[time-nuts] TDC-GP22 vs TDC7200

[Li Ang]

Hi
    I got 5 samples of TDC7200 from TI a few months ago. I have made a board to test it with TDC-GP22 from ACAM. Actually it's a new board of my frequency counter. The CPU system is changed from MCU to a OrangePi board. The digital part is still a Cyclone 4 FPGA. 


Verilog: http://www.qsl.net/bi7lnq/Projects/freqcnt4.2/100ns_noise_test.v
Picuture: http://www.qsl.net/bi7lnq/Projects/freqcnt4.2/5.2.jpg
Result: http://www.qsl.net/bi7lnq/Projects/freqcnt4.2/100ns_stdev.xls


   The DC-DC part works at 4MHz (MP2560), it generates a lot of noise. (stdev of 1000 measures is around 300ps).  It's a mistake to put that on board.  However, if the 5v is supplied by a HP E3630A, the performance is much more stable.
   The 100ns start-and-stop signal is generated by FPGA. I do not know if it's a stable way to do that. The 10MHz ref clock is from one FE180 OCXO and shaped by NC7SZU04.
   The STDEV of 1000 measurements by TDC_GP22 is 70ps. (45ps double resolution mode, STDEV is not given by the datasheet)
   The STDEV of 1000 measurements by TDC7200 is 20ps. (datasheet: 55ps resolution, 35ps STDEV RMS  )


    So I think the TDC7200 is a better choice for my homebrew frequency counter. It's much more friendly to solder(TSSOP vs QFN). It's much more esay to config(2 Hours of coding vs 2 weeks of debugging). It's a little more expensive than TDC-GP22(9$ vs 4$).






de BI7LNQ
73