[time-nuts] time-nuts, frequency counters

CHazlitt chazlitt at mtaonline.net
Wed Sep 26 17:41:30 EDT 2007


I just joined the group a couple of weeks ago and have one remark about an 
HP counter that I bought on ebay and also one question. The HP 5386A I own 
incorporates a 12-character display which works out wonderfully when using 
the 10 MHz output of a Rubidium source to clock the counter. The one I 
bought goes up to 3 GHz but there is a less expensive version of the same 
thing which although doesn't go to 3 Gig, has the same resolution. I found 
this unit to be a great way to compare the Rubidium's frequency with an HP 
GPS clock source.

I bought a Frequency Electronics FE 5680A Rubidium card on ebay too and 
found that when comparing it's output at 10 MHz to the output of the HP GPS 
standard that they did not agree when comparing the two using the above 
mentioned HP counter (using the HP GPS as the clock for the counter). The 
Rubidium I purchased is a few years old but locks up every time in just 
three minutes so I know it is in good shape but yet it didn't agree to 
the -9 on the frequency counter.

So, here is my question, do Rubidium standards drift that much over a period 
of years to where they have to be brought back on frequency? If so, what is 
tuned on the Rubidium to do so, C-field? I didn't touch anything on the 
physics package of the FE 5680A but the unit has a DDS divider built into it 
so I was able to adjust the Rubidium to agree with the HP GPS standard 
through an RS 232 serial connection and hexidecimal inputs to change the 
frequency of the dividers output. The divider they use can adjust the 
frequency far finer than the best resolution of any Rubidium standard so I 
thought that would be a good way to match the output of the GPS standard. 
I'm new to time standards, GPS or Rubidium units and purchased it for some 
telecom work so I'd appreciate any suggestions or comments anyone here might 

One last question in regard to counters. Is it possible, or does anyone make 
a frequency counter which has 12 digits but only displays 1 Hz and below 
regardless of the frequency which is fed into it? i.e. 1 Ghz reads out as 
1.000 000 000 00 (if on frequency) only displaying the last cycle of the 
count down to -11 ?

Thanks for the bandwidth

73 K L 7 Fox Bravo.

----- Original Message ----- 
From: <time-nuts-request at febo.com>
To: <time-nuts at febo.com>
Sent: Wednesday, September 26, 2007 8:00 AM
Subject: time-nuts Digest, Vol 38, Issue 36

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>   1. Re: FMT on October 13 (Magnus Danielson)
> ----------------------------------------------------------------------
> Message: 1
> Date: Wed, 26 Sep 2007 13:15:15 +0200 (CEST)
> From: Magnus Danielson <magnus at rubidium.dyndns.org>
> Subject: Re: [time-nuts] FMT on October 13
> To: time-nuts at febo.com, didier at cox.net
> Message-ID: <20070926.131515.-1607496526.cfmd at bredband.net>
> Content-Type: Text/Plain; charset=us-ascii
> From: "Didier Juges" <didier at cox.net>
> Subject: Re: [time-nuts] FMT on October 13
> Date: Wed, 26 Sep 2007 05:57:14 -0500
> Message-ID: <006201c8002c$00044520$6501a8c0 at didierhp>
> Didier,
>> I just wanted to point out that with reciprocal counters, you can get
>> resolution much better than the 1Hz/s you would get with conventional
>> frequency counters, even though the actual accuracy of the measurement 
>> may
>> be way off.
> These days conventional counteras are reciprocal counters. It is only the
> old-school counters which is not reciprocal. Nothing wrong with 
> old-school,
> but a conventional counter of the shelf today is probably a reciprocal 
> jobbie.
>> The original question seemed to imply that with a short transmission 
>> time,
>> you could not guarantee a frequency accuracy of 1e-6 Hz, which you 
>> probably
>> can't anyhow, but the limit is not the resolution of the instrument or 
>> the
>> measurement method.
>> I do not know how far off calibration my HP 5370s are, but the 20pS
>> resolution is at best only usable under some circumstances that I have 
>> not
>> isolated yet, due to jitter.
>> When measuring a 3.5 MHz signal (@1dBm) from my HP 8657B through 1 meter 
>> of
>> good coax cable (with counter and generator phase locked to the 
>> Thunderbolt
>> GPSDO) in Frequency mode with a 1s gate time, the resolution is 1e-5Hz, 
>> with
>> about 1e-3Hz p-p variation. When measuring over 1 period with 10,000 
>> periods
>> sample size, the resolution is only 1e-1Hz with a standard deviation of 
>> ~400
>> Hz (or about 0.1%). Of course, over the air, it will be much worse due to
>> noise, let alone propagation, fading and multipath.
> When measuring over a longer period you see a different spot on the 
> curve. Chances are that you are more unstable there for an OCXO. Both 
> linear
> and noise products will make things harder. It can be a challenge to 
> separate
> the drift rate due to signal path shifts and that of the OCXO.
> Cheers,
> Magnus
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