Voltage gain or power gain

[Leiser]

Hi all,

I have a question. I have an RF buffer for AM reception. Its input impedance is high and the ouput impedance 50R. This is because the antenna is so far from resonance that this is the only way to get something from it.

Well, if I connect the buffer to the network analyzer and measure the S21, I believe then that this is the power gain, am I correct? So that is the power gain calculated as the power the NA receives at the port 2 minus the power that generates the NA at the port 1. But because the input impedance of the buffer is very high the actual input power would be nearly 0.

Anothe question is how can I measure the voltage gain of the amplifier with the NA.

Thanks to all!

 

[zappedagain]

On the simplest level, the NA assumes everything is a 50 ohms system, so you can reference your power levels to 50 ohms (V^2/50 ohm). For example, when the NA puts out -16 dBm, you'll see 100 mVpp across a 50 ohm load; if you really have a high impedance you'll see 200 mVpp. The rest is voltage gain from there.

Your output voltage is straight forward; take the output power (S21 * input power) and translate that the voltage across 50 ohms.

For your input voltage, you should measure S11 first and see what your input impedance looks like. Then you can treat it as a complex voltage divider and calculate the voltage from the 50 ohm source.

Others may know more subtleties to the test, but that is where I would start.

 

[VEBill]

When you design a buffer with a very high input Z, it almost doesn't make sense to try to discuss, let alone specify and attempt to measure, 'power' gain.

The RF characteristics to specify would perhaps be the input Z (high at AM), the voltage gain (modest), and the output Z (low) - all versus frequency. And then all the secondary performance characteristics such as PS rejection, ESD susceptibility, noise figure at FM band, strong signal overload, etc., etc.


PS: I wouldn't assume that car radios antenna sockets are anything close to 50-ohms input Z. I'm not sure, but I'll bet most aren't even close (on AM band). Car radios would be optimized (if optimized at all - LOL) to work with plain (unamplified) car antennas at the end of a short length of that funny Hi-Z coax that car antenna makers sometimes use. Input Z might be fairly high for AM band, and maybe 75-ohms (roughly) for FM band. Your design could easily accommodate this by providing a stable low Z output. High VSWR in the AM band isn't going to matter in a very short cable.

 

[Leiser]

Hi,


Thanks zappedagain and VE1BLL. Very useful comments and suggestions. I spent few hours thinking and measuring some of your comments. It was good fun seeing how everything came together.

Keep in touch.