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HELP:Measuring refresh rate of PCBs
#242553 - 01/04/11 01:14 AM
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Hi all,
I built a simple circuit using the 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-de-m...ignal-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)
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italie |
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MAME owes italie many thank yous, hah
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Re: HELP:Measuring refresh rate of PCBs
[Re: caius]
#242587 - 01/04/11 04:22 AM
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> Hi all,
> I built a simple circuit using the 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-de-m...ignal-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)
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?
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Re: HELP:Measuring refresh rate of PCBs
[Re: italie]
#242632 - 01/04/11 03:55 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.
Edited by caius (01/04/11 05:57 PM)
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MAME owes italie many thank yous, hah
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Re: HELP:Measuring refresh rate of PCBs
[Re: caius]
#242641 - 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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Re: HELP:Measuring refresh rate of PCBs
[Re: italie]
#242649 - 01/04/11 08:01 PM
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> 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
Ok thanks, I'll try filtering the input..but capacitors must be polarized or normal ceramic?
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Re: HELP:Measuring refresh rate of PCBs
[Re: caius]
#242659 - 01/04/11 08:40 PM
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> > 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
>
> Ok thanks, I'll try filtering the input..but capacitors must be polarized or normal
> ceramic?
Ceramic is fine.
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Re: HELP:Measuring refresh rate of PCBs
[Re: italie]
#242775 - 01/05/11 10:38 PM
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I have tried adding the resistor (670 ohm, 620 ohm doesn't exist) and capacitor (510pF) as in the schematic of the datasheet but nothing has changed.The only thing I noticed is if I use the "attenuation X20" button on my frequency counter the reading from the PIN3 becomes stable and not more floating.
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Re: HELP:Measuring refresh rate of PCBs
[Re: caius]
#242777 - 01/05/11 10:50 PM
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> I have tried adding the resistor (670 ohm, 620 ohm doesn't exist) and capacitor
> (510pF) as in the schematic of the datasheet but nothing has changed.The only thing I
> noticed is if I use the "attenuation X20" button on my frequency counter the reading
> from the PIN3 becomes stable and not more floating.
So you no longer expirience any issues?
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Re: HELP:Measuring refresh rate of PCBs
[Re: italie]
#242780 - 01/05/11 11:09 PM
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> So you no longer expirience any issues?
The reading on PIN3 of the LM1818 is stable now but it depends from the attenuation I applied and not from the filtering with the resistor and capacitor.Infact I tried another same circuit without capacitor and resistor and the problems remains without the attenuation;with the attenuation it disappears but I don't know if the reading is right though it's stable.
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| italie |
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Re: HELP:Measuring refresh rate of PCBs
[Re: caius]
#242782 - 01/05/11 11:22 PM
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> The reading on PIN3 of the LM1818 is stable now but it depends from the attenuation I
> applied and not from the filtering with the resistor and capacitor.Infact I tried
> another same circuit without capacitor and resistor and the problems remains without
> the attenuation;with the attenuation it disappears but I don't know if the reading is
> right though it's stable.
So the obvious answwer seems to be "use attenuation". Would appear to be measureing equipment setup then, rather than the compnents being measured.
Why do you assume the readings aren't correct if they are now stable? I'm now a little unclear on what exactly the problem is that you are trying to solve.
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Re: HELP:Measuring refresh rate of PCBs
[Re: italie]
#242784 - 01/05/11 11:34 PM
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> Why do you assume the readings aren't correct if they are now stable? I'm now a
> little unclear on what exactly the problem is that you are trying to solve.
Because on some PCBs I obtain different values from the ones already in MAME
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| TheGuru |
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MAMEDev Dumper
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Re: HELP:Measuring refresh rate of PCBs
[Re: caius]
#242832 - 01/06/11 09:39 AM
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> > Why do you assume the readings aren't correct if they are now stable? I'm now a
> > little unclear on what exactly the problem is that you are trying to solve.
>
> Because on some PCBs I obtain different values from the ones already in MAME
the LM1881 is not the best option. use a EL4583 sync splitter from Elantec and make the circuit shown in the pdf for the dev board they also sell. add a low-pass filter and you'll get the same readings as MAME (+- 2.0% depending on your equipment)
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Re: HELP:Measuring refresh rate of PCBs
[Re: TheGuru]
#242835 - 01/06/11 10:47 AM
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> the LM1881 is not the best option. use a EL4583 sync splitter from Elantec and make
> the circuit shown in the pdf for the dev board they also sell. add a low-pass filter
> and you'll get the same readings as MAME (+- 2.0% depending on your equipment)
Hi Guru and thanks for replying.
Are you referred to this demo board ciruit:
http://www.intersil.com/data/tb/tb430.pdf
I have to build both circuits?
Is it the same EL4583CS-EVAL part that they sell? (but now it's not available and seems to cost a lot)
Please, do you know where I can find it?
Thanks
Edited by caius (01/07/11 01:11 AM)
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| TheGuru |
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MAMEDev Dumper
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Re: HELP:Measuring refresh rate of PCBs
[Re: caius]
#242985 - 01/07/11 09:49 AM
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yes that's it. just build the circuit on the lower left of page 2
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Re: HELP:Measuring refresh rate of PCBs
[Re: TheGuru]
#242999 - 01/07/11 12:20 PM
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> yes that's it. just build the circuit on the lower left of page 2
Ok, thanks Guru.Last things:is the low-pass filter already included in this circuit?
If you look at the circuit the 'C2' capacitor has no value, instead has an 'OPEN' writing.What does it mean?What is his value?
Edited by caius (01/07/11 10:21 PM)
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