed to compare with a GPSDO. I've watched them improve markedly over a week. I've played with a lot of OSC5A2B02 second hand and they are not really stable for days. The design of my GPSDO (a different thread) is different from all others in that I allow the OCXO to free run for a period then apply a fixed change to the control voltage. It is a way to determine if the oscillator is running OK as it is comparing but not adjusting. The results below are a log of a OSC5A2B02 that has been powered and not moved for months. Adjustments are made at 1024 second intervals (about 17 minutes). The absolute accuracy is unknown, but an upper estimate can be made by the size of the adjustments. The results indicate it is running better than 10MHz ± 1mHz (the largest change is 0.036 parts per billion or 0.36mHz).

 Time 012123 UTC. Ctrl 1.641806 0.026 ppb
 Time 013827 UTC. Ctrl 1.641828 0.017 ppb
 Time 015531 UTC. Ctrl 1.641834 0.005 ppb
 Time 021235 UTC. Ctrl 1.641847 0.010 ppb
 Time 022939 UTC. Ctrl 1.641836 -0.008 ppb
 Time 024643 UTC. Ctrl 1.641812 -0.020 ppb
 Time 030347 UTC. Ctrl 1.641800 -0.009 ppb
 Time 032051 UTC. Ctrl 1.641785 -0.012 ppb
 Time 033755 UTC. Ctrl 1.641804 0.015 ppb
 Time 035459 UTC. Ctrl 1.641805 0.001 ppb
 Time 041203 UTC. Ctrl 1.641804 0.001 ppb
 Time 042907 UTC. Ctrl 1.641833 0.023 ppb
 Time 044611 UTC. Ctrl 1.641826 -0.006 ppb
 Time 050315 UTC. Ctrl 1.641805 -0.017 ppb
 Time 052019 UTC. Ctrl 1.641792 -0.010 ppb
 Time 053723 UTC. Ctrl 1.641765 -0.021 ppb
 Time 055427 UTC. Ctrl 1.641760 -0.004 ppb
 Time 061131 UTC. Ctrl 1.641785 0.020 ppb
 Time 062835 UTC. Ctrl 1.641790 0.005 ppb
 Time 064539 UTC. Ctrl 1.641779 -0.009 ppb
 Time 070243 UTC. Ctrl 1.641762 -0.013 ppb
 Time 071947 UTC. Ctrl 1.641752 -0.008 ppb
 Time 073651 UTC. Ctrl 1.641797 0.036 ppb
 Time 075355 UTC. Ctrl 1.641769 -0.022 ppb

Ringmodulator:
Hi  Electro Fan,

the topic of timing, accuracy and stability is a much wider and deeper field, as it looks from the ouside (the rabbithole of time nuttery).
And I am in no means an expert in this.

If you feed the couter input with its 10 MHz outut, you see the "hard limit" of the counter. Your results can never be better and it does not mean, that your results can be as good.
If you feed the counter with a signal form a generator and both are synced to the same soure, imho there is not much to gain from that.

If you want to evaluate an oscillater, you need another one which is at least a magnitude better.

The BG7TBL GPSDOs are rumored to have an inherent error in the range of about 2 mHz. It is not clear to me which models are affected.
I have a BG7TBL and a Samsung gpsdo and I have found this to be the case for my unit, but I have not done extended tests.

I suggest to check out the tinyPFA Phase Frequency Analyzer:
https://www.tinydevices.org/wiki/pmwiki.php?n=TinyPFA.Homepage

I got a used NanoVNA-H4, which can be flashed with the TinyPFA firmware.
I found this to be a very interesting low cost device to experiment with GPSDO, OCXO and counters.

The FA-2/FA-3 and the TinyPFA do work with the Timelab software.
This can keep you busy for quite some time :)

Regards
Chris


Electro Fan:

--- Quote from: Ringmodulator on June 21, 2024, 05:40:21 pm ---Hi  Electro Fan,

the topic of timing, accuracy and stability is a much wider and deeper field, as it looks from the ouside (the rabbithole of time nuttery).
And I am in no means an expert in this.

If you feed the couter input with its 10 MHz outut, you see the "hard limit" of the counter. Your results can never be better and it does not mean, that your results can be as good.
If you feed the counter with a signal form a generator and both are synced to the same soure, imho there is not much to gain from that.

If you want to evaluate an oscillater, you need another one which is at least a magnitude better.

The BG7TBL GPSDOs are rumored to have an inherent error in the range of about 2 mHz. It is not clear to me which models are affected.
I have a BG7TBL and a Samsung gpsdo and I have found this to be the case for my unit, but I have not done extended tests.

