Electronics > Projects, Designs, and Technical Stuff
Voltage comparator
soldar:
One of the first instruments I designed and built myself around 1975 was a frequency counter and I still have it and it still works. I used TTL SN7490 decade counters, SN7475 latches, SN7447 bcd to 7 segment decoders and 7 segment displays.
Crystals were extremely expensive so I used one that I got for 5230 KHz and used a lot of digital circuitry to divide by 5230 and obtain a 1 KHZ signal used as timebase.
The signal input amplifier/conditioner was really, really badly designed but I was just starting and that's all I could do. It had two parts. The low frequency part went up to about 1 MHz and a separate amplifier was switched when counting higher frequency. I have sometimes thought I could improve the instrument by replacing this part and leaving the rest as it is.
Conditioning the signal is not as obvious or straightforward as it might seem at first. How many cycles or pulses do you want to count in the attached graphic?
OM222O:
--- Quote from: soldar on April 30, 2019, 07:03:04 pm ---One of the first instruments I designed and built myself around 1975 was a frequency counter and I still have it and it still works. I used SN7490 decade counters, sn7475 latches, SN7447 bcd to 7 segment decoders and 7 segment displays.
Crystals were extremely expensive so I used one that I got for 5230 KHz and used a lot of digital circuitry to divide by 5230 and obtain a 1 KHZ signal used as timebase.
The signal input amplifier/conditioner was really, really badly designed but I was just starting and that's all I could do. It had two parts. The low frequency part went up to about 1 MHz and a separate amplifier was switched when counting higher frequency.
Conditioning the signal is not as obvious or straightforward as it might seem at first. How many cycles or pulses do you want to count in the attached graphic?
--- End quote ---
ideally 4 (actual value) but there are spikes all over the shop and my hysteresis would be close to ground, so it would be counting 5 :-// (spike on the 3rd wave causing a false count)
soldar:
These things can get very tricky. Do you count only zero crossings in one direction? with how much hysteresis? Too much and you can miss some in low amplitude waveforms. Too little and you can get false triggering in higher amplitude waveforms. Ideally you would first normalize the signal.
What about when you have something like a high frequency signal summed with a lower frequency signal? Do you want to count one or the other?
IDEngineer:
The two waveform examples suggest filtering is important too. I did a search and this post is the first in this thread with the word "filter" in it.
I acknowledge that if the frequency counter is measuring something that another circuit will depend upon, that other circuit may also be fooled by unexpected frequency components. But that doesn't mean they can be ignored! If your counter can be fooled, so too may your actual circuit be fooled.
It looks (pun intended) like observing the signal with a scope or SA might be really good insurance before relying on the accuracy of the frequency counter. The latter may be fooled by spurious frequency content, but the scope should be able to image it (presuming sufficient bandwidth). Lacking that, perhaps we should include a discussion of front end filters while talking about frequency counters....
EDIT: I suppose one could design a pulse discrimination circuit that would only count the transitions that occur on a periodic basis, but I think I'd rather use standard filters.
OM222O:
I'm not trying any fancy waveforms like you mentioned (two summed up or etc) no frequency counter that I know of can handle those (except with adding filter for frequency range, etc which I don't want to do!) and it will be close to zero (only rising edges) so it's fine. signals aren't as noisy as you made them seem.
my main question about the input still remains however:
1) what's the jfet used for?
2) Is it a good idea to include a common emitter amplifier on the input?
edit: inputs are well behaved, I don't want to discuss edge cases / where the input signal is not known / not well behaved
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