Frequency Input Circuit

[Suraj143]

I made a frequency counter using a PIC & its working fine.Range is 1H to 50Mhz.

For the time being I directly applied frequency to the PIC pin.This is not good.

I need an interface circuit that will couple frequency to the PIC pin safely.

Please help me.

 

[Nigel Goodwin]

Instead of asking here why just not use google?.



Is one of the early hits for "PIC frequency counter", and is a fairly good choice

 

[Suraj143]

Oh no that is too much of parts.I need to make one with discrete components

 

[Mr RB]

The PIC schmidt trigger CCP input is a good FET type input, and you can connect it to most things just using a resistor, and/or through a cap. However the signal needs to be >3.5v or so peak to peak.

Otherwise for small signals it is common to use a single FET to amplify the input wave before going to the PIC pin. Many of the freq meter projects do this, and as Nigel suggested a google search will show you heaps of schematics or good working designs.

 

[Suraj143]

The PIC schmidt trigger CCP input is a good FET type input, and you can connect it to most things just using a resistor, and/or through a cap. However the signal needs to be >3.5v or so peak to peak.

Otherwise for small signals it is common to use a single FET to amplify the input wave before going to the PIC pin. Many of the freq meter projects do this, and as Nigel suggested a google search will show you heaps of schematics or good working designs.

Very nice Mr RB I understood what you said.

I have a small question.Why most people adding a capacitor (ex:-470pF) in front of a transistor base in many frequency counters?

 

[Nigel Goodwin]

Very nice Mr RB I understood what you said.

I have a small question.Why most people adding a capacitor (ex:-470pF) in front of a transistor base in many frequency counters?

Because frequency counters count AC, so you don't need a DC connection.

The link I posted was a descrete design, you only need to use the front part.

 

[Suraj143]

ok Nigel but if I feed a square wave clock frequency to my frequency input will it count? Because most of pulse streams on workbenches are square wave not sine waves.Will I still able to measure them with that small front end cap?

 

[Nigel Goodwin]

For really low frequencies it's probably best to use DC coupling.

 

[colin55]

Nothing will couple the input to the PIC better than direct coupling when it comes to picking up high frequencies. Just use a 1k resistor and make sure the signal is not higher than 6v.

 

[hugo]

Hi Suraj,

Take a look here :

https://www.rfcandy.biz/communication/transcendent.html
https://www.rfcandy.biz/communication/pic/transchm.gif

https://electronics-diy.com/50MHz_Frequency_Meter_Counter.php

 

[Suraj143]

Ok guys here is the interface circuit I'm going to use.

Can somebody tell me whats the purpose of that 33K resister?

 

[ljcox]

The 33k provides a DC bias current to the base & it provides negative feedback that makes the amplifier a transresistance amp.

In other words, it has a low impedance input & a low impedance output due to the feedback.

 

[Suraj143]

Ok I understood.

Now I need more professional details on the signal input.

Whats the minimum voltage level needed on the incoming pulse stream etc......so this amp can amplify the signal to a known signal level that can seen by the micro controller.

What about input impedance stuff!!

 

[ljcox]

A transresistance amp needs a current input. Not a voltage input.

 

[Nigel Goodwin]

Ok guys here is the interface circuit I'm going to use.

It's really crap - what are you wanting it to do?.

 

[colin55]

I just told you that NOTHING will operate better than the input of the micro.

 

[Nigel Goodwin]

I just told you that NOTHING will operate better than the input of the micro.

Which is just as wrong as last time you said it

The essential points of input processing are to give a high input impedance and to make reading of lower level signals possible, if you have TTL level logic signals then nothing is required and the PIC input itself is fine.

 

[colin55]

I need an interface circuit that will couple frequency to the PIC pin safely.

No-where did he mention that he wanted to detect signals less than TTL level.
He just asked for "safely."

 

[Nigel Goodwin]

No-where did he mention that he wanted to detect signals less than TTL level.
He just asked for "safely."

As it's a for a frequency counter it's obviously far more likely, and thus a better assumption that it's for use with more than TTL levels - a further pointer is the requirement for 'safely'.

 

[Suraj143]

Ok guys I modified some parts.Can somebody tell me whats the purpose of that two diodes?

Thanks