Hi Everyone,
This is my first post here!
I've been working on making an AVR based reciprocal frequency counter. I'm using a MC74AC4040N and MC74AC132N to be able to support frequencies of 0.5 Hz to 100 Mhz. One of my goals is to have the same precision whether I am measuring a fast or slow signal which is why it is a reciprocal type of counter. The digital part of this works very well.
Analog I am weak in. What I have been trying to do is come up with a decent analog input stage for it. I would like it to be able to handle small or large signals such as 50-100 mV rms on the low end and mains on the high end. It also needs to handle slow (0.5 Hz) and fast (100 MHz).
I've had some decent results using the input stage from this (upper left of it up to C5):
But, it doesn't amplify enough and I don't think it handles higher frequencies well (>40 Mhz).
I've also tried a MAX997 ultra fast comparator and again, analog being weak for me, I was hoping it would be a magic solution only to find that my experience with it on the breadboard was also with its issues. It got much better after I connected a cap across the +/- inputs like the datasheet said was sometimes necessary, but the noise from its switching still introduces quite a bit of noise on the source signal causing issues.
I know I'm working on breadboards with parasitic capacitance, and perhaps I am trying to do too much. What I would love to have is a simple input stage with as few components as possible that can handle a wide range of signal strengths and frequencies...
At this point, I'm trying to back away from it and rethink what my goals are.
I've come up with:
In a perfect world, I want:
no loading of the signal (very high input impedance)
to be able to work with even small signals such as 50 mV RMS
to be as tolerant as possible of noise so it doesn't affect the measurement
not to introduce noise into the signal
handling of very slow (0.5 Hz) to very fast (100 MHz) signals
output is very fast and clean switching and switching from 0V to 5V
Ideas I have that I think I need to:
perhaps use a low pass filter that removes high frequency noise (>100Mhz).
strong enough hysteresis that noise on a signal will not cause false changes
What do you guys recommend?
Thanks,
Alan
This is my first post here!
I've been working on making an AVR based reciprocal frequency counter. I'm using a MC74AC4040N and MC74AC132N to be able to support frequencies of 0.5 Hz to 100 Mhz. One of my goals is to have the same precision whether I am measuring a fast or slow signal which is why it is a reciprocal type of counter. The digital part of this works very well.
Analog I am weak in. What I have been trying to do is come up with a decent analog input stage for it. I would like it to be able to handle small or large signals such as 50-100 mV rms on the low end and mains on the high end. It also needs to handle slow (0.5 Hz) and fast (100 MHz).
I've had some decent results using the input stage from this (upper left of it up to C5):
But, it doesn't amplify enough and I don't think it handles higher frequencies well (>40 Mhz).
I've also tried a MAX997 ultra fast comparator and again, analog being weak for me, I was hoping it would be a magic solution only to find that my experience with it on the breadboard was also with its issues. It got much better after I connected a cap across the +/- inputs like the datasheet said was sometimes necessary, but the noise from its switching still introduces quite a bit of noise on the source signal causing issues.
I know I'm working on breadboards with parasitic capacitance, and perhaps I am trying to do too much. What I would love to have is a simple input stage with as few components as possible that can handle a wide range of signal strengths and frequencies...
At this point, I'm trying to back away from it and rethink what my goals are.
I've come up with:
In a perfect world, I want:
no loading of the signal (very high input impedance)
to be able to work with even small signals such as 50 mV RMS
to be as tolerant as possible of noise so it doesn't affect the measurement
not to introduce noise into the signal
handling of very slow (0.5 Hz) to very fast (100 MHz) signals
output is very fast and clean switching and switching from 0V to 5V
Ideas I have that I think I need to:
perhaps use a low pass filter that removes high frequency noise (>100Mhz).
strong enough hysteresis that noise on a signal will not cause false changes
What do you guys recommend?
Thanks,
Alan