frequency counter : reference clock frequency - Page 1
Forum section: electronics / projects

author: electros6
author_rank: Regular Contributor
timestamp: June 06, 2014, 03:47:23 pm
content: I am tried to do a hardware frequency counter upto 10Mhz. I most of the commercial frequency counter used high frequency clock to generate reference time base. (eg) HP used 10MHZ reference clock and philips used 4.1.194304MHZ clock. what is the need of such high frequency reference. why don't use 32.768KHZ to generate reference clock , even 32.768KHZ can be used to generate 1s time base reference.

author: jpb
author_rank: Super Contributor
timestamp: June 06, 2014, 04:02:12 pm
content: Counters don't just count cycles in a 1 second window (or gate). Having a higher frequency clock (my TTi uses 50MHz internally but will frequency lock to an external 10MHz reference) allows better resolution especially at lower frequencies - they count how many high frequency clock cycles match with a whole number of cycles being measured within the gate period. The more expensive counters will also interpolate between clock cycles for better resolution. For example the top of the line Agilent (I forget the model number) has a resolution of 20 picosecs though its internal clock is probably of the order of 100MHz.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 06, 2014, 04:06:59 pm
content: Two reasons. 10 MHz clocks are industry standard for synchronizing multiple instruments. If you have a spectrum analyzer and a frequency counter that need to use the same time base you would do that via the 10 MHz reference inputs. If you need them both to have higher stability than the built in reference, you buy an external 10 MHz OCXO or Rb oscillator and connect it to both.The second reason is for accurately measuring low frequencies. If you are measuring 60 Hz by counting the cycles in a 1 s gate you only get 2% accuracy from the +/- 1 count. Instead you do either period counting counting or reciprocal counting which count cycles of the reference clock. A faster reference clock then results in greater accuracy. Pretty much all general purpose frequency counters now use reciprocal counting because if you have a micro to do the division it is easy to do and provides good accuracy over the entire frequency range without having to explicitly switch from frequency to period counting.

author: dannyf
author_rank: Super Contributor
timestamp: June 06, 2014, 04:40:56 pm
content: Quote1. very difficult to find TCXO or OCXO at low frequencies;2. a high frequency internal clock allows the possibility to use the external signal to gate the internal oscillator - very useful to measure low frequency signals;3. allows quicker gate time.4. cheap and easy to find.

author: KJDS
author_rank: Super Contributor
timestamp: June 06, 2014, 05:11:22 pm
content: 32kHz clock crystals aren't very stable. Common AT cut crystals used for general purpose higher frequency use also have limited stability.ST cut crystals provide the best stability and good phase noise and these are generally used for test equipment references.Quote from: ejeffrey on June 06, 2014, 04:06:59 pmThere are ways of considerably improving on that accuracy. but it takes a lot of effort