I suggest to check out the tinyPFA Phase Frequency Analyzer:
https://www.tinydevices.org/wiki/pmwiki.php?n=TinyPFA.Homepage

I got a used NanoVNA-H4, which can be flashed with the TinyPFA firmware.
I found this to be a very interesting low cost device to experiment with GPSDO, OCXO and counters.

The FA-2/FA-3 and the TinyPFA do work with the Timelab software.
This can keep you busy for quite some time :)

Regards
Chris

--- End quote ---

Hi Ringmodulator, 

So, if a person had one of these:

https://www2.randl.com/index.php?main_page=product_info&cPath=16_16990&products_id=75961

Theoretically it would enable such a person to determine which of his devices (GPSDOs, generators, counters, etc.) was the most accurate with respect to frequency measurement?  (In other words, you think the TinyPFA is more accurate than a BG7TBL GPSDO and the FA-2?)  Or the TinyPFA would just provide more data (regarding phase relationships, etc.) for the rabbit hole?  :)  Thx

Edit:  It looks like the answer is the TinyPFA isn't going to do much to discern frequency accuracy....
"ATTENTION: The tinyPFA does not contain an internal high stability reference. It will only perform a high accuracy phase difference measurement between two input clocks."

PS, Also, just to confirm, you think a BG7TBL GPSDO might be accurate to ~2 mHz? 

PSS, "If you feed the couter input with its 10 MHz outut, you see the "hard limit" of the counter. Your results can never be better and it does not mean, that your results can be as good."
- These are fascinating ideas. :)  :-+ :-+
Thx again!

Electro Fan:
Ringmodulator,

Here is the BG7TBL FA-2 Counter with the internal Ref output feeding the Channel 1 input.

"If you feed the couter input with its 10 MHz outut, you see the "hard limit" of the counter. Your results can never be better and it does not mean, that your results can be as good."

- if I understood this^ correctly, it means that the hard limit of this particular FA-2 is ~0.000,015,6 Hz (or given the Min and Max shown for the Pk-Pk variation, the unit could be approximately +/- 8 or 9 micro Hz (on the hard limit).  To your full point, it can never be better, and it does not mean that it as good as this - but it is a datapoint.  Yes?  If so, it seems plausible that the FA-2 can do better than 1 mHz, and likewise I'm pretty sure the BG7TBL GPSDO does better than 1 mHz (just my guess).  If this happened to be correct, we'd just be down to whether they are truly in sync including not only frequency but also phase.  Assuming they might be close but not identical on frequency, why would we potentially care about some amount of phase offset?  It seems like we would only care about phase offset if we were within some specified amount of frequency, ie to some Hz or some fraction of a Hz.... yes/no?  Thanks

MIS42N:

--- Quote from: Electro Fan on June 22, 2024, 07:39:29 pm --- If this happened to be correct, we'd just be down to whether they are truly in sync including not only frequency but also phase.  Assuming they might be close but not identical on frequency, why would we potentially care about some amount of phase offset?  It seems like we would only care about phase offset if we were within some specified amount of frequency, ie to some Hz or some fraction of a Hz.... yes/no?  Thanks

--- End quote ---
When two sources identical in frequency are compared, there is a constant phase offset (which may be zero, in which case they are in sync). If they are not identical in frequency, then the phase offset changes at the rate of the difference in frequency. If the two frequencies are wide apart, say a difference of 100Hz, then general terminology sees the difference as a beat frequency of 100Hz or a heterodyne. If the two frequencies are within a few Hz then the terminology talks about change of phase differences (in radians or degrees). For instance, comparing 10MHz with 10,000,000.1Hz the phase changes 1/10th of a circle or by 36 degrees a second. There's no difference between the fundamentals of the two examples, just the terminology changes.

So when comparing two close frequencies, it is more practical to measure the change in phase offset over some period than by counting cycles. For example if the difference is 10µHz then it will take 100,000 seconds for one signal to have one more cycle than the other. The period of a 10MHz signal is 100ns (nanoseconds) and a 9,999,999.99999Hz signal is 100.0000000001ns. So the difference between say the zero crossing of each signal is changing by 0.0000000001ns per cycle (a ridiculously small amount of time). But over a 1 second period the difference is .001ns - 1ps (one picosecond). This is measurable with a nanoVNA configured as a Phase Frequency Analyzer.

So phase offset changes are of importance when comparing two close frequencies. From the specification of the BG7TBL FA-2 Counter it would appear it uses phase change to measure frequency as it claims a resolution of 0.0001Hz@10MHz with a 1s gate. However, this says nothing about its accuracy. That is determined by whatever reference signal it is using.

Does that make sense?

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