[Elecraft] dB

[Jim Brown]

On Wed, 13 Oct 2004 07:27:52 EDT, G3VVT at aol.com wrote:

>3dB gain reference to what?
> 
>Without a reference decibel values are meaningless. 

Horse puckey.   Without a reference they do not tell you the voltage or power in a circuit, 
but they can certainly tell you the differences between two voltages or powers, and that 
can be QUITE meaningful. For example, you are in the UK and I am in Chicago, and I 
increase my transmitter power by 4:1 (6 dB) and change nothing else. My signal at your 
location will get 6 dB stronger, independent of the reference used by the voltmeter you 
choose to measure them, and the signal to noise will also improve by 6 dB, again, 
independent of that voltmeter's reference. If you must have a reference it is the signal 
before I increased by power!  

Once you use the word "gain" you make dB quite meaningful. The most formal definition 
of gain using dB is for power -- 10 * log (P2/P1), where P2 is the output power and P1 
is the input power. But if the impedance is the same for the two powers,  the gain in dB 
is 20 * log (V2/V1). BOTH of these expressions are VERY meaningful, and no 
reference must be defined!  Or, looking at it another way, P1 (or V1) is the reference. 

Also, many circuits operate as voltage amplifiers, where power is essentially 
insignificant. Thus it is also entirely correct to speak of the voltage gain in dB, and that 
is also 20 * log (V2/V1). And, again, no reference level need be defined. 

Further, if we wanted to MEASURE the gain in dB, we could measure the voltages and 
do the math, or we could use a voltmeter calibrated to ANY voltage reference for dB, 
subtract the input voltage in dB from the output voltage in dB, and that would be the 
gain. Again, the reference is unimportant (so long as we don't change streams in mid-
horse). 

On the other hand, if we want to measure power gain, we would, indeed, need to 
consider the resistance at both input and output. 

While the telecom industry was the first to define the Bel and did so in terms of power, 
virtually all modern instrumentation that gives a readout in dB with respect to any 
reference is REALLY a voltmeter. That voltmeter may include a load resistor (with a 
switch to remove it from the circuit), or it may not. 

Hope this helps. 




Jim Brown  K9YC