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Design Notes

AGC System:

In this example, the result of multiplying a WHITENS source with a PPULSE source is used to drive the input of an Automatic Gain Control (AGC) system. The power setting of the AGC's input signal is set by the variable StepPwr. The PPULSE’s amplitude varies from 1 to 10^(StepPwr/20) and has a 50% duty cycle.  Setting SetpPwr to 40 is equivalent to setting the signal’s channel power seen by the system's bandpass filter (BPF) to 37dBm. The second column of the table "PWR" shows the measured channel power at the BPF’s input.

The AGC loop attenuates the input signal when its power is high and amplifies the signal when its power is low. The graph "Waveforms" displays the AGC's input signal (in brown), the control signal (in blue), and the output signal (in green).

The control signal takes on positive values (in dB) during the low-power cycle of the input and takes on negative values in dB during the high-power cycle of the input. 

The gain stage "gain K" just before the VGA determines the relative power levels of input and output. If you set “gain K"   to -6 dB the control waveform becomes the same as the lowpass-filtered signal. This can be seen in the graph "Control WVFM."  The control waveform is in blue and the "low pass filtered signal" is in gray.

The "Power Setting" constant (at the far right off the system diagram) can be set to a number in dB, and is tunable between 0 and 10. This number sets the average overall output power scaling (10 leads to higher overall output power, 0 leads to lower overall output power).

The system diagram "Bandpass Filter Response" is used to display the gain and phase response of the bandpass filter. The filter characteristics can be seen in the graph "Gain and Phase Response of Bandpass Filter"

System Diagram - Bandpass Filter Response

System Diagram - AGC System

Graph - Control WVFM

Graph - Waveforms

Graph - PWR

Time (ns) DB(PWR_vsT(TP.in1,1,0,1,0,0,1000,0,10,0,-1,0,-1,1,0,0,0.5,0,0,0,0,0)) (dBm) AGC System DB(PWR_vsT(TP.OUT1,1,0,1,0,0,1000,0,10,0,-1,0,-1,1,0,0,0.5,0,0,0,0,0)) (dBm) AGC System
56883.2 36.9913 30.3182

Graph - Gain and Phase Response of Bandpass Filter