Frequency meter input stage

[Ronald]

Hi,

Ive been trying to make myself a frequency counter for some time. I have
made a few attempts, but they have usually been more like random number
generators, because i cant get the input stage right. I dont have much
experience with analog electronics.

I though that before wasting my time and money on building the new
design, maybe some of you could check my latest design out and tell me
if it is OK or what should i change. The input stage schematic is at :
www.zone.ee/emzee/freq.PNG
Or any other ideas for a good input stage?

My goal would be to get it read up to 100MHz or more and have input
impedance of 1MOhm.

Ronald

 

[Andrew Holme]

Hi,

Ive been trying to make myself a frequency counter for some time. I
have made a few attempts, but they have usually been more like random
number generators, because i cant get the input stage right. I dont
have much experience with analog electronics.

I though that before wasting my time and money on building the new
design, maybe some of you could check my latest design out and tell me
if it is OK or what should i change. The input stage schematic is at :
www.zone.ee/emzee/freq.PNG
Or any other ideas for a good input stage?

My goal would be to get it read up to 100MHz or more and have input
impedance of 1MOhm.

Ronald

You've got no hysteresis on that comparator - the positive feedback via R6
is decoupled by C4.

Have you tried looking at the comparator output on a 'scope?