Octavio Nogueira wrote:
>
> >The posted circuit is incomplete, for a more sensitive frequency
> >counter, it needs a pre-amplifier and convert its output to TTL level.
>
> Does anyone have a nice and simple pre-amplifier schematic for this kind
> of application?

I'll try on ascii:

                       +---------+---O +5V
                       |         |
                      3K3       1K
                     ohms      ohms
                       |         |
                   +---+         +-----+
                  +|   |         |    +|
             2.2uF =  10K       820    = 2.2uF
                   |  ohms      ohms   |
                  GND  |         |    GND
                       |         |                  OUT
                       |         +-----||----+----O TTL
                       |         |    10nF   |
IN  10nF         10nF  |       |/            |
O----||----+--+---||---+-------|   MPSH10   10K    adjust for DC
           |  |        |       |\           Ohms   level be in the
   1N4148  V  -       3K3        v         trimmer middle of TTL
     X2    -  ^       ohms       |           |     thresholds
           |  |        |         +----+      |
         GND GND      GND        |    |     GND
                                120   |+
                                ohms  = 10nF + 2.2 uF in parallel
                                 |    |
                                GND  GND


This circuit could work from to 0.02-50 MHz.

Reggie