Re: [IRCA] Selecting XTALs for synchros

[Rick Kunath <k9ao@xxxxxxxxxxx>]

On Saturday 15 July 2006 03:58 pm, Dan Strassberg wrote:
> Charles A Taylor wrote:
>
> You simply must know that 99.9% of AM transmitters have a frequency trimmer
> associated with the crystal.
>
> ------
> Yes, but the existence of a trimmer is not 100% relevant to the discussion.
> Even if you have to trim the frequencies of the two crystals to get the
> frequencies within, say, 0.5 or 0.1 Hz of each other, if they don't drift
> in the same direction at approximately equal rates as a function of time,
> temperature, and any other relevant parameter (oscillator power-supply
> voltage, for example), the frequencies soon enough won't match well.
>

Yes exactly.

You've got two really good choices for frequency synchronization. 

Old way, grab an Efratom atomic rubidium frequency standard, and net the two 
standards together for a while. A week is long enough. Then move them to the 
transmitter sites. I have a pair of these I use for ham radio applications, 
and 9 more of them I have in use out in the field on a trunked FM simulcast 
radio system (Motorola SmartNet II.) They have been bulletproof over the 
years.

New way, grab a GPS based frequency standard and install it, after netting the 
two units together. Much better long term accuracy. Pretty much install it 
and forget it.

>
> I would also say that if the transmitters aren't the same model, you might
> be well advised to retrofit one of them with an oscillator circuit
> identical in every respect to that of the other. Also, unless the
> oscillator circuit's power-supply-rejection ratio is really good, provide
> local regulation for both oscillators. 
> --

Yep, differing equipment design is a definite hurdle to overcome, but not 
impossible. It would be the weak link I would think.

> If you can't ensure that the thermal 
> environment for the oscillators in the different-model transmitters is
> comparable, you probably should also pay the extra cost of
> temperature-controlled oscillators.

Not required with the Efratom or GPS based equipment.

>
> This is a classic analog-circuit problem that AFAIK has to be solved with
> rather tedious old-fashioned technques. I don't think there are any
> shortcuts. Well, maybe use a clock received from a satellite to drive local
> direct-digital-synthesis-based frequency dividers. I suspect that kind of
> high-tech approach has problems of its own that I have never thought about.
> But if you don't know about the problems, it sounds great;>)
>

Actually it's pretty reliable if done well. I have 144 transmitters that use a 
system like this. While I have had PA issues with these now and then, the 
occasional other issue, in 6 years I have never had the synthesizer portion 
fail, though I have had one Efratom unit die due to a direct lightning hit a 
few years back. 

Rick Kunath
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