HMC8100 is an integrated receiver IC. It has two AGC modes:
1) External AGC mode
2) Internal AGC mode.
To operate the IC in internal AGC mode, we need to set the desired output IF power via SPI interface and IC adjusts its gain according to input RF power to produce a constant IF at desired power.
Now Datasheet shows the graph of noise figure versus RF input Power. (test conditions: IF output power= -9 dBm).The noise figure starts to increase very rapidly as the input power exceeds -50 dBm. My questions are:
1) Can anyone explain this?
2) I want the noise figure around 7-8 dB through out the input power. Is it possible with internal AGC mode?
3) Can noise figure be improved if I set different IF output power i.e, Is noise figure also related with output power ?
RF System Engineer
You are correct. With higher input levels the AGC adds attenuation at the RF stage (1st stage) so it stays linear, this attenuation adds directly to the noise figure. This increase in noise figure is not an issue because the SNR is large at these higher input levels. What's important is the noise figure at sensitivity, here the AGC will be operating at max gain and the noise figure will be at it lowest (5 dB).