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Can I use Picoscope as a realtime frequency counter?

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Can I use Picoscope as a realtime frequency counter?

Postby hillp » Mon Oct 01, 2007 3:43 am

I'm trying to read an inductive pickup that gives me zero-going voltage pulses at about 50 to 100 Hz, but my automotive multimeter (set on the frequency range) updates or averages so slowly (several seconds) that I have an unacceptable lag. ( I want to manually actuate another device, a manual valve, when the frequency reaches a certain value while reading the frequency ramping up from 0 Hz to 50-100 Hz. It takes about 5 seconds or so to get to the target frequency from 0 Hz.)

I have an ADC-40 I can press into service; I believe it should be quick enough to see part of each pulse; the system geometry is such that at 100 Hz the voltage signal should drop off to zero for about 0.5 msec every cycle (versus the 9.5 msec the signal is high).

Can the PS software (as a spectrum analyser, maybe?) read these pulses and display frequency or period quickly enough for it to be essentially real-time? I can't take a trace and then go back and inspect it.
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Postby PeterF » Mon Oct 01, 2007 2:11 pm

Hi,
You should be able to get a good frequency reading out of an ADC-40 in these circumstances. You will need to display at least 3 cycles of the pickup output at the lowest frequency you need to measure. Goto Settings\Measurements. In the measurements list go to Add. In the measurement details, select frequency, whole trace, Ch A. (You can even try to set alarm limits to get audible indication).
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Postby hillp » Tue Oct 02, 2007 3:36 am

Very nice. Works like a charm. The audible alarm is perfect for my application; I operate my mechanical device when I hear the beep.

I did notice that if I chose a very long timebase, such as 10 seconds for a trace, the numbers are way low. Why is this? I have it set now at 10 msec for the whole display.

A few more questions:

How much averaging is set up in this measurement, and where do I configure that? Is it linked to the timebase?

Can the frequency numbers be saved, or just the voltage vs time trace?

Is there a way to make the font bigger (as seems to be done in the "meter" function)? The voltage vs time trace is of little interest to me at the moment.

Thanks.
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Postby PeterF » Tue Oct 02, 2007 1:37 pm

Hi,
The averaging is set up in the same dialogue box where you chose "Whole Trace", with options for averaging between cursors etc. All that is also linked to the timebase speed as well. You can display frequency also on the meter display if you wish. I don't think the frequency numbers can be saved as they are calculated on the fly.
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meter display reads 0 Hz while scope shows live numbers?

Postby hillp » Sat Oct 13, 2007 1:14 am

Is there any reason the meter display set to frequency would work diferently than the scope display with frequency calculated at the bottom right? It seems that as I ramp up a signal from 0 to 50-100 Hz in a few seconds, the meter display stays at 0 Hz for quite a while, then starts displaying the ramp-up at 50 Hz or more. Switching to the scope display(with the much smaller frequency readout pasted on the lower right of the scope screen), it starts reading the ramp-up much earlier.
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meter display of frequency is slower than scope display

Postby hillp » Wed Oct 17, 2007 6:31 pm

I've come to the conclusion from reading a number of earlier posts that I should be using the frequency readout on the scope display, not the meter, if I want to get a decent response time.

One statement from an earlier post says

"The meter does not know whether your voltage is just DC or contains AC frequencies. To ensure an accurate measurement it has to collect data for a long time to ensure it can pick up low frequency AC changes."

Thanks.
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