Re: [time-nuts] Equipment question: OCXO versus GPSDO + XTAL

[Bruce Griffiths]

Since the 53320A TCXO option has a 1s ADEV spec of 1E-9 whereas the OCXO 
option has a 1s ADEV spec of 1E-11 and the PLL bandwidth is unlikely (no 
info on the PLL bandwidth available from datasheet) to be very large the 
OCXO contribution to measurement uncertainty will likely be 
significantly larger than that of the OCXO even when phase locked to an 
external standard particularly for short gate times (less than the 
reciprocal of the PLL bandwidth i.e. probably below a few tens of 
millisec). For sufficiently long gate times (> 1s??) their will likely 
be no discernable difference between the the OCXO and the TX) options 
when locked to a sufficiently quiet external frequency standard (ie one 
with an ADEV << 1E-11 or so for Tau > 1s).

Locking the counter timebase to an external standard isnt as flexible as 
a direct input where one could use something exotic like a low noise 
OCXO simultaneously locked to a hydrogen maser and a Caesium frequency 
standard (or GPSDO) to achieve low ADEV over a wide range of Tau values. 
Implementing such a system doesnt necessarily require that the hydrogen 
maser be phase locked to the Caesium standard (or GPSDO).

In some cases trigger jitter may dominate as it isn't easy (but not 
impossible for low noise sources: 
http://www.holzworth.com/Spec_sheets/HX2410_Web_Datasheet.pdf ) to 
produce subpicosec trigger jitter when converting from sine wave inputs 
to logic level signals.

Bruce


Bob Camp wrote:
Hi

Alan variance / short term stability / medium term stability is what will impact your 
measurements "noise". That's not to surprising - noise on the standard gives 
you noise on the measurement.

If for some reason an internal standard is more quiet than an external standard, the 
internal may give you "better" measurements. This is not a so much function of 
the oscillator, but of the PLL used in the instrument.  If the PLL changes between your 
choice A and choice B, that may impact the result, if there is much noise on your 
external standard.

As you measurement interval gets shorter, the noise level of all sources will 
go up. Fortunately, the noise tolerance of the measurement does the same thing. 
Depending on exactly which specific oscillators are being compared (not just 
types, and possibly not just models) there may be a cross over between devices. 
That could also give you a cross over between choice A and choice B.

All that said, normally you are better off with the highest priced standard. 
It's going to have the lowest intrinsic noise. It also should  be locked 
through the narrowest PLL. That's a most of the measurements most of the time 
judgement.

Bob


On Mar 18, 2011, at 10:18 PM, Wolfgang Wieser wrote:

   
Hello all,

first, thanks for all the answers!

Maybe let me ask the more fundamental question: Completely independent
of use scenarios and so on: Assume device A uses standard reference tied
to ultra-stable zero-drift magic 10 MHz refclock. Device B uses TXCO option.

Is there any measurement type with any strange setup that you could imagine
where B outperforms A just because the combo "standard XO + refclock"
is inferior in any respect compared to "TXCO". (Lab/shop equipment,
no portability needed.)

I mean - not necessarily for this particular Agilent 53230A but for anything
of this type and class.

@Bill:
thanks - exactly the type of advice/opinion I was hoping to get.

@Paul Swed:
     
Unfortunately in the first round of the thread you did not say what you are
actually doing with the system and whats important to you.
       
Measuring time and frequency? Maybe there are some unknown issues that I am
not aware of but what I would like to know is whether for any usual
measurement made with a timer/freq counter the choice of XO+refclock could
be inferior to TXCO. If there are significantly different scenarios in
your mind - it would be nice to let me know for what type of measurement
you think that the one is better and for what not.
Anything except portability issues?

     
The last answers actually the best considering the base system initial
investment.
       
Well, once a year I buy something more expensive...
However, while 2500 EUR seems reasonable for the counter, additional
1000 EUR seems a bit much for the OCXO option.

@Joe
     
You will have to look at the stability measurements of the OCXO versus the
GPSDO to choose that one.
       
OK, do you have an educated guess about the outcome of that?
Because acutally doing this comparison exceeds my scope ATM.

@Greg
     
My opinion is that first you need to tell more about how you
are going to use this counter. For work in the shop where
a distributed reference is always available (...)

       
Hm.. OK.. I re-formulated into the question on the top.

     
I am suspicious of the note that this OCXO upgrade can only
be done at the factory.  See if you can get the control sequence
to cause reference autocal.
       
I've got a test unit ATM and there actually is a security code to
unlock the calibration. I don't know the code right now.

- Wolfgang, DL1SKY

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