On Wed, 13 Oct 2004 07:27:52 EDT, [EMAIL PROTECTED] 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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
