[time-nuts] Homebrew frequency counter, need help

lllaaa lllaaa at gmail.com
Thu Nov 27 14:08:11 UTC 2014


Hi guys,
    I've just get my homebrew counter working. And the resolution seems 10x
better than my RACAL DANA 1992.
    This counter is heavily inspired by the idea from Kasper Pedersen.
http://n1.taur.dk/permanent/frequencymeasurement.pdf
    STM32F051RB & EMP240T100C5 do the control and counting job. TDC-GP22 as
the interpolator. Linear regression is done by CPU.
    There are no fancy analog front for both signal path and refclk path.
I'm using two SN75ALS176 and the schmitt input of CPLD to do the job.
    I've noticed that the 10s gate does not get more meaningful
digits(looks worse than 1s gate). So here are the questions:
    1) I'm wondering if I could say this is an 11 digits/s counter?
    2) How can I improve that? Is it limited by the 485 transceiver? I can
switch to a faster MCU, that gets more measures per second, but I think
that only gets no more than 2 stable bits.



Rb clock as ref, trimble GPSDO (the old bigger one) as the signal to
measure:
gate=1s #=9039  freq=10.000000005 MHz
gate=1s #=9040  freq=10.000000004 MHz
gate=1s #=9041  freq=10.000000003 MHz
gate=1s #=9041  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9041  freq=10.000000004 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9041  freq=10.000000004 MHz
gate=1s #=9040  freq=10.000000005 MHz
gate=1s #=9040  freq=10.000000002 MHz
gate=1s #=9041  freq=10.000000004 MHz
gate=1s #=9040  freq=10.000000004 MHz
gate=1s #=9039  freq=10.000000004 MHz
gate=1s #=9040  freq=10.000000004 MHz
gate=1s #=9039  freq=10.000000004 MHz
gate=1s #=9040  freq=10.000000004 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9038  freq=10.000000003 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9038  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9038  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000004 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9038  freq=10.000000003 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9040  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9038  freq=10.000000003 MHz
gate=1s #=9038  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9038  freq=10.000000003 MHz
gate=1s #=9039  freq=10.000000003 MHz
gate=1s #=9038  freq=10.000000003 MHz


