TTL Signal Conditioning

[olsenn]

I currently have a frequency counter made up, sitting on my breadboard, but the input currently has to be a TTL signal (0v - 5v square wave) as it goes directly into my AVR microcontroller to be sampled.

I would like to be able to input a wide range of signals, including for example, a -1v to +1v peak-to-peak sinusoid. I first tried a simple RF NPN (BF199) transistor as a switch, but that only worked up to about 100kHz (even though the datasheet says it has a current-gain product of 1100MHz and I tried various values of base/collector resistors). I then tried an op-amp comparator, but that had a performance that was about the same.

I would like to be able to convert signals from 0.1Hz to 40MHz, and with p2p voltages (for sinusoids) of as low as 1.5v - 2v. If the input voltage goes above 5v I'm sure I can just use a clipper or a zener overvoltage protection circuit.

[SeanB]

74HC14 used as a linear amplifier, one stage driving the second as a clipper. Ac couple in via 100n capacitor.

[MikeK]

How about this: