[Antennaware] Antenna amplifier noise

[Andy Ikin]

Hello Folks,

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"!!!

73

Andrew