italie |
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MAME owes italie many thank yous, hah
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Reged: 09/20/03
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Posts: 15246
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Loc: BoomTown
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Re: HELP:Measuring refresh rate of PCBs
01/04/11 06:11 PM
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> > Sounds like Hsync values.... What PCB are you testing? Do these results show up on
> > multiple PCBs?
> >
> > What does pin 7 on the LM1881N read?
>
> I'm testing various PCB, I'm using two different frequency counter, an assembled kit
> and a low-cost professional counter (model VC3165 from Victor).In both I have bad or
> floating reading on PIN3 of the LM1881 IC.In this moment I'm testing a Legendary
> Wings bootleg PCB and I obtain on pin3 of LM1881 a floating value from 55 to 60 Hz.I
> thought V-Sync must be stable signal or I'm wrong?
>
>
> P.S.
> The VC3165 counter has also a 'period' function (instead of normally used 'frequency'
> function) and a 'gate time' knob but I don't know how to use them.
Floating values could be a function of the PCB, tthe quality of the input signal, the connections, the impedance/capacitance of the probe wire, improper coupling/grounding...etc.
If you don't have access to a scope, I'd start by filtering the input pin as shown on page 7 (below) with a 620 Ohm resistor and a 510pf capacitor to make sure your imput is clean.
http://www.national.com/ds/LM/LM1881.pdf
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