dB Measurement.

[lord loh.]

Here is a multimeter that measures gain in dB. (https://aplab.com/tmi/1087.htm)

Now, gain in dB is measured with respect to a reference. I have never seen a multimeter measuring gain in dB. Can anyone tell me how the reference is set. I have seen analog meters with d'Arsonval movements having dB caliberated scales. What is the standard load against which the dB is measured?

:? :? :?

 

[Nigel Goodwin]

Here is a multimeter that measures gain in dB. (https://aplab.com/tmi/1087.htm)

Now, gain in dB is measured with respect to a reference. I have never seen a multimeter measuring gain in dB. Can anyone tell me how the reference is set. I have seen analog meters with d'Arsonval movements having dB caliberated scales. What is the standard load against which the dB is measured?

There is no 'standard', dB is just a ratio, you can reference it to anything you like. However, for audio purposes, 0dB is often assumed to be 0.775V into 600 ohms - I think?.

We were told years ago at college, that "only people who don't know what they are talking about talk in dB's" - I've never found any reason yet to disbelieve that statement!.

Incidently, a while back now I was playing with using a PIC to replace the LM3914/5 chips. By using a 16F819 (or now the 16F88), you could create a PIC that could replace both of them, and even make it two channel (for a stereo VU meter). I wrote a Delphi program that generated the required log values to give dB readings from the A2D readings.

 

[JimB]

The Decibel is a ratio. The original idea was to be able to express the ratio between two powers in a logarithmic manner.
But, if you define the reference point, it is possible to use dB as a unit.

For audio signals it is usual to express the power with respect to 1mW, usually in a 600 ohm matched line.
Look at the picture of the meter on the Marconi Distortion Factor Meter, 0dBm is equivalent to 0.775 volts in 600 ohms. (Just as Nigel said).

For RF siganals the usual reference point is 1mW in 50ohm. Also referred to as 0 dBm. Be carefull not to mix them up!

Look at the pictures of the meter on the signal generator and the range controls on the spectrum analyser, they all have dB scales. The screen of the spectrum analyser (not shown) is also calibrated in dB.

So, to follow the argument placed by the lecturer at Nigels college, the engineers at Hewlett Packard and Marconi Instruments dont know what they are talking about.
Mmmm... let me think about that for a bit.

Post edited with better quality pictures.

JimB