What Does a VTVM Do That a DVM Can't?
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steerpike2
Super Member
DVMs sample at about 300mS intervals. On a changing signal that makes it very difficult to see what the trend of the signal fluctuations is. A moving coil meter is so much easier to interpret.
Further, most DVMs have a frequency response limit of about 1kHz, whereas a VTVM can cope with 50kHz or higher, depending on the range selected.
Further, most DVMs have a frequency response limit of about 1kHz, whereas a VTVM can cope with 50kHz or higher, depending on the range selected.
whoaru99
Epic Member
VTVM typically has very high input impedance so very sensitive circuits aren't affected by the meter becoming part of the circuit when you test it.
This was more a factor in the past than it is today with modern DVM that typically have high input impedance too. But, if you're looking at old service manuals when analog VOM was common that may be the reason.
This was more a factor in the past than it is today with modern DVM that typically have high input impedance too. But, if you're looking at old service manuals when analog VOM was common that may be the reason.
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DVMs tend to go nuts around RF sources. If you have to adjust an oscillator circuit to some specific voltage, you'll often see the DVM read voltage in ancient Egyptian where a VTVM just reads the correct value. I also find them easier to use for reading a peak vs watching the numbers. Finding zero is nice too if you're doing an FM discriminator alignment the old fashioned (aka crappier) way with a fixed frequency sig gen. You can set the VTVM for 0 at center scale and land it there. Its much more difficult to do with a DVM switching back and forth from positive to negative volts.
Basically each tool has its place and a lot of it is your preference. For some things, I much prefer a needle meter. For other things, the digital readout is easier. If you need a specific voltage or value, the digital is usually easier. I own both meters, and a regular VOM. They all get used for different jobs.
Basically each tool has its place and a lot of it is your preference. For some things, I much prefer a needle meter. For other things, the digital readout is easier. If you need a specific voltage or value, the digital is usually easier. I own both meters, and a regular VOM. They all get used for different jobs.
steerpike2
Super Member
No. Typically true-RMS is only done on expensive meters, so the byproduct is that you are getting a generally higher quality meter, but it doesn't operate any differently from the cheaper ones. It may have wider bandwidth, etc, but you still are stuck with a seven-segment display.
Retrovert
Addicted Member
Aeons ago, I posed this very question to very wise engineers with much gray hair and grumpy dispositions. When I asked it was in terms of why an FET VOM was better than a DMM, and why a VTVM would be better than either.
The upshot was exactly what steerpike2, whoaru99, and gadget73 noted. Another comment was that at the time VTVMs were still being used, the newer VOMs were not yet high-impedence, as FETs were still expensive and rare in test equipment. So that was an artifact of the time.
Two other important benefit exists.
(1) It can easier to fry a DMM when measuring tube voltages than it is to cook a VTVM. That alone is worth the price of admission.
(2) It is often far easier to read an analog meter at a glance than process a digital display. This is why I don't like digital speedometers.
VTVMs are fun things with their huge meters and clunky switches. I have my father's EICO VTVM that he assembled in the 60s and was going to get rid of when he got a brand new shiny DMM. It needs to be recapped and calibrated. I can't remember if it's that one or the 1960s Lafayette (in a bakelite case) that needs a weird mercury battery replacement voltage circuit built.
The upshot was exactly what steerpike2, whoaru99, and gadget73 noted. Another comment was that at the time VTVMs were still being used, the newer VOMs were not yet high-impedence, as FETs were still expensive and rare in test equipment. So that was an artifact of the time.
Two other important benefit exists.
(1) It can easier to fry a DMM when measuring tube voltages than it is to cook a VTVM. That alone is worth the price of admission.
(2) It is often far easier to read an analog meter at a glance than process a digital display. This is why I don't like digital speedometers.
VTVMs are fun things with their huge meters and clunky switches. I have my father's EICO VTVM that he assembled in the 60s and was going to get rid of when he got a brand new shiny DMM. It needs to be recapped and calibrated. I can't remember if it's that one or the 1960s Lafayette (in a bakelite case) that needs a weird mercury battery replacement voltage circuit built.
