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

Gerard PG5G pg5g at b737.co.uk
Sat Feb 27 09:36:02 UTC 2010


   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
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