Measure signal in dBm

[chongwui]

I noticed in some of the older Boss service manual, the input/output signal is mentioned in the unit of "dBm". May I know what tool can I use to measure signal in this unit? Is there some sort of "dBm meter" exists?

[Manfred]

In this case dBmV is meant by dBm.
So you can convert the dBmV values into voltage values that can be measured with an audio voltmeter or with a True RMS voltmeter.
Audio voltmeters usually have an additional dBmV scale.
The True RMS voltmeter must be suitable for the audio frequency range.
Online dB to voltage conversion calculator:
https://www.antennas.ca/calc_db_volts.htm

[george giblet]

When I see dBm (as opposed to dBmV) I read it as dB relative to a reference *power* level of Pref = 1mW,

dBm = 10*log10(P / Pref) = 10*log10(P / 1mW)

For audio we actually want voltages and we can't convert voltage to mW unless we know the reference impedance. For audio the reference impedance is 600 ohm but for RF 50 ohm is more common. When specifying voltages with dBm you
use 600 ohm for audio even when the actual circuit is 10k or 100k, or whatever. You would only use 50 ohm for audio if it is clear this is the reference impedance.

P = Vrms^2 / Rref

and,

Pref = Vref_rms^2 / Rref ,

For 1mW and 600 ohm, the reference level for the dBm is,

Vref_rms = sqrt(Pref * Rref) = sqrt(1mW * 600) = 0.7745 V rms

So now an rms voltage V can be computed in dBm as,

dBm = 20*log10(V / Vref) = 20 * log10(V/0.7745)

When you are given dBm you can calculate the corresponding rms voltage as,

Vrms = 0.7745 * 10 ^ (dBm/20)

For example when Boss documents quote a -10dBm signal level then that means a voltage of,

Vrms = 0.7745 10^(-10/20) = 0.7745V * 0.3162 = 0.2449V rms ; in loose terms 250mV rms

FWIW, for RF and a reference impedance of 50 ohms we would get different voltages for Vref and for the voltage corresponding to -10dBm.

As it happens there's a calculator for dBm to volts,
https://coretechgroup.com/dbm-calculator/

[Manfred]

There are different reference levels for 0db.

0 dBu: Uref = 0.775 Vrms is the voltage that converts 1 mWeff of power at 600 ohms.
This reference level of 0.775 Vrms can be found in studio technic and telecommunication.

0 dBV: Uref= 1 Veff
0 dbmV: Uref = 1mVeff
0 dbuV: Uref = 1uVeff

I assume that these values in the schematic are meant independent of the 600Ohm terminating resistor and thus the reference voltage level is 1 Vrms.

[george giblet]

I assume that these values in the schematic are meant independent of the 600Ohm terminating resistor and thus the reference voltage level is 1 Vrms.

dBm in audio is essentially dBu (ref voltage 775mV rms)

For dBm, the 600 ohm reference is an "artificial" scaling factor. The 600 ohm value is used for the reference impedance in calculations even though there's no termination or there may even be some other impedance in the actual circuit. It's kind of a weird thing to do but that's how it is done. It makes no sense physically as far as the circuit is concerned. (I don't like it or agree with its use but somewhere in the past it was defined like that.)

https://duckduckgo.com/?q=dBm+in+audio& ... t=1&ia=web

https://www.soundonsound.com/sound-advi ... -explained

[mauman]

For dBm, the 600 ohm reference is an "artificial" scaling factor. The 600 ohm value is used for the reference impedance in calculations even though there's no termination or there may even be some other impedance in the actual circuit. It's kind of a weird thing to do but that's how it is done. It makes no sense physically as far as the circuit is concerned. (I don't like it or agree with its use but somewhere in the past it was defined like that.)

The power of Bell Labs in setting defacto standards! 600 ohms was the nominal Z for twisted pair copper trunk circuits between telecom switching offices. Amplifying those for long-distance voice traffic was an early application of setting levels, which required standards.