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# Simple Frequency Counter

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#### chingyg

##### New Member
I'd like to build a very simple frequency counter for VHF (about 100MHz) I have seen this design on the net.
but the maximum range of this is only 25MHz, Is there a way to maybe divide the time base, or the input frequency or maybe to change the Reference crystal to make this device able to measure frequency up to 120MHz?

hi,
You could divide the incoming frequency by 10, the freq resolution would be poor.

The 74F74 series will clock at approx 125MHz, look at the 74F series devices.

EDIT: 74F74 at the front end , followed by say, a 74F163

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• 74F163.pdf
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Last edited:
I only want a resolution of about 500kHz, not that fine.

So I must just change the 74HC4060 and the 74HC4040 to
74F74 , 74F163.

Is that all, I am a newbie in electronics, and how would you divide a frequency by 10?

Edit: but the 74F163 is only a 4 bit counter, isnt that too small?

Last edited:
I only want a resolution of about 500kHz, not that fine.

So I must just change the 74HC4060 and the 74HC4040 to
74F74 , 74F163.

Is that all, I am a newbie in electronics, and how would you divide a frequency by 10?

Edit: but the 74F163 is only a 4 bit counter, isnt that too small?

hi,
Not that change.

You have the 74F dividers before the rest of the existing circuit.

So the input to the existing circuit has been divided by 10.

Whats the source of the 100MHz signal.?

hi,
Not that change.

You have the 74F dividers before the rest of the existing circuit.

So the input to the existing circuit has been divided by 10.

Whats the source of the 100MHz signal.?

Why, does it only work well for square waves?

Last edited:

So its a low level 100Mhz signal.

It will need to be amplified by a suitable transistor before it clocks the 74F74.

The 74F74 requires a 5V TTL level.

I could use the final output signal of the LC resonator, it is amplified by 2 transistors, around about 500mW. Is that strong enough?

I could use the final output signal of the LC resonator, it is amplified by 2 transistors, around about 500mW. Is that strong enough?

hi,
Its the voltage levels of +5V equal to peak of the RF sinewave and the 0V equal to the zero crossing point of the sinewave, with the negative swing removed.

So effectively we require a 5v/0v square wave for the 74F inputs. OK

It's probably also a good idea to match the impedance of the frequency counter's input to that of the coaxial cable or whatever you're driving it from to avoid miscounts caused by reflections in the unmatched line.

I was gonna say...a prescaler. The most common are the dual modulas prescalers. They just divide it down to something workable at HMOS or TTL speeds. The selectable modulas is used to change the divider ratio such as the one in the above document which looks like a tri-modulas.

Anyway, I am getting ready to build one myself and I was curious as to the scheme you are using??? Are you using a PIC? I don't really need to go above 30MHz and I am planning to use a PIC. I looked and looked for the maximum frequency I could input to the port change interrupts on the mid range PICs. Say I'm using a 20MHz clock. Anybody have any ideas what the frequency might be? Roughly speaking, just a ball park figure.

You don't connect the incoming freq to the port change interrupts, you connect it to the TMR1 external clock input so the incoming freq increments TMR1. Then when TMR1 overflows, you make an int and you compare that period against another timer like TMR2 which is clocked by the PIC main oscillator. That's the way most of those one-chip PIC frequency meters do it as the incoming frequency is only limited by the TMR1 external clock max freq.

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