1
\$\begingroup\$

I'm looking for a smallest design of a frequency counter time base uses a crystal with lower frequency that can give rates of 1Hz, 10Hz, 100Hz after dividing, I intend to use it in RF circuit, the reason for a lower frequency crystal is trying to avoid interference in the RF circuit, for example can the 32.768khz crystal, is it a better choice for the time base ? if not any other suggestions with help of a better design diagram ? I would appreciate it, thanks

\$\endgroup\$
  • 1
    \$\begingroup\$ While 32000Hz crystals aren't as popular as 32768Hz crystals, they manufactured as standard parts and should not be difficult to obtain. Given a 32000Hz crystal, it should be easy to extract a 1000Hz signal and from there to use a couple of two-digit decimal counter chips to get stable outputs at power-of-ten frequencies. \$\endgroup\$ – supercat Sep 13 '16 at 17:14
  • \$\begingroup\$ In the UK you can find 60kHz crystals (thanks to its use as the MSF broadcast frequency reference. uk.farnell.com/citizen-finetech-miyota/cfv206-60-000kazf-ub/… \$\endgroup\$ – Brian Drummond Sep 13 '16 at 17:17
  • \$\begingroup\$ You should tell us more about your counter. if mimimal space is the concern, an FPGA is the way to go, but I doubt that's in you budget. So, are you planning to use separate counter ICs and displays? How about displays with built-in counters? And how, exactly, do you plan to count RF frequencies? Even 100 MHz (American FM radio stations) will require a certain amount of competence. Or are you planning to use a divider chip to get down to a usable frequency? \$\endgroup\$ – WhatRoughBeast Sep 13 '16 at 17:29
  • 3
    \$\begingroup\$ the reason for a lower frequency crystal is trying to avoid interference in the RF circuit That is not by definition the best approach ! A squarewave signal of 32 kHz will contain a spur every 32 kHz. A 10 MHz one only every 10 MHz. Keep the rise/fall times under control as this determines how strong those spurs will be. Choosing the right crystal frequency is also crucial. \$\endgroup\$ – Bimpelrekkie Sep 13 '16 at 17:44
1
\$\begingroup\$

Seems to me you're most interested in reducing electromagnetic coupling from counter (and its display of frequency) to a RF-sensitive circuit (receiver?).
The time-base oscillator is just one source of RF. Counter chains and display drivers are other radiating sources. Use a metal-case crystal (like HC-49) and ground its case.
HC-49 crystal Integrate your time base into a microcontroller that also drives the frequency display. A crystal-controlled microcontroller master clock can also serve as the counter's time base. The tiny microcontroller area is easy to isolate, bypass, and shield. Discrete counter chains of chips are to be avoided.
Use a LCD display of frequency, rather than LED or other high current displays. Transient currents to each LED segment radiate RF energy.
Sometimes choosing a high-frequency crystal allows you to place its frequency in a remote out-of-the way spot that will cause you less grief. Harmonics of a low-frequency time base can cause grief over a w-i-d-e frequency range.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.