gate=10s #=90132  freq=10.00000000341 MHz
gate=10s #=90337  freq=10.00000000334 MHz
gate=10s #=90333  freq=10.00000000317 MHz
gate=10s #=90324  freq=10.00000000409 MHz
gate=10s #=90315  freq=10.00000000353 MHz
gate=10s #=90320  freq=10.00000000478 MHz
gate=10s #=90321  freq=10.00000000480 MHz
gate=10s #=90315  freq=10.00000000530 MHz
gate=10s #=90304  freq=10.00000000362 MHz
gate=10s #=90372  freq=10.00000000347 MHz
gate=10s #=90386  freq=10.00000000404 MHz
gate=10s #=90383  freq=10.00000000409 MHz
gate=10s #=90368  freq=10.00000000474 MHz
gate=10s #=90366  freq=10.00000000435 MHz
gate=10s #=90370  freq=10.00000000426 MHz
gate=10s #=90365  freq=10.00000000381 MHz
gate=10s #=90364  freq=10.00000000541 MHz
gate=10s #=90364  freq=10.00000000504 MHz
gate=10s #=90363  freq=10.00000000469 MHz
gate=10s #=90369  freq=10.00000000088 MHz
gate=10s #=90351  freq=10.00000000216 MHz
gate=10s #=90355  freq=10.00000000250 MHz
gate=10s #=90357  freq=10.00000000309 MHz
gate=10s #=90346  freq=10.00000000366 MHz
gate=10s #=90344  freq=10.00000000397 MHz
gate=10s #=90343  freq=10.00000000350 MHz
gate=10s #=90339  freq=10.00000000378 MHz
gate=10s #=90336  freq=10.00000000374 MHz
gate=10s #=90330  freq=10.00000000457 MHz
gate=10s #=90322  freq=10.00000000439 MHz
gate=10s #=90314  freq=10.00000000387 MHz
gate=10s #=90318  freq=10.00000000319 MHz
gate=10s #=90328  freq=10.00000000343 MHz
gate=10s #=90313  freq=10.00000000443 MHz
gate=10s #=90314  freq=10.00000000157 MHz
gate=10s #=90287  freq=10.00000000321 MHz
gate=10s #=90377  freq=10.00000000425 MHz
gate=10s #=90366  freq=10.00000000403 MHz
gate=10s #=90372  freq=10.00000000366 MHz
gate=10s #=90371  freq=10.00000000391 MHz
gate=10s #=90379  freq=10.00000000359 MHz
gate=10s #=90376  freq=10.00000000378 MHz
gate=10s #=90364  freq=10.00000000318 MHz
gate=10s #=90374  freq=10.00000000288 MHz
gate=10s #=90368  freq=10.00000000411 MHz
gate=10s #=90364  freq=10.00000000366 MHz
gate=10s #=90362  freq=10.00000000377 MHz
gate=10s #=90354  freq=10.00000000335 MHz
gate=10s #=90359  freq=10.00000000278 MHz
gate=10s #=90354  freq=10.00000000317 MHz
gate=10s #=90353  freq=10.00000000323 MHz
gate=10s #=90348  freq=10.00000000348 MHz
gate=10s #=90348  freq=10.00000000369 MHz
gate=10s #=90338  freq=10.00000000306 MHz
gate=10s #=90335  freq=10.00000000329 MHz
gate=10s #=90336  freq=10.00000000299 MHz
gate=10s #=90329  freq=10.00000000340 MHz
gate=10s #=90326  freq=10.00000000399 MHz
gate=10s #=90333  freq=10.00000000307 MHz
gate=10s #=90312  freq=10.00000000321 MHz
gate=10s #=90320  freq=10.00000000316 MHz
gate=10s #=90323  freq=10.00000000361 MHz
gate=10s #=90311  freq=10.00000000539 MHz
gate=10s #=90306  freq=10.00000000184 MHz
gate=10s #=90326  freq=10.00000000335 MHz
gate=10s #=90389  freq=10.00000000373 MHz
gate=10s #=90365  freq=10.00000000362 MHz
gate=10s #=90376  freq=10.00000000360 MHz
gate=10s #=90377  freq=10.00000000389 MHz
gate=10s #=90385  freq=10.00000000364 MHz
gate=10s #=90371  freq=10.00000000366 MHz


WAVE Factory NF1946A(0.01uHz resolution, but no refclk in!!!!!!)
CH2 10MHz as refclk and CH1 9.87654321012345 MHz as signal:
with reciprocal method, I get almost 7 stable digits
with the help of interpolator, I get 7 stable + 2 = almost 9 stable digits
with the help of linear regression, 9 stable + 2 = almost 11 stable digits.
gate=1s #=9039 linear=9.8765432100 MHz Interpolated=9.876543210
Reciprocal=9.8765433
gate=1s #=9040 linear=9.8765432103 MHz Interpolated=9.876543217
Reciprocal=9.8765427
gate=1s #=9040 linear=9.8765432101 MHz Interpolated=9.876543213
Reciprocal=9.8765434
gate=1s #=9038 linear=9.8765432102 MHz Interpolated=9.876543211
Reciprocal=9.8765431
gate=1s #=9040 linear=9.8765432102 MHz Interpolated=9.876543209
Reciprocal=9.8765430
gate=1s #=9039 linear=9.8765432101 MHz Interpolated=9.876543210
Reciprocal=9.8765425
gate=1s #=9039 linear=9.8765432100 MHz Interpolated=9.876543207
Reciprocal=9.8765430
gate=1s #=9040 linear=9.8765432102 MHz Interpolated=9.876543213
Reciprocal=9.8765429
gate=1s #=9039 linear=9.8765432101 MHz Interpolated=9.876543212
Reciprocal=9.8765426
gate=1s #=9040 linear=9.8765432102 MHz Interpolated=9.876543203
Reciprocal=9.8765436
gate=1s #=9039 linear=9.8765432102 MHz Interpolated=9.876543201
Reciprocal=9.8765437



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