Moin, On Tue, 24 Nov 2015 14:04:32 -0000 "Thomas Allgeier" <th.allge...@gmail.com> wrote:
> We want to use this chip to measure the period of a square wave, of > around 13 kHz i.e. in the 70 µs range. As the application is potentially > high-accuracy we need to know the period to within 1 ns or better. That's some modest requirement and should be doable with the GPS22 quite easily (or any other TDC for that matter). BTW: when specing something like this, please make sure to mention whether 1ns is 1sigma, 3sigma or worst case/peak-to-peak. These 3 are quite different requirements. May I ask why you want to verify the specs of the GP22? The specs say that it does something between 39ps and 70ps (1sigma), which is probably way better than what you need. And as Acam is a german company, I expect the datasheet to be accurate. BTW: 1ns over 70us is approximately 14ppm. The GP22 uses the attached crystal for absolute calibration. Please be aware that 14ppm will require at least a TCXO to reach that level over the whole temperature range, and depending on what TCXO you use, you might need to calibrate the TCXO post-production and again after a couple of years of use. Even if you don't need calibrate, I would add a TCXO frequency measurement to the production test. BTW2: the "we have a x ppm TCXO" value is usually misleading, as that's the best-case, pre-soldering, pre-aging, pre-anything value. The end-value can be 3 times as large... easily. (unless you happen to choose one of the more honest manufacturers, for example, like Abracon) > In order to evaluate the chip I was planning to replicate John A’s > experiment with the coaxial delay line from the HP5370b For those wondering: "John A" is John Ackermann and the experiment in question is documented at http://www.febo.com/pages/hp5370b/ > – but as my interest > is in “measuring range 2” of the GP22 I need a delay of 500 ns or more > (actually 1 µs sounds a better start). This is the equivalent of a 200 m > length of cable. I fear trouble with this: Am I not getting unwanted > inductivities if I use a coil of that size? The coax is a transmission line. Yes you have inductance and capacitance, but it does not make that much sense to talk about that anymore, the impedance is the right thing to talk about. Your output will not be as sharp as your input due to dispersion, but that can be easily recovered using some buffer gate. Please make sure that your coil is reasonably temperature stabilized and, if it's cheaper cable, also humidity stabilized, as both parameters change your delay. (putting it into an isolated box should be good enough for this kind of measurment). Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.