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+--- Thread: frequency counter mod (/thread-756.html)
frequency counter mod - VK5KAF - 14-03-2019
i have an older 1 ghz frequency counter i built many years ago(will count to about 1.7ghz) .in general it works pretty well but suffers from frequency drift and lack of input sensitivity which i think can be improved by adding an elcheapo ebay wide band amplifier, My question is i will soon have a gpsdo at my disposal if i replace the 10 mhz crystal with 10 mhz gpsdo will this improve frequency stability and if so where and how should i inject 10 mhz at pin 24 (iICM7216A) or remove the existing crystal and replace with gpsdo
Cheers and thanks for any input
John
VK5KAF
RE: frequency counter mod - VK4ADC - 14-03-2019
Why not make both options available ?
Take the output from the existing 10MHz to a DPDT switch (back panel is probably fine), a lead from a rear BNC socket (via a DC blocking capacitor) to the other pole connection and the centre feeds back to the 7216 counter clock pin. Short shielded leads preferred eg miniature coax, 50 or 75 ohm, or even shielded audio cable (for short lengths only {it's only 10MHz})..
When you have your GPSDO warmed up and stable (/ phase locked), switch to the external 10MHz signal for better accuracy. Make sure you have plenty of GPS signal.
My GPSDO runs 24x7 ( and has for the last 3-4 years) so is always ready to use. From power-on, it can take up to an hour or more to be really stable hence always being on. I use it to feed into my synthesised sig gen, my 1GHz counter and also my 18GHz counter so these units are ready to use immediately after powering on.
RE: frequency counter mod - VK5KAF - 14-03-2019
Thanks Doug you have given me a bit more confidence in attempting this and yes i plan to have the gpsdo on continuously and next stage is converting 10 mhz gpsdo to 1 mhz( going to try a video dist amp to feed multiple outputs) to replace 1 mhz reference in my old systron donner signal generator
Cheers and thanks
John
RE: frequency counter mod - VK4ADC - 14-03-2019
A 74HC4017 will go up to about 50MHz, divides by 10, 1:1 high to low duty cycle. Put a schmitt trigger squarer in front (assuming the GPSDO outputs 10MHz sine wave) and put that into your dist amp - or alternatively steal the LM7171 output section of https://www.vk4adc.com/web/general-projects/34-frequency-stabilisation/72-lctcxo in lieu.
https://www.ti.com/lit/ds/symlink/cd74hc4017.pdf
10MHz fed to CLOCK, pin 14
1MHz out from Terminal Count, pin 12.
ground on pins 8, 13, 15.
Supply to pin 16.
Schmitt squarer : HC14, hex inverter
http://www.ti.com/lit/ds/symlink/sn74hc14.pdf
----- 1Mohm----
| |
INPUT o-- C ------- INV -------------- INV ------- HC4017 ----- C ----o O/p at /10 (SQ wave)
Clk TC |
|------- INV ---- C ----o Option: 4 sq wave isolated outputs @ 1MHz
|--------INV ---- C ----o
|------- INV ---- C ----o
|------- INV ---- C ----o
10MHz input level is assumed to be at least 1V Pk-Pk.
C is DC block, 103n to 104n, actual value not critical
1Mohm makes the single schmitt inverter act slightly more linearly ie. accepts signals more sinusoidal in form
Make sure you add 104n caps to "ground" as RF bypasses directly on the VCC supply pins of the two ICs, pin 14 of HC14, pin 16 of HC4017.
Optionally add another HC14 INV section after the HC4017 TC output to give isolation protection. Alternatively tie all 4 spare INV inputs to the TC pin and that gives you 4 separate sq wave outputs at 1MHz.....
Supply voltage is not critical but >= 5VDC for fast switching speeds.
RE: frequency counter mod - VK5KAF - 14-03-2019
Thanks doug well thats given me some good leads to follow
RE: frequency counter mod - VK4ADC - 15-03-2019
I have updated my previous post with more details. A new day gives better ideas
