In measuring amplifiers with high IP2 values, the spectrum analyzer is the
limiting factor; it will generate its own second harmonic. In addition, even a
very good signal generator will have a second harmonic 60 dB or so down from the
fundamental.
Two bandpass filters are necessary to increase the dynamic
range of the IP2 measurement process.
First, a bandpass filter on the signal
generator output. (A low pass filter work just as well, but I don't have a
suitable low pass filter.) The test frequency is 5350 KHz and the bandpass
filter reduces the second harmonic by approximately 60 dB. The HP8640B's
second harmonic is down 60 dB, so the net is an amplifier input test signal
with the second harmonic down approximately 120 dB.
Second, a bandpass filter on the spectrum
analyzer input. (A high pass filter would work just as well, but I don't
have a suitable high pass filter.) This filter, centered at 10700 KHz in my
test setup, is necessary to prevent the strong 5350 KHz signal from
overloading the R3463 spectrum analyzer and generating spurious second
harmonics. In order to ensure the amplifier under test sees a 50 ohm
termination at both the fundamental and second harmonic, a 10 dB attenuator
at the amplifier's output is necessary.
In calculating the IP2, it is necessary to reference the
various signal levels to a common point, the amplifier output.
Thus, the fundamental is:
Signal generator output
- bandpass filter loss
+amplifier gain.
In this example:
Signal generator output = 0 dBm at 5350 KHz
5350 KHz bandpass filter loss 3.5 dB
Amplifier gain: 11 dB.
Thus the amplifier under test has a 5350 KHz output of
+7.5 dBm.
Likewise, we go backwards from the spectrum analyzer
towards the amplifier output:
The second harmonic is:
Spectrum analyzer level measurement
+bandpass filter loss
+attenuator
In this example:
Spectrum analyzer reads 10700 KHz signal at -97 dBm
+3.5 dB 10700 KHz bandpass filter loss
+10 dB attenuator
Thus, referenced back to the amplifier output, the
10700 KHz signal level is -83.5 dBm.
The harmonic is thus 91 dB below the fundamental or the
"harmonic ratio" is 91 dB.
IP2 is calculated as:
Fundamental + harmonic ratio.
In this example:
+7.5 dBm + 91 dB = +98.5 dBm
Hence, the Z10040A tested has an IP2 of +98.5 dBm.
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