EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: kronos on November 02, 2021, 06:48:11 pm
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Hello,
I have an RF amplifier, and want to measure its gain and plot it using a nanoVNA. But I am stuck with a problem I found.
First step is to perform a calibration; when calibrating the through step, the connection looks like this (I am ignoring an attenuator at the amplifier input to avoid overload, for simplicity):
[attachimg=1]
The through-cable has 0 dB gain (or loss); the voltage at its input is Vi=Vs/2, that is, 6 dB lower than Vs: like a 6 dB loss. The nanoVNA measures the voltage at its input (S21) and calibrates knowing that the cable is 0 dB, so that although the total gain Vl/Vs is -6 dB, it shows 0 dB: it adds 6 dB to the measurement.
So I now connect the amplifier. Assuming its input impedance Zin is 50 ohm, flat, the connection is now this:
[attachimg=2]
The amplifier has a flat gain of G db (G=Vl/Vi). Vi has a loss of 6 dB with respect to Vs, as before, so when the nanoVNA measures Vl/Vs it sees a total gain of G-6; it adds 6 dB, and shows a gain of G. All is OK.
But now I connect an amplifier with a high input impedance; in the extreme case, Zin = \$\infty\$, so that Vi=Vs. There is now no loss w.r.t. Vs, the nanoVNA sees a signal that is 6 dB higher than the previous case, and it then shows a gain of G+6, which is wrong. This is actually what I see, the nanoVNA shows an excess gain of 6 dB (compared to the gain I measure with the scope).
Does this mean that I can only measure amplifier gain when Zin=50 ohm? Or am I missing something?
But even if that is the case, that input impedance is not flat, it decays with frequency, distorting the Bode plot. So I can never measure amplifier gain with the nanoVNA! is this correct? It cannot be!