"Caiusfabricius" wrote in message
...
Hi all,
I built a simple circuit using te famous "sync separator" IC LM1881 in
order to measure the refresh rate (Vsync) of JAMMA PCBs.I follow this
scheme (but also the one form NATIONAL manufacturer is fine):

http://www.gamoover.net/tuto/schema-...gnal-composite

The problem is that when I try to measure with a frequency counter the
vertical sync output on PIN 3 of the LM1818 I obtain bad values (in order
of Khz not in Hz as it should be.Usually it should be about 60Hz)
To say that I check the circuit and is OK as well as my frequency counter
that works fine when I measure, for example, the clock of the various CPU.

Any suggestions?I'm going crazy...

P.S.
I use the VIDEO Sync (pin P of solder side of the jamma connector) as
COMPOSITE VIDEO IN (pin 2 of the LM1881)


Have you looked at the output of your sync separator chip on a 'scope to
verify that *only* field sync pulses are appearing at the pin that you are
connecting to your counter ? Is the power supply clean ? No hash or high
frequency ripple that might be appearing on the output ? Have you tried a
simple R-C low pass filter between the chip output and the frequency counter
input ? (pure off the top of my head guess, try 10k series resistor followed
by 1000pF shunt capacitor). Does your counter have appropriate sensitivity
for the level coming out of the chip ? Too little level (under-drive) or too
much level (overload) can cause counters to produce erratic and erroneous
readings. Also remember that a field sync pulse train is *highly*
asymmetric, being a series of short pulses with long gaps between. Can your
frequency counter handle that sort of waveform, and interpret it as a
'frequency' ? If the counter has a "time" function on it, you may do better
to try using this, and just work out the apparent field frequency from the
time between pulses.

Have you tried measuring the odd / even output to at least see if that gets
you in the right ballpark ? If it is an interlaced sync pulse train, then
the vertical rate should be twice the odd / even rate.


You probably need to do a bit more research as to what exactly the problem
is, before reporting back so that we can see if we can help you figure how
to make it work as you want it to.

Arfa