[time-nuts] 10MHz standard for comms receivers

Dana Whitlow k8yumdoober at gmail.com
Mon Oct 1 13:44:09 EDT 2018


Isn't it the case that computer clocks these days are subject to two
influences that make them worthless for timing?

1. Deliberate random FM to spectrally spread RFI leakage.

and

2, Wild variations of clock speed according to usage needs of the moment,
    in order to reduce average power consumption and thermal loading.

Dana


On Mon, Oct 1, 2018 at 12:37 PM jimlux <jimlux at earthlink.net> wrote:

> 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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