[R-390] R390 Dave N3DT, ask and then what is the input Z of the IF?

[Jacques Fortin]

Hi Dave,

A "standard" signal generator adjusted for, say, 150µV output really behaves as a 300µV source with a series resistance (output impedance, if you like) of 50 ohms.
When another 50 ohms resistance (load) is connected to the output of the generator, 150µV will then appear across because of the voltage division between the two 50 ohms resistors.
And, btw, 150µV related to 300µV is 6 dB lower (20 x log (V1/V2))
In this case, we can tell that the generator output is adjusted to either 150µV (terminated in 50 ohms) or 300µV EMF (terminated in whatever impedance you want).
Of all the RF generators I have around, only my HP 8657A can be set at a given value of EMF voltage, all the others gives the output value as the terminated (in 50 ohms) voltage.

dBm is another way to specify the generator output, but valid only when it is terminated in 50 ohms.
0 dBm (one milliwatt) in 50 ohms is 223.6 mV.
And the level is then expressed in dB from that reference, in dBm (dB relative to 1 mW).

There was (in the old times) signal generators that were designed for 75 ohms output impedance, like the Marconi TF-955 series, for which there was a "termination pad" to transform the output impedance to 50 ohms for North-American use...

And yes, you can specify an output in V p-p EMF: that will be 2.828 x the EMF rms value, open circuit, or 5.657 x the rms terminated value !

My point over all this is that if a certain RF generator output level is specified, it is normally applicable to the level obtained in a resistive 50 ohms load (the terminated level).
If it is the open circuit level of the generator that is specified, it should be noted as a EMF value.

Sorry to be too academic...

73, Jacques, VE2JFE