[time-nuts] 10MHz standard for comms receivers

jimlux jimlux at earthlink.net
Mon Oct 1 13:36:26 EDT 2018


On 10/1/18 9:43 AM, Bob Betts wrote:
> Hi All. Over the years, I have experimented with WWVB and GPS and Rubidium timing to establish a 10MHz Standard for LO sync in communications receivers. My career has kept me away from the hobby for some time and, frankly, I am not very well versed in some of the timing techniques that I read here...actually I'm just a hacker. Anyhow the thought occurred to me that with some of these newer computer clock timing systems, that a (for example) a 3GHz clock could be divided down by 300 to obtain a fairly tight 10MHz reference. It does have an effect on SDR pgms, but I'm curious about analog systems as well. It would seem (to me) that whatever error there may be at 3GHz, that it would also decrease to a tighter tolerance during the "divide by" process.


ppm scales with frequency - a 1 GHz oscillator with 1ppm will divide by 
100 to 10 MHz with 1ppm.


Where it gets interesting is with phase noise.  Assuming no other 
sources of noise (not generally valid, but a good starting place):

If you multiply or divide a source by N, the phase noise (at a given 
offset from the carrier) changes by 20 log N.  So if phase noise at 1 
GHz is -80dBc at 10kHz away, if you divide by 100, the phase noise will 
be -120dBc at 10kHz away.

In general, the quietest sources (in terms of phase noise) are crystal 
oscillators in the 5-10 MHz range - to a first approximation, it's 
because the physical "rock" is big.

So, it's more likely you'd get a quieter 1 GHz source by multiplying 
10MHz up, than the other way around.

That's for close in noise...(within some kHz of the carrier)  For far 
out noise, that's not necessarily the case.  You might find that the 
microwave oscillator has better noise at 1 Mhz out than the 10 MHz 
multiplied up (and degraded by 40 dB).  Maybe, maybe not - you need to 
look at your particular oscillator.

In general, the wider the tuning range, the worse the noise.


I work a lot at 8.4 GHz - taking something like a HMC506, the phase 
noise is -50dBc at 1kHz, -105dBc at 100khz and -125 dBc at 1 MHz

If I lock that to a crystal at 10 MHz.. let's take an inexpensive tiny 
OCXO with good but not special noise..
-160 dBc at 1kHz, -165 at 10kHz, and probably the same out to 1 MHz.

I'm going to need to multiply up by 845. so 20logN is 58 dB

At 1kHz, the multiplied up crystal is -102dBc, which is a lot better 
than my VCO at -50.
At 100kHz, though, my crystal is -107dBc, and my VCO is at -105 -so 
that's pretty much a wash.

At 1 MHz out, my crystal is still -107, but my VCO is down at -125, so 
it's quieter.

If I were building a synthesizer, I'd probably make the loop bandwidth 
right around 100kHz.



> Okay, so I've been an RF and Audio products designer for 55 years and really (probably) shouldn't venture into territory unfamiliar to me. But this part is a hobby...so how's a guy supposed to learn.
> 
> Will someone please explain the errors in my thinking? it would be appreciated.
> Bob, N1KPR http://www.bobsamerica.com  http://www.youtube.com/n1kpr
> 
> Engineering: Where Enigma meets Paradox
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