[time-nuts] Spectrum Analyzer Specifications

Bob Camp kb8tq at n1k.org
Wed Mar 23 18:56:50 EDT 2016


Hi

The “spurs and noise” you see plotted on an OCXO are the output of a 
phase noise tester. No spectrum analyzer or SDR out there has enough 
dynamic range to look at the phase noise on a good OCXO. You are dealing
with +10 dbm signals so “sensitivity” is not the issue. The range between 
the signal and the noise with that signal applied is the issue (= dynamic range). 

A 40 year old surplus counter for < $100 from eBay will get you 9 digits of 
frequency in 1 second. It will get you to 10 or 11 digits before you get totally 
bored waiting for the reading. For about 5X that, you can add another digit. 

The counter by it’s self does not have a lot of use. You need some sort of
reference for it. A GPSDO (again I’d recommend surplus commercial gear) 
is the best bang for the buck there. There are a lot of candidates in the $100 
to $200 range. Once you have that, a telecom Rubidium standard can be 
bought for about the same amount of money. 

Before I’d spend $20,000 to $50,000 on a good spectrum analyzer, I’d get the counter
and standard covered. If you want to do spurs, spend $40 and build a phase
noise test set that will drive the sound card on your PC. 

Lots of choices ….

Bob

> On Mar 23, 2016, at 8:45 AM, timenut at metachaos.net wrote:
> 
> Charles,
> 
>>> When using a spectrum analyzer to examine the output of a frequency 
>>> standard, what are the minimum specification needed? Bandwidth, 
>>> resolution, sensitivity, etc?
> 
>> It very much depends on what you hope to see with it.  That said, 
>> virtually no SA will do anything useful in this application without a 
>> fair amount of preprocessing.
> 
>> . . .
> 
>> I am all for having one or several SAs around the lab, but I fear
>> that an SA may not be very useful for the particular tasks you are 
>> hoping it will perform.
> 
> Being new to this, I don't actually know what I need to see with it. However,
> I see charts that show the "spurs" on the output of an oscillator. I assume
> that I will want to do similar tasks. As I understand it, those are the main
> problem when multiplying the frequency by very much because they get
> multiplied as well. Thus, one of the differences between a "good" OCXO and a
> "bad" OCXO.
> 
>> Pay attention to ergonomics -- suffering with an SA that doesn't
>> think like you do will drive you crazy.  For example, I prefer 
>> HP/Agilent SAs over Tek SAs because the HPs allow you to specify a 
>> start and stop frequency *or* a center frequency and span, while the 
>> Teks only allow you to specify a center frequency and span.
> 
> As an example, I currently have a broken MTI 260 which I am repairing. I
> currently have it cleanly disassembled so that it can be reassembled with no
> damage. I know what needs to be done to fix it (at least the first pass) and
> am waiting on a tool that I need. But, if I succeed in repairing it, what
> test equipment do I need to determine the quality of output?
> 
> I can measure frequency to a point (7 digits, uncalibrated - 8 once I have a
> frequency standard). That is not enough to determine very small deviations
> from 10Mhz, but it will tell me that I have a sine wave at approximately the
> correct frequency. The last one I (sort of) repaired, gave me a sine wave but
> with a duty cycle that wasn't exactly 50-50. So there must be some distortion
> present. Not surprising in that case because of the hacking that I did.
> 
> However, I understand that the quality of the OCXOs can vary widely. What
> tools do I need to determine the quality of output? I assumed that a spectrum
> analyzer would be a first step - to look for large spurs, overtones and
> consistent noise (e.g. 120 kHz components) at unusual frequencies. Even jitter
> and phase noise should show up that way - at least with sufficient resolution,
> which might be unrealistic.
> 
> 
> Mike
> 
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