Measuring the period of a signal

Look up "time to amplitude converter" (TAC) or "frequency to voltage converter". e.g. LM2917 or LM2907.

Today, if the desire is to get a digital number out, you'd just start running a timer at one edge and stop at another edge. HP has sold instruments that do this for a good chunk of a century now. Note that you do not specify your desired resolution, nor even whether you want to know period, time high, or time low.

Tim.

Well, it isn't clear how accurate you want your measurement to be. If you need high accuracy (1% or better), mostly it's recommended to use a digital frequency counter. If, on the other hand, your

5% number is the frequency TOLERANCE, consider using a phase-locked-loop chip. 74HC4046 can lock onto this kind of signal and has an output pin whose voltage varies linearly with the frequency it locks to.

With a good quality timing capacitor and some 1% program resistors, precision is possible. Accuracy follows after you calibrate the analog output with some known-frequency sources. Either a calibration graph, or a gain adjustment to 1.000V/MHz, can be useful.

one way would be to charge up a capacitor with a constant current source, then discharge the capacitor with a mosfet with a short pulse at the start of the period. then average the value with a low pass filter.

the pulse could be via a cr network, or as you sugest using half the frequency.

depends how accurate you want it.

If your going to digital anyway why not just simply count the pulses and work out the period from 1/freq ?

Colin =^.^=

Clue #1: the period is the inverse of the frequency. Clue #2: one can start a counter that is driven from a fixed frequency at (say) the rising edge, and stop the counter at the next rising edge. Clue #3: many frequency meters have that period funcion also, but if not, refer to clue #1.

Thanks for the answers. I will try to go with the simple counter first. Hopefully it is accurate enough. If not, i guess i will have to find a better solution. Amish

Microcontrollers are good at this kind of thing, but the frequency that you are talking about is kinda high for low-end parts. For example with a PIC running at 4MHz, you can easily and accurately measure the length of pulses to 1uS precision. That's good resolution for lots of things, but for a 5MHz signal it's unusable. On a larger (read more expensive) you could clock it at 40MHz and increase resolution to 100nS. Better, but probably still pretty far from being precise enough for your frequency of interest. I believe that you can actually acheive 25nS resolution with those models, but I'm not sure on that. Not that it would help that much.

You probably want pS resolution, so the easiest thing is to just count the frequency and do the math.

Of course, you may wish to verify this with Mr. Black. ;-)

Im using a dspic30 wich as you say can be clocked at 40mhz, but actually this is the instructions per second, the actual clock is 160mhz from a PLL inside, although the counter/timers clock of the 40mhz tho so that stil gives me

25ns, trying to get 0.1ps resolution seems kind of absurd but thats what im trying todo.

anyone know what other MCU has faster clocking timer/counter ? if a 2ghz pentium had timer counter now that would be nice :)

Colin =^.^=

You mean to say that you have not heard of the FRDTSC command?