Is the following, a valid method to either measure amplifier excess noise or
make a meaningful comparison?
Assume one measures the S/N of a AM Rx to be say 10dB S/N at 30 percent
modulation for an input signal -101dBm.
Then one re-measures the Rx S/N with a 20dB noiseless pre-amp. Would the Rx
sensitivity, 10dB S/N now be a theoretical -121dBm????
If so, then, if the 20dB amplifier had a NF of 3dB, the measured Rx sensitivity
for 10dB S/N would be 3dB less i.e. -118dBm!!!!!
For example, the well respected DXeng RPA-1 HF Pre-amp. has gain of 15.8dB at
500kHz. This amplifier increases the RX sensitivity 10dB S/N to -112dBm. If the
amplifier was noiseless then the RX sensitivity for 10dB S/N would be
-116.8dBm! So the difference between the noiseless amplifier and the measured
sensitivity is 4.8dB. Can I assume that 4.8dB is the NF??? I would have expect
a NF of over 4dB as this amplifier uses resistor feedback.
I also have a transformer feedback amplifier, gain 14.6dB at 500kHz. This
amplifier increases the RX sensitivity 10dB S/N to -113dBm. If the amplifier
was noiseless then the RX sensitivity for 10dB S/N would -115.6dBm. So the
difference between the noiseless amplifier and the measured sensitivity is
2.6dB. Can I assume that 2.6dB is the NF???
The Rx is an NRD525. A HP 8568B Spectrum Analyser and Marconi 2019A signal
generator is used for amplifier gain measurement and Rx sensitivity.
I assume that there is probably a flaw in the above rational, but the figures
don't seem too "way out"!!!
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