HELP:Measuring refresh rate of PCBs

May 22, 2010 2 Replies
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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):



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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)


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What kind of counter are you using? What are its trigger voltage levels (adjustable or fixed)? Have you monitored the input and output signals with a scope while you have the counter connected? What do the signals look like (voltage levels, noise, etc.)? Have you adeqquately decoupled the LM1818 power supply pins?

David dgminala at mediacombb dot net
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"Dave M" ha scritto nel messaggio news:q8GdndOC6PEx_2XWnZ2dnUVZ snipped-for-privacy@giganews.com... > What kind of counter are you using? What are its trigger voltage levels > (adjustable or fixed)?

I use this counter:

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I don't have a scope

Have you adeqquately decoupled the LM1818

What did you mean for 'dcoupled'?Adding capacitors? No, ground and power pins are directly connected to the ground and +5 Volt of my JAMMA test rig.

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