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bg7tbl gpsdo master reference
Electro Fan:
--- Quote from: MIS42N on June 23, 2024, 02:06:36 am ---
--- 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?
--- End quote ---
Thanks for all the info, it makes sense, mostly. The only thing I'm wrestling with is why we would care much about phase offset if two signals are more than 1 Hz apart (other than possibly to observe drift such as jitter)? I can see the value in examining phase offset if two signals are less than 1 Hz apart. Thx again.
Electro Fan:
--- Quote from: Electro Fan on June 24, 2024, 03:43:14 am ---
--- Quote from: MIS42N on June 23, 2024, 02:06:36 am ---
--- 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?
--- End quote ---
Thanks for all the info, it makes sense, mostly. The only thing I'm wrestling with is why we would care much about phase offset if two signals are more than 1 Hz apart (other than possibly to observe drift such as jitter)? I can see the value in examining phase offset if two signals are less than 1 Hz apart. Thx again.
--- End quote ---
Hi MIS42N and Ringmodulator,
I found the info below. It's all helpful but the first link* especially answers my question(s) about how phase can be used to tune more than just within one Hz. This is a very cool technique using a very cool device. Looks like next step is to use a tinyPFA and TimeLab to try to do some ADEV measurements.
Thanks again for the guidance.
---
For anyone wanting to find a door deeper into the rabbit hole, here are some breadcrumbs :)
*
https://www.tinydevices.org/wiki/pmwiki.php?n=TinyPFA.Homepage
https://www.miles.io/timelab/beta.htm
G8UUG:
Hi the valule is 1K ohm, any idear wht the GSM modual is?
KR
Andrew
G8UUG:
Hi I have a BG7TBL 2019-07-05 the GPS unit has faled, does anyone know which GPS unit is used in this module. Mine is blank, Pin 1 seems to be in use, many U-BLOX data sheets say this is pin is unused or N/A
KR
Andrew Lenton
G8UUG
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