[time-nuts] My DIY frequency counter and a request for help

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Feb 27 12:36:41 UTC 2010

Gerard PG5G wrote:
>     Hello all,
>     First post here, so I'll start with a quick introduction. I trained as
>     an electronic engineer but don't work in that field any more, which has
>     given me the appetite back to do some electronic engineering as a
>     hobby. I have been a licensed ham for over 25 years (more than 60% of
>     my life I realised the other day) and used to be rather active on HF as
>     PA3DQW. At the moment I live in the UK where I am licensed as M0AIU.
>     I recently designed and build a frequency counter and I need some help
>     with verifying its performance. I believe it gives me 11 digits in 1
>     second. I say believe because I have not got the hardware to verify
>     this. At the moment my assumption is based on calculations and limited
>     testing with the equipment available to me.
>     My counter is a "continuous time stamping reciprocal counter". I
>     implemented this as a USB powered device, with the hardware taking the
>     time stamps and sending it over USB to a windows PC. Some software
>     written in C++ takes care of analysing the data.
>     The hardware takes 5000 time stamps per second using a high speed TDC.
>     The hardware is a single PCB measuring about 50 by 80 mm. it requires
>     an external 10MHz reference and apart from using this as the time base
>     it also uses this for self-calibration of the TDC. The unit requires no
>     further calibration.
>     The PC software takes these time stamps and the associated counts and
>     uses regression to calculate the slope. This slope represents the
>     frequency of the input signal. I am sure people on here are familiar
>     with the counters made by Pendulum, and I have to confess that their
>     marketing material was helpful in putting this thing together.
>     Since the hardware is true zero dead time, the final capabilities of
>     this counter are determined by software. At the moment I can
>     simultaneously display the input at multiple gate times (see the
>     attached screen shot). For gate times over 1 second I have the option
>     to use overlapping gates, so that the display gets updated every
>     second.
>     Because there is no dead time I can also calculate Allan Deviation. The
>     two displays at the bottom of the page show both normal and overlapping
>     Allan deviation at tau=10s. I am still working on the software to do
>     this at multiple tau in real time and display it as a graph and a
>     table.
>     So, after this lengthy introduction here is my request for some
>     assistance. Is there somebody on the list who can assist me in
>     verifying the performance of this frequency counter? Ideally somebody
>     with access to two highly stable and known frequency sources. I can
>     send the hardware by mail, but if there happens to be somebody with
>     this kind of gear not too far from where I am (50 north of London) I
>     will travel. In exchange you get to keep the hardware and will be
>     supplied with whatever software I come up with.
>     Thanks in advance and regards,
>     Gerard, PG5G
Are you calculating ADEV and MDEV using the slopes determined by the 
regression fit?
If so, what you calculate isn't ADEV or MDEV.

You need to use the raw timestamps taken at a rate of 5000/sec directly 
to produce estimates of ADEV, MDEV.
What is the resolution of the TDC?


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