Any mathematicians in the house? - Page 1
Forum section: electronics / rf-microwave

author: purpose
author_rank: Frequent Contributor
timestamp: June 24, 2021, 01:54:48 pm
content: Afternoon all,Mathematics is a language that I've never been able to master.I'm trying to find the frequency difference between two 10MHz sine waves, without an accurate counter.One is from my ghetto gpsdo and the other from one of those cheap Chinese pll gpsdos.Triggering on the ghetto, the pll gpsdo is wandering one 50ns division on the scope in 120 seconds.My tiny mind can't wrap my head around where to begin.Any guesses, or formulas welcome.CheersPeter

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 24, 2021, 02:03:21 pm
content: So it's drifting 50ns/120s = 0.42 ppb, or times 10MHz, a difference of 4mHz? Just simple ratios. For such a small difference, it won't much matter which one is being measured, but you'll need to know which one is leading/lagging to find which one is higher/lower.Tim

author: purpose
author_rank: Frequent Contributor
timestamp: June 24, 2021, 02:15:34 pm
content: Quote from: T3sl4co1l on June 24, 2021, 02:03:21 pmTim, many thanks and may you live for a thousand years.4mHz I can live with. Had it been 5, I would have to abandon my project and take up knitting.Which one is leading... A question for another time.CheersPeter

author: radiolistener
author_rank: Super Contributor
timestamp: June 25, 2021, 03:39:22 am
content: Quote from: purpose on June 24, 2021, 01:54:48 pmWhat results do you need?Value in Hz?Or maybe analog voltage which indicate the difference between two sine waves from separate sources? Such way with voltage difference is used for a fine tuning low noise VCXO by a more stable reference clock. In such way you can use less noise oscillator and still keep frequency stability on it with help of stable but more noisy reference

author: radiolistener
author_rank: Super Contributor
timestamp: June 25, 2021, 03:52:01 am
content: Quote from: purpose on June 24, 2021, 01:54:48 pmIf you multiply two sources in a time domain on DSP it will be equals to a frequency shift in a frequency domain.For example:cos(2πf1t) ∙ cos(2πf2t) = 0.5 cos[2π(f2− f1)t] + 0.5 cos[2π(f2+ f1)t]wheref1 - frequency of source 1f2 - frequency of source 2you can use it to get the difference between two source.The main issue here is that we have sum of two components f2-f1 and f2+f1 and they are mirrored from a 0 Hz, because there is no negative frequency in real world (no way to see the direction of vector rotation in quadrature if you have just a single projection on the axis) and negative frequencies are folded on positive.This is why you can listen mirror channels on the receiver. But you can avoid such frequency mirrors agains 0 Hz border if you use multiplication for a signals represented in a quadrature because it allows to keep information about negative frequencies.Read this article for details:https://www.dsprelated.com/showarticle/192.php

author: sarbog
author_rank: Contributor
timestamp: June 25, 2021, 04:18:36 am
content: From Trigonometry if you multiply two sine waves together you will get the sum and difference frequency sine waves and the original but they are all the same. What are you trying to do?

author: purpose
author_rank: Frequent Contributor
timestamp: June 25, 2021, 11:30:35 am
content: Quote from: radiolistener on June 25, 2021, 03:39:22 amMr. Radiolistener,I wanted to know how frequency accurate myghettogpsdo (without expensive OCXO) was compared to something commercially available. Both sine waves were input to the scope and observed.It turns out that 4mHz was the answer, even though clear skies (with two different antennas) improved the result to an almost perfect lock between the two.As for using one to tune the other.... You're getting into the realms of making me use my brain. I haven't done that in years.Many thanksPeter

author: purpose
author_rank: Frequent Contributor
timestamp: June 25, 2021, 11:35:26 am
content: Quote from: radiolistener on June 25, 2021, 03:52:01 amSir,The title of this thread precludes me from even looking at that.I know I asked for it, but you really didn't have to.ConfusedPeter

author: purpose
author_rank: Frequent Contributor
timestamp: June 25, 2021, 11:41:31 am
content: Quote from: sarbog on June 25, 2021, 04:18:36 amMr. Sarbog,I would also like to avoid any kind of arithmetic.I'm just trying to make a gpsdo without spending a fortune. Turns out that by spending a fortune, you can make something cheap.CheersPeter

author: ejeffrey
author_rank: Super Contributor
timestamp: June 25, 2021, 02:46:07 pm
content: Make sure to observe it over time. Very long term two correctly functioning gpsdo should ideally have zero frequency offset. They may wander quite a bit over shorter periods in either direction. You want to make sure your 0.4 ppb is representative

author: purpose
author_rank: Frequent Contributor
timestamp: June 25, 2021, 03:50:32 pm
content: Quote from: ejeffrey on June 25, 2021, 02:46:07 pmThanks, Mr. ejeffrey.From my observations so far, the clearer the sky, the less drift. Still minor when overcast.I'm guessing that the pll gpsdo (with an ocxo from 2009) is the drifter, but I merely wanted to verify the accuracy of my ghetto contraption.The ghetto has a LEA M8T gnss timing chip and the pll gpsdo purportedly has a NEO M7N. Both gnss capable, but getting info from the pll gpsdo does not appear to be straightforward.CheersPeterP.S. Right now it is taking about five minutes to transition 50ns.

author: Miti
author_rank: Super Contributor
timestamp: June 26, 2021, 09:26:01 pm
content: If they drift relative to each other, you have one or two GPS and O but you’re missing the D in between the two.

author: purpose
author_rank: Frequent Contributor
timestamp: June 26, 2021, 10:23:46 pm
content: The D fell into the carpet and disappeared.

author: purpose
author_rank: Frequent Contributor
timestamp: June 26, 2021, 11:06:55 pm
content: How about 2 ghettos...Slight movement to and fro (roughly 15ns either side), but not continuously in one direction as with the pll.Had to invert one waveform.

author: Miti
author_rank: Super Contributor
timestamp: June 27, 2021, 01:09:54 am
content: Now you found the D in the GPSDO...

author: purpose
author_rank: Frequent Contributor
timestamp: June 27, 2021, 01:19:42 am
content: Quote from: Miti on June 27, 2021, 01:09:54 amI did... It fell into the dog.