HELP:Low-pass filter on frequency counter

A simple RC filter gives only 6dB/8ve rolloff. You probably need something much sharper, which would require active filters.

"William Sommerwerck" ha scritto nel messaggio news:ihcj1s$hfc$ snipped-for-privacy@news.eternal-september.org...

Ok, thanks, I imagined this.Is it difficult to build an active low-pass filter (with a 50KHz cut-off frequency)?

It's easy to design one. But... with such a high cut-off, you'll op amps with a very high gain-bandwidth product to get a stable filter (ie, one whose characteristics don't change much with variations in component values).

Caius-

Your problem may not be related to high frequencies unless you are measuring low level, high impedance signals in the presence of high frequency electromagnetic fields.

One possibility is that you are over-driving the input to the counter. Using an attenuator may help.

Another possibility is that the signal actually varies in frequency over time.

Fred

"Fred McKenzie" ha scritto nel messaggio news: snipped-for-privacy@5ad64b5e.bb.sky.com...

My counter has a X20 button attenuaor but form manjla I didn't understand if it on channel A or B Anyway if I press down the reading become stable on low frequecies>

I think not.Because , for example, now I'm measuring the pin5 of the EL4583 (an IC sync separator) , this pin provides vertical sync output.As video input I'm using a video composite sync, so I should obtain a stable reading of 60Hz on pin 5 of EL4583.The reading is floating ( goes up and down from

60 to 63 Hz) instaed.

My counter has a X20 button attenuator but from manual I didn't understand if it's on channel A or B Anyway if I press down the button the reading become stable on PIN5 of the EL4583, may it's as you said?.This is the manual:

Anyway a friend of mine uses another counter which has a 50KHz low-pass filter and measuring the PIN5 of the EL4583 the reading is stable with the low-pass filter switched on:

From this manual:

"For Input A, a low pass filter with a cut-off frequency of 50kHz is user selectable to ensure stable readings at low frequencies"

So, I'm bit confused..do I need a low-pass filter or an attenuator to obtain stable readings on low frequencies using my counter?

Sorry for the previous post sent from 'Fabio', by mistake I used another account form another computer but I'm always 'Caius' the author of the post

Time period measurement is more accurate at low frequencies, as it reduces the timing errors. You measure the time period, then calculate the reciprocal.

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"caius"

** Errr - as you only want to measure 60Hz, why not have a LP filter at say 100Hz ??

A 10kohm resistor and a 0.15uF film cap ought to work - plus keep the input level down to something the frequency counter likes.

..... Phil

"Michael A. Terrell" ha scritto nel messaggio news:ZbSdnTlf2LOflKfQnZ2dnUVZ snipped-for-privacy@earthlink.com... > Time period measurement is more accurate at low frequencies, as it

Yes, I have this period function on my counter but I didn't understand how to use it, if you could help me.

From the spec sheet that you posted on your counter, the X20 button is not a filter; it's an attenuator. The description says that channel B has an attenuator (X1,X20), & AC/DC coupling for a low-pass filter. It attenuates the signal from the B input (selected by the function switch) by a factor of

1. According to the table at the top of the page, in order to measure frequencies below 100Hz, you should be using DC coupling. AC coupling is really a high-pass filter, which, from the table, attenuates frequencies below 100Hz. Which input are you using to measure your 60Hz sync signal? If you're tring to use the B input, of the A input with the Function set to the 2-50MHz range, then it's no wonder that you're getting erratic readings. It's a wonder that you're getting any reading at all.

From the specs on the channel A input, you should be OK to measure a high level signal (up to 30VP-P. The fact that your reading is varying from

60-63 HZ, that would indicate that your signal is mixing, or being overwhelmed by mains coupling. Try isolating the counter's ground from the power line. That brings another question... Where are you connecting the counter's signal ground? Are you relying on the power line ground for the connection? If so, that's a very bad idea. Use a scope probe or shielded cable, and connect the probe's shield to the signal ground of the circuit that you're testing. I really doubt that a low-pass filter is going to help you here, unless the signal is not clean, that is, has a lot of noise riding on it. How clean is the power supply that is powering the circuit under test? If it has a lot of ripple, that could explain your erratic readings. Make sure the Vcc rail is clean from ripple and noise.

--
David
dgminala at mediacombb dot net

Download this manual. It is for a '60s HP 5245L, but it has good desriptions on how varous functions work, and how to use them. Period measurment starts on page 22. Then ask any questions you still have and I'll try to help you. It is a scan of an old printed manaul, so I can't copy the text for you. It is a 16.1 MB file of the 1973 version of that manual.

--
You can't fix stupid. You can't even put a band-aid on it, because it's
Teflon coated.

"Phil Allison" ha scritto nel messaggio news: snipped-for-privacy@mid.individual.net... > ** Errr - as you only want to measure 60Hz, why not have a LP filter at say

Ok, but where exactly i have to put this low-pass filter?In the circuit to be measured (before the input) or between the counter and the probe?The circuit is this:

R3 and C2 in this circuit should be a simple RC circuit, right?

Did you mean the input impedance?According to the manual of my counter, Channel A has 1Mohm of input impedance.How can measure it on the circuit?

"caius"

** Your are obviously a babe in the woods.

The RC filter goes at the *input* to the counter.

A counter is a dumb animal that tries to count whatever you feed it - so if you expect to see a stable reading then make sure you are feeding it a stable SINGLE frequency.

Any signal that contains harmonics of the fundamental or hum or noise is bad news.

Look at the signal on a scope.

You have one of them - right ??

** So you have no idea of the difference between " input level" and "input impedance" ??

The woods is a dangerous place for you - pal.

..... Phil

"Dave M" ha scritto nel messaggio news:wOOdnXELXJK6i6fQnZ2dnUVZ snipped-for-privacy@giganews.com... >

According to the table, for measuring 60Hz I'm using the Channel A (from

0.01Hz to 50 MHz) with step 3 and DC coupling pressed

Yes, I'm connecting the counter ground alligator clip to the ground of the power supply (from which the circuit gets power and ground also).So signal ground of the circuit is the same of the power supply.Is it wrong?

Excuse me the ignorance but how can I verify this?I have no scilloscope...

Then you're shooting in the dark. In order to effectively troubleshoot this type of problem, you need the right equipment. At this point, I'd suggest that you Google for a PC sound card scope, There are a couple freebies on the internet that you can download. Bear in mind that they're very basic, uncalibrated, and low frequency (audio) response. An example is at

Do you have any friends, neighbors or relatives that can help? Your responses to other postings indicate that you're a novice in the world of electronics. You really need to learn more of the basics in order to understand the intracies of dealing with measuring and evaluating complex signals using basic test equipment. Google the net for tutorials on basics of circuit theory, using basic test equipment, evaluating waveforms, etc.

--
David
dgminala at mediacombb dot net

On 1/21/2011 3:30 PM caius spake thus:

Input level = input *voltage*. Forget about impedance (for the time being).

And beware Phil: he can be dangerous (or at least appear to be) if he's off his meds and you somehow offend him ...

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"Phil Allison" ha scritto nel messaggio news: snipped-for-privacy@mid.individual.net... > ** Errr - as you only want to measure 60Hz, why not have a LP filter at say

I added a RC filter with a a 10kohm resistor and a 0.15uF film (cut-off frequency at 106.2 Hz) but now I get no readings at all.

plus keep the input

The sensivity of my counter is:

"AC" 100Hz-50MHz