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Of the analog and digital meters, which one should be a preferred choice when...

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PG1995

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Hi

Of the analog and digital meters, which one should be a preferred choice when dealing with high frequencies? Suppose that analog meter uses permanent magnet for the field instead of field coils. I think digital meter would be more accurate because the analog one would be greatly affected by inductive effects and hysteresis losses. Please let me know. Thank you.

Regards
PG
 
Analog meters have faster response so you might see some transients better and are easier to see the approximate value in a quick glance.
 
Of the analog and digital meters, which one should be a preferred choice when dealing with high frequencies?
What an impossible question!
First of all what are "high frequencies".
Talk to a power engineer and high frequencies are 400 or 1000hz as used on aircraft.
Talk to an audio engineer and 20khz is high frequency.
Talk to a radio engineer and high frequencies are defined as 3 to 30 Mhz.

Suppose that analog meter uses permanent magnet for the field instead of field coils. I think digital meter would be more accurate because the analog one would be greatly affected by inductive effects and hysteresis losses.
Most (but not all) analogue meters are moving coil permanent magnet types.

Assuming that we are talking about voltage and current meters here, when measuring AC of any frequency, the AC is rectified to DC before being used the drive a moving coil meter.

I think that you need to refine your question if you want some specific answers.
I can't help but think that you are suffering from too much theory and not enough exposure to practical real world equipment.
(But don't stop with the questions, keep them coming).

JimB
 
Thanks for the replies.

What an impossible question!
First of all what are "high frequencies".
....

Most (but not all) analogue meters are moving coil permanent magnet types.

Assuming that we are talking about voltage and current meters here, when measuring AC of any frequency, the AC is rectified to DC before being used the drive a moving coil meter.

Sorry. I had only voltage and current meters in mind. In other words, if AC's frequency was 1000 Hz instead of 50/60 Hz which one should be a better choice? I hope it's clear now. Thank you.

JimB said:
I can't help but think that you are suffering from too much theory and not enough exposure to practical real world equipment.
(But don't stop with the questions, keep them coming).

Yes, you are right. I still try to do my best to make sense of everything. And with the help of guys like you I have progressed somewhat. Thanks.

Regards
PG
 
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PG:

Let's say you were buying a multimeter for use on an Aircraft whose power is 400 Hz. What might you use there?

On a different note, what if you were interested in Audio frequency response 0-100 Khz?
 
Hi KISS

On an aircraft I believe using a digital meter would be a wise choice because there are going to be vibrations, oscillations etc. which makes it difficult for the analog to produce reliable results. Thanks.

Regards
PG
 
Hi,

Who here remembers the old VTVM's of the past? They could do audio.

Most of today's meters are made for line frequencies. Meters that can be used on higher frequencies will be more expensive.

My analog meters might do audio but they were not made for that so i'd have to check, but one of my digital meters has calibration data for much higher frequencies up above 100kHz. So it really depends on the meter. Which one would i use here for say 20kHz? I'd use the digital because it has calibration data for frequencies even higher than that.
 
Hi

I should have mentioned this earlier. Anyway, I was actually asked this question that between of the two meters, analog and digital, which you would use for high frequencies. My choice was digital meter because an analog one uses a coil which is an inductor and for an inductor reactance X_L=2*pi*f*L. Therefore, as the frequency goes high inductive resistance increases; moreover hysteresis loss also increases. Just to be on safe side I also stated that eddy current loss would also increase!

Regards
PG
 
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Anyway, I was actually asked this question that between of the two meters, analog and digital, which you would use for high frequencies.

Who asked you that question?
The question is completely misguided and irrelevant.

Generally speaking, the form of the display has no relevance to the quantity being measured.
As has been alluded to here already, AC is rectified before it gets anywhere near the meter movement.
The VTVM (Vacuum Tube Volt Meter) of days gone by used a moving coil meter and would measure AC from a few Hz to many MHz depending on the model.

Have a look at these links to see some examples produced by one of my former employers:

https://sites.google.com/site/marconiinstrumentslimited/tf2600-sensitive-valve-voltmeter

https://sites.google.com/site/marconiinstrumentslimited/tf2603-rf-electronic-millivoltmeter

JimB
 
But does this mean my reasoning doesn't carry any merit in your view?
What you said is quite correct.
But, the basis of the discussion is pointless.
If you were looking at the frequency response of a moving iron meter, or the electro-dyno thingy discussed in another thread where the field coil is excited by the supply, as is the moving coil, then yes there is merit in what you say.

But to try and compare a moving coil meter with a digital display meter is like trying to compare a car (automobile) with a boat, which one is best?
A car is faster than a boat, but if you want to go across the water, what good is a car?

Just because a meter is digital, does not mean it will work better at high frequencies that the moving coil meter.

JimB
 
Hello again,


This question is really an old question which originated back when there were very few types of digital meters out there, right around maybe when the digital meters just started coming out becoming more popular.

Today the only answer is that you have to have the right meter for the job.

The difference today between the analog and digital is mostly that the analog can average measurements and provide a quick reading, and show some overshoot or general short variation a little better than a digital which would either not show anything or just a bouncing digit. But today's digital meters are so advanced it's hard to say anything negative about them except that they might not be that cheap if you want it to do everything. Some even have a peak readout now which would show a quick transition.
Of course the big advantage to using digital is the accuracy and resolution. I can read 1.234 volts easy on all my meters but with the analog ones im lucky if i can read 1.2 or so. On one meter i can read 4.9876v, try that with an analog meter without an adjustable calibration standard source.

