Murray Greenman wrote: > Hi all, > > We have a problem at work where one of our engineers needs to measure > the start-up chirp in an oscillator destined for a GPS application. > > We are looking for ppb level frequency resolution with sample periods > around 10 - 20us (yes, that's us, not ms!). The chirp is all over well > inside 1ms. > > The measurement is on a 40MHz oscillator, which is mixed down to about > 198kHz to improve resolution. We can count the down-converted frequency > in a conventional manner with no problem. The problem is of course that > at 198kHz the period is ~5us so even an interpolating counter will > struggle. > > Preliminary tests with a Pendulum CNT90 counter give results that > deviate +/- 16Hz when the gate time is reduced to 10us. We are using > GPIB to capture multiple repeated samples, and trigger the counter when > the oscillator is powered up. We also have an Agilent 53181A counter > available. > > Amplified mixer noise could easily account for this if the post mixer amplifier/filter system isnt appropriately designed. Optimising the post mixer amplifier and filter will drop this noise to well below the counter resolution. > Any suggestions (a) how to make the measurement; (b) what is causing the > counter result to degrade in this way? > > Thanks, > Murray Greenman ZL1BPU >
Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
