[time-nuts] mixers for frequency measurement

Ed Palmer ed_palmer at sasktel.net
Mon Jan 23 14:08:30 EST 2012


Hi Ulrich,

I've only had my 5370B for a couple of months so I'm still learning it's 
tricks.  How do I set up this "high resolution mode"?  I've looked 
through the manual and app notes and I'm just not getting it.

Ed


On 1/21/2012 11:13 AM, Ulrich Bangert wrote:
> Magnus,
>
>> The end result will be that the instrument limit slope hits the level of the stable source much earlier.
> Can you elaborate this claim a bit more? I Think I do not understand it in the correct way.
>
>> It's more of a practical limitation of getting all those readouts that I wonder about.
> It is not "all those readouts"! The counters do the averaging inside, giving an overall measurement rate of 1/s. the external arming of 1000/s is just for keeping Tau exactly at 1s.
>
>> I also have another project on a FPGA ongoing with a DDMTD test, but last time I tried things I ended up with a tool problem.
> I would never claim that FPGAs are bad for time nuts projects in general but my own experiences (I tried things like programmable dividers, linear phase comparators and ps TI interval measurements) with FPGAs have all shown heavy problems because of unwanted "analogue like" interactions inside the FPGA that are difficult to deal with since we lack to opportunity to put a blocking C here ore there inside the FPGA. These effects in the sub nanosecond region are irrelevant for all other kind of electronics including VERY fast logic but the can be a disaster for time nuts.
>
> Best regards
> Ulrich
>
> Am 21.01.2012 um 17:27 schrieb Magnus Danielson:
>
>> On 01/20/2012 02:54 PM, Ulrich Bangert wrote:
>>> Bob,
>>>
>>>> ~2x10^-10 you can do this with a good frequency counter, no
>>>> mixers needed. ~2x10^-11 you can do this with a very good
>>>> /hard to find / expensive frequency counter. 1.0x10^-11
>>>> pretty easy, nothing very fancy required for a single mixer
>>>> approach. 1.0x10^-12 works fine with an RPD-1 and some care,
>>>> but not a lot of crazy stuff 1.0x10^-13 you need some
>>>> attention to detail, and may need a better mixer. 1.0x10^-14
>>>> can I come live at your house? If you have this sort of
>>>> stuff, the cost of a fancier test setup should be a minor
>>>> issue. 1.0x10^-15 indeed people do measure this stuff.
>>>> Proving accuracy at this level involves a lot of work on
>>>> secondary effects.
>>> In general I agree to all these numbers. I just want to point to the fact
>>> that a HP5370 or a SR620 allow for a kind of "high resolution mode". This is
>>> a mode in which the counter is externally armed to make 1000 TI measurements
>>> per seconds and display the mean of them. Which gives a SQRT(1000)
>>> improvement of all counter related non systematic errors. My experiments
>>> with a SR620 indicate a 6E-13 noise floor for Tau = 1s without any need for
>>> mixing for two 10 MHz sources. Naturally this works ok only for source
>>> frequencies>= 1000 Hz ( The higher the frequency the less trigger noise ).
>>>
>>> For those of us who have no access to H2-masers or BVA-oscillators as a
>>> reference this may be not exactly an overkill but quite good to characterize
>>> HP10811/FTS1000(1200) or the like not to mention anything worse than that.
>> Such block averages will indeed improve resolution, but one has to recall that the block-summing causes a pre-filtering shifting the counters effective bandwidth. Once that is considered (really just an issue for sources of noise over-shining the instruments contribution) it will be fine. The end result will be that the instrument limit slope hits the level of the stable source much earlier.
>>
>> I will try to spend some more effort to see if there are further theoretical limitations to consider.
>>
>> It's more of a practical limitation of getting all those readouts that I wonder about. I consider hooking up a FPGA board on the back of the HP5372A high speed interface to see what that gives me.
>>
>> I also have another project on a FPGA ongoing with a DDMTD test, but last time I tried things I ended up with a tool problem.
>>
>> Some of the GPIB interfaces used, can have serious rate limits. So, do elaborate some of what you have done.
>>
>> Cheers,
>> Magnus
>>
> Ulrich Bangert
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> 27243 Gross Ippener
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