[time-nuts] OCXO Voltage Input?

[Bob Camp]

Hi

By far the most common way to check low level stuff on an OCXO is to measure it’s phase noise. There are a variety of approaches. The lowest cost approach is usually a dual oscillator into a mixer / quadrature lock. Feed the output to a preamp and then into some sort of audio spectrum analyzer. 

If you truly want to look at FM as FM, then you can multiply the output up to microwaves and look at it there. After multiplication to 10 GHz, your 20 Hz on 10 MHz is going to look like 20 KHz. 

If you want to stay off the shelf, then something like a TimePod will do a fine job of plotting low level low modulation rate stuff. It will run fast enough that you can follow the modulation over many decades. 

For higher rates, some of the HP modulation analyzers are quiet enough to properly display 10 or 20 Hz deviation FM. If you are below that level, a simple 5X frequency multiplier will get you running with one. 

Yes there are likely another two or three dozen ways to do it. Those four are the most common. 

Bob

On Sep 5, 2014, at 10:54 PM, dan at irtelemetrics.com wrote:

> Hi Bob 
> 
> You have some good observations. Spread spectrum clocking is one I hadn't considered when looking at this problem. In that case the crystal is pulled a bunch. (It's also cheating in my opinion!)  Correcting for mechanical vibration in aircraft would also tend to indicate it's possible
> 
> In the schematic for the 10544 that Tom posted the link to, it appears that when both of the EFC lines are available, only two 20K resistors are in series with the varactor. One would guess that changes on the EFC line could quite easily modulate that varactor. Assuming a similar scheme in other oscillators, one would think that modulation could quite easily happen there also. 
> I supposed that the manual telling the operator to avoid noise on the EFC line, due to FM modulation happening, supports this theory as well! (Big Exclamation point there!) 
> 
> Once the question is asked, you have to ask how does one measure such a modulation? At least with simple equipment easily available. Some searching didn't result in anything promising, at least for what I have access to. Initially my thoughts are because the varactor is acting on the crystal to change frequency the assumption is the modulation is FM (Again, the HP manual backs this up). The specified peak to peak deviation is only +/- 20Hz at most. No matter what the modulating frequency is the FM modulation index is virtually zero, so there are no side bands to look for! If they are there, they are very close together. With a lack of FM side bands, one would postulate the low deviation modulation is going to look like just like phase noise. Obviously very hard to measure without a lot of good equipment. 
> 
> Is there a way to tease the data out, to at least get a frequency response plot by disturbing the EFC line with a signal generator? Maybe for low frequencies, a TIC and known reference could do it. It's something I'd like to test, but fear it requires more equipment than is easily available. 
> 
> The other thought which you brought up is spread spectrum. If spread spectrum is taken as an example, the amplitude of the 10Mhz may change with modulation on a spectrum analyzer. It's an easy enough test to try. 
> 
> The bottom line is, at this point, the examples on line and provided here point towards the fact modulation can happen. Given this background information it only makes sense to keep the EFC line as clean as possible. 
> More reading is necessary to understand what the implications are. Any other input regarding real numbers, or actual testing is very welcome!
> 
> Dan
> 
>  
>   
> 
>> 
>> Some VCXOs actually specify their bandwidth.  High audio is sometimes useful.  I haven't seen anything beyond that, but I'm just listening to discussions like this one.  There could well be applications that use a higher frequency. 
>> 
>> One application is correcting for mechanical vibrations.  This is interesting in radar used on helicopters.  (They do Doppler filtering to remove clutter.  The lower speed of objects that can get through the filter depends on the clock stability.)
>>  PCs often FM modulate their clocks.  It's a hack to get past the FCC EMI requirements.  It spreads a spike in the frequency domain into a blob with a lower peak.  I think 30 KHz is typical.  The PCI specs were tweaked to allow this so they probably say something about the legal frequency limit. 
>> PCs probably don't use expensive OCXOs, but that technology might get used in other applications. 
>> 
>> How do FM modulators work?
> 
> 
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