Hi,

I'm trying to piece together a frequency counter using a circuit from the MC14553 datasheet and a page from a Forrest Mims book. The schematic of what I have so far is attached. The purpose of the frequency counter is to use it with my signal generator so I don't have to use my scope to get an approximate frequency.

I'm not to sure about the frequency input. As shown the CD4011 may switch as 0.3*VDD or 0.7*VDD so it may not count a 1V p-p sinewave. Surely there needs to be some sort of comparator at the input, with as little hysteresis as you can get away with to make it as sensitive as possible? Or, since my sinewave generator sucks anyway and I'm going to redo it, build a rail to rail output in it to be used by the counter. Is that a common thing to do or should this problem be solved in the counter?

As an aside, how can we find the scan frequency that is set by C13?

 

The display multiplex frequency set by C13 is non-critical.

 

The purpose of the frequency counter is to use it with my signal generator so I don't have to use my scope to get an approximate frequency.

What frequency range are you expecting it to work for? At 5V, you'd be lucky to get more than 1MHz.

I'm not to sure about the frequency input. As shown the CD4011 may switch as 0.3*VDD or 0.7*VDD so it may not count a 1V p-p sinewave.

1V peak to peak as in centered about ground? Whatever the case, you'll likely violate the maximum rise time for CD4011. The counter has a Schmitt input, so it will tolerate slower edges, but you still need an appropriate voltage swing.

As an aside, how can we find the scan frequency that is set by C13?

You can measure it with your scope.

 

since my sinewave generator sucks anyway and I'm going to redo it, build a rail to rail output in it to be used by the counter.

Yes, that would simplify the counter since you wouldn't have to worry about the signal amplitude (it would even work down to a 0V output setting.

 

A 4069UB wired as an amplifier (10M resistor from output to input, and 10k resistor and 1uF capacitor in series with the input) would make a good input amplifier - it would work with both logic and sinewaves. You can use the rest of the 4069 as buffers to make the amplified signal into a nice squarewave, or you could wire a pair of them as a Schmitt trigger -
very similar to the amplifier circuit, except that the 10M resistor goes around a pair of inverters.

 

What frequency range are you expecting it to work for? At 5V, you'd be lucky to get more than 1MHz.

I will probably use as 12V supply. With a 1 Hz clock and 6 digits a 1 MHz limit is fine. Yes, I would think I need to bias the input to mid rail.

A 4069UB wired as an amplifier (10M resistor from output to input, and 10k resistor and 1uF capacitor in series with the input) would make a good input amplifier - it would work with both logic and sinewaves. You can use the rest of the 4069 as buffers to make the amplified signal into a nice squarewave, or you could wire a pair of them as a Schmitt trigger -
very similar to the amplifier circuit, except that the 10M resistor goes around a pair of inverters.View attachment 234532

I like that idea! I'll test it on the breadboard and consider this vs a rail-rail output on the signal generator.