[time-nuts] Considerations When Using The SR620

Volker Esper ailer2 at t-online.de
Sun Dec 2 22:36:37 UTC 2012


I was the one who wrote those nasty things about a fantastic instrument.

I bought such a counter a few weeks ago. When I first peeked into it 
(because the fan was running at top speed after a few minutes) I 
noticed, that the arrangement of the components was anything but optimal.

Examples: The osc is only one or two inches away from the linear voltage 
regulators. These are "cooled" by a 1mm iron sheet. The iron sheet has 
some cooling slots where the air can flow through, but these slots are 
miles away from the regulators. The sheet gets so hot that you won't 
touch it voluntarily. If at least the air could flow directly along the 
regulators - but they sit in a corner and the air flows diagonally 
through the case, avoiding contact with the regulators.

Since I use an external osc, that doesn't bother me too bad.

However, while trying to adjust the trigger circuits, I noticed, that 
they are very sensitive to temperature variation. Unfortunately, the 
warmed air (which is coming from the cooling slots) flows over these 
parts... see picture.

The thermistor that controls the fan is located amidst the air flow - 
thus resulting in a very nervous control loop. The fan is continously 
speeding up and down. It's not to hard to imagine, that this has effect 
on all parts where the air flows (-> Trigger...).

If I had to construct the thermal design for a 4000 Dollar counter I 
would do totally different.

That's why I feel a little displeased.

If you own a counter, that you can't modify (due to calibration seals) 
heat it up and do not switch it off. Let the cooling air flow by giving 
the counter enough space around it. Stabilize room temperature and don't 
place your counter near a door or a window or a place where air 
temperature is often varying.

I'm shure, you will run a great instrument.

If I own a totally atypical counter and Time Nuts have different 
experience please stop me from spreading rubbish.


Am 02.12.2012 21:12, schrieb Bob Camp:
> Hi
> Some of the problem comes from the OCXO. Some of it also comes from the stability of the internal circuitry. They do some interesting things with delay lines and the like in the SR620. Best bet is to let it warm up for a while before you need it to perform at it's top level of performance.
> Bob
> On Dec 2, 2012, at 2:45 PM, Paul DeStefano<paul.destefano at willamettealumni.com>  wrote:
>> All,
>> 	The following comment appeared on this list recently and it scared me a little:
>>> Though the SR620 TIC is a great instrument when hunting the pico seconds we have to realize, that it's a thermal design desaster (I have to apologize to all sr620 friends). I have to run it for at least 12 hoursif not 24 to be shure, that every single part is at a more or less stationary thermal state. Some (NERC) say "...never switch it off".
>> I assume this instability is due to the instability of the internal frequency standard.  There are two options for the SR620: the standard oscillator and an ovenized oscillator.  In fact, in our measurements, we plan to use a Cesium frequency standard as the timebase to our SR620. Does this anecdotal warning apply generally to the instrument or mainly to the use of the internal standard oscillator?
>> We are using the SR620 to measure the interval between 1PPS signals from two clocks.  One is the Septentrio PolaRx4 GPS receiver and the other is a Rubidium clock.
>> Many Thanks,
>> Paul
>> --
>> Paul DeStefano
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