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: [email protected] [mailto:[email protected]] 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