Two of my digital meters have a 'relative' setting, where you effectively can null out a reading so that all future readings are what they are minus the reading when you nulled. So for example when measuring a 4.123v battery with a meter with a max count of 3333 it would degenerate to a readout of 4.12 (cant use the first digit because it only goes up to 3) but if i null it with 1.000v then it reads 3.123v which can use all four digits, so i dont loose any resolution. To do this with an analog meter we'd have to have a battery laying around we could connect in series with the point under test (if it was a voltage measurement of course).

One place where i really like an analog is for the car voltage. When starting the engine i can quickly get an idea how the battery is behaving (ie getting old or what) by watching the meter needle while i start the car. If it dips too low the battery is getting old.
With most digitals it would be too fast to measure. Yeah the more expensive ones, but i dont want to have to buy that for the car just for the battery.
 
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I was actually asked this question that between of the two meters, analog and digital, which you would use for high frequencies. My choice was digital meter because an analog one uses a coil which is an inductor and for an inductor reactance X_L=2*pi*f*L. Therefore, as the frequency goes high inductive resistance increases; moreover hysteresis loss also increases. Just to be on safe side I also stated that eddy current loss would also increase!
Measuring AC (fast AC) with a meter:
If you meter is set to DC it will read zero. Meters have a averaging function. The average of a sign wave is zero. In a digital meter the average may be a RC. A analog meter finds average by the L of the meter and by the slow movement of the meter.
If you meter is set to AC there is a diode, or four diodes, or diodes and op-amp to turn AC into DC.
In the case of simple diodes: Input to the meter is 100V sign wave, peak to peak is -141 to +141. After the diodes the peak to peak is 0 to 141. The average is about 100V. (If the frequency is slow enough that the diodes are working correctly, OR, slow enough that the op-amp+diodes rectifier is working correctly, then the meter part of the volt meter will see DC. The DC has ripple but the meter will find the average of the DC.

For high frequency I use a "RF Probe" where fast diodes. The output of the probe is DC.

The only time I can think of where "AC" is a problem is when a voltage (AC or DC) changes value randomly. If you have a voltage that changes from 100 to 128 and back several times a second. Many digital meters have a real problem displaying this. They might display 100 on top of 128 and the display will look like 188. While an analog meter will dance between about 100 and 130, more or less. (or may 110 and 120) Watching the meter shake gives you the idea the voltage is not constant. Displaying 188 is not right! I have several meters with LCD displays that display numbers and have a bar graph on the side or across the top , much like a mechanical meter. This way you get the best of both worlds.
 
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I wanted to provoke the concept of the right meter for the right job. In reality I wanted PG to search for a product that:

1. One might use on airplanes
2. One might use for audio testing.

A. An industrial DVM. This one is messy which is why I didn't include it.
B. An automotive tech
C An HVAC repair person

Unfortunately, you may not know enough about the disciplines to know what you need, I know little about airplanes, but I do know the power frequency is 400 Hz and DC. I'm not well versed in automotive or HVAC, but I am EPA Certified in automotive AC/small appliance.
 
What you said is quite correct.
But, the basis of the discussion is pointless.
If you were looking at the frequency response of a moving iron meter, or the electro-dyno thingy discussed in another thread where the field coil is excited by the supply, as is the moving coil, then yes there is merit in what you say.

But to try and compare a moving coil meter with a digital display meter is like trying to compare a car (automobile) with a boat, which one is best?
A car is faster than a boat, but if you want to go across the water, what good is a car?

Just because a meter is digital, does not mean it will work better at high frequencies that the moving coil meter.

JimB

Hi

This has really helped me. When the instructor returned the answer sheets, he had awarded me zero marks for my answer to the question. But as I knew my reasoning carried a little bit weight so I discussed with him what I had written and also said to him that the question was an open ended one. At the end, I only said this, "Don't you think there is a merit in my reasoning too?". Well, then he awarded me 65% marks for the question.:) Thank you.

By the way, he was telling me that it has something to with biasing etc. In his view, analog meter should be used. Well, it could be his personal 'scientific' opinion!

Regards
PG
 
Like JimB said, the real problem here is that you (or at least your instructor) is trying to make to broad of a generalization. Meters, like everything else, need to be chosen based on the specific characteristics of the meter itself.

Whether analog or digital, a meter that is designed and optimized for RF work, will be more accurate in its reading of RF values, than a meter designed for DC, or even line frequencies.

It sounds like your instructor views the 'right' answer to be based whatever point he made in last weeks lecture on the subject.
 
It sounds like your instructor views the 'right' answer to be based whatever point he made in last weeks lecture on the subject.

Rule #1. What gets you points is spitting out what the instructor wants which may not be the correct answer. It depends on your knowledge. I told you this long ago.

Suppose I displayed an analog dial on a computer screen and a digital readout and I got my data from a D/A converter. the program I used to do this is LabVIEW. Are these analog or digital meters?

They are no more than an HMI or Human Machine Interface.

Some of the really nice Analog meters or D'Arsonval based meter movements have a mirror scale, so that when the human reads the mirror, their is no reflection of the pointer in the mirror. It avoids paralax.
 
Hi

KeepItSimpleStupid said:
Rule #1. What gets you points is spitting out what the instructor wants which may not be the correct answer. It depends on your knowledge. I told you this long ago.

It wasn't that long ago! :) Actually when I read ChrisP58's reply, it reminded me of what you had suggested me not very long ago.

Best wishes
PG
 
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