[time-nuts] Newbie questions

[Bob Camp]

Hi

Ahhh, that jogs a few of the tired old brain cells....

You can run the gate off of the GPIB input to get all kinds of silly long
gates. Then you start to get into the cpu overflow issues. Since it's a MSB
overflow you can usually clean it up in software *if* you know it's
happening. 

Since the GPIB is simply telling the counter "start about now" and "finish
it up now" the accuracy of the GPIB timing does not get into the result. The
counter still uses it's time base as the standard of comparison.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Magnus Danielson
Sent: Wednesday, January 06, 2010 9:28 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Newbie questions

Bob Camp wrote:
> Hi
> 
> The 5335 is a pretty nice counter, you can use the math functions to get
just about anything you want displayed. One simple example is to just
subtract 10 MHz from the count and display the difference. 

Indeed. The 5335 can display 12 digits but does not overflow the display 
as some other counters but display the 12 most significant digits. The 
math function allows you to remove the most significant digits in order 
to display more digits. Good math-functions is necessary for good 
counters, I regularly use them. Correctly used you may often crank out 
the numbers you are interested in fairly quickly.

> The problem you run into is overflow in the counter chip. Somewhere around
100 seconds you run out of storage with a 10 MHz input. 

The Gate Time is specified from 100 ns to 10 Ms. I think the CPU handles 
the overflow flag in the MRC and clears it.

The Gate Time can be set up to 4 seconds on the front (I really love the 
directness of the Gate Adjust knob, miss that in many other counters) 
and "about 1 second" using GPIB GA command. There are three ways to get 
longer gate time, Manual Trigg on the front panel, External Arm input at 
the back and using the GO (Gate Open) and GC (Gate Close) GPIB commands.

The single shot resolution is about 1 ns, each of the interpolators is 
good for about 500 ps, but since they always combine the result is 1 ns.

While increased time-base will give increased frequency resolution, the 
stability of the reference time-base becomes an issue. To measure 
stability the way we like (Allan Deviation and friends) they vary with 
the gate-time (which we call tau in time-nut-speech), so a counters 
resolution can be expressed as resolution divided by tau as a single 
figure of merit (which is an oversimplification). Another figure of 
merit is the single-shot resolution, the time resolution for a single 
trigger event. There is actually the hardware resolution and that which 
includes trigger-jitter (which is more usefull). The SR-620 is at about 
25 ps, but the hardware counting is in units of 4 ps. These numbers 
correlate to some degree with the Allan Deviation of the instrument for 
shorter taus, but as taus is allowed to increase various limits kicks in.

> It's been *years* since I ran one, I could be off on the 100 seconds ....
> 
> Most of the ones you see for sale have at at least one input channel blown
out.. They are easy to fry and the front end chips aren't anything you can
get off the shelf. 

It's a nice bench-counter thought and is fairly flexible.

The HP5334A is the economy version of the HP5335A but has an additional 
feature which is usefull for time-nuts... the binary output dumps the 
unprocessed MRC values and can do that in a steady stream, allowing for 
time-stamp records to be recorded. Together with picket fence it allows 
a higher rate recording than the HP5335A.

Hmm, I have a HP5335A to repair. Probably the PSU.

Cheers,
Magnus

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