AUTOMATIC GAIN CONTROL LOOP

FOR SCOTTY’S SPECTRUM ANALYZER

Sam Wetterlin

swetterlin@comcast.net

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Last Updated: July 18, 2007

 

This page describes an alternative method for measuring the magnitude of the IF2 signal in Scotty’s Spectrum Analyzer, whether the SSA, MSA or VSA.

 

The IF2 signal is 10.7MHz, and its magnitude measures the strength of the spectrum analyzer input at the frequency to which it is tuned, or in the case of the vector network analyzer extension, it indicates both the magnitude and phase of the response signal received from the Device Under Test (DUT).

 

The original method of measuring the amplitude of this signal, as well as amplifying it to a strong level at which it can be used for phase measurement, relies on the AD8306 log detector.  This is a very straightforward and accurate method, but has the disadvantage that the dynamic range is limited somewhat by the noise that enters the log detector.

 

In this alternative method, a variable gain amplifier (VGA) with about 85 db of useable range is controlled by an AD8367, which contains both a 40db VGA and a magnitude detector.  The combination is used to create an AGC loop in which the AD8367 adjusts the AGC voltage as necessary to cause the AD8367 output to equal 354 mVrms.  The higher the AGC voltage, the lower the total gain.  The AGC voltage ends up being proportional (in db) to the strength of the input signal, and becomes the magnitude measurement.  The output signal, of fixed amplitude, is sent to the phase detector.

 

In between the VGA and AGC control (the AD8367) is a noise filter.  This severely limits the noise bandwidth of the noise coming out of the VGA.  Because of the extremely high gain of the VGA, even a small amount of noise entering the VGA adds up to a lot at the VGA output, if you count all the noise in the full bandwidth of the VGA.  The noise filter eliminates most of that noise.  There are two selectable bandwidths for the noise filter, roughly 1 MHz and 7.5 KHz.  The narrower the bandwidth, the less the noise.  But for reasons I will explain later, it is important that the noise bandwidth not be less than the resolution bandwidth of the spectrum analyzer.

 

These modules are based on a fully tested AGC loop, but have been modified to fit in Scotty’s new modular format.  In the process, a few previously available options have been eliminated, since they turned out not to be useful.  The new modules have not been tested. Test results for the previous version, using a 12KHz noise filter, are shown here.

 

And now for the actual modules. Both the schematics and PCB layouts shown below can be downloaded in ExpressSchem and ExpressPCB format. The programs to read these files are available free at ExpressPCB.com.

 

 

VGA

 

The VGA module is in Scotty’s 1.2x2.4 inch format.  The schematic and PCB layout are shown below.  The schematic includes references to other modules, to show how this module fits in the overall scheme.

 

The VGA module consists of two AD8330 variable gain amplifiers, whose gain is determined by the AGC voltage fed back from the AGC module. Transformers at the input and output allow impedance matching, and also allow either differential or single-ended connections.  For Scotty’s devices, both input and output of the VGA are single-ended.

 

A band-pass filter between the two AD8330 modules, tuned to 10.7 MHz, provides enough noise filtering to keep the input noise from overloading the second AD8330, and also limits self-oscillation possibilities.

 

VGA Schematic

 

 

VGA Board Layout

 

 

Noise Filter

 

The noise filter is a 1.2” x 1.2” module containing two RF switches which select one of two band-pass filters,

one a simple LC filter and the other a monolithic crystal filter.  The output contains an op amp which makes impedance matching for the filters easier, and also provides enough gain that the overall board insertion gain/loss should be near 0db.

 

Noise Filter Schematic

 

 

Noise Filter Board Layout

 

AGC Module

 

The AGC module is 1.2” x 1.2”.   It is simply an AD8367, which combines about 40db of VGA capability with the ability to measure the output signal, and provide a feedback AGC signal to adjust the gain.  The gain is adjusted so as to result in an output of 354mVrms.  This level is based on actual power measurement, and for a pure sine wave corresponds to 1 volt peak-to-peak.  U2 is used to adjust the AGC voltage fed back to the VGA module, which uses a slightly different AGC voltage range than the AD8367. U3 simply buffers the output of the AD8367, so that when it is fed to the digital phase detector, any digital “kick-back” does not disrupt the AD8367.  Q1 is used to clamp the AGC voltage, which otherwise can go very high if the AD8367 input is overloaded, and can get stuck there.  This is properly classified as a “bug” in the AD8367, but in any case Q1 takes care of it.

 

AGC Schematic

 

 

AGC Board Layout

 

The Effects of Noise

 

The following photos show the effect of noise on the operation of the AGC loop.  The spectrum analyzer photos show the signal and noise in a 1.5MHz band centered at 10.7MHz, at 10db per vertical division.  The display line shows the output level of the AGC loop with a relatively strong signal.  The absolute level of the display line is meaningless because of external attenuators.  But note that when the input is reduced from -60dbm to -90dbm, the signal output remains unchanged, but the surrounding noise creeps up around the signal.  The topmost oscilloscope photo shows the output waveform, showing significant noise.  The triggering action of the scope tends to fix the location of the first zero crossing, which is why no horizontal jitter shows—in effect, the scope eliminates phase noise but not amplitude noise.

 

When the input signal is reduced to -100dbm, the output signal drops below the display line.  The AGC loop is still maintaining its desired output power level, but now a significant part of that power is in the noise output.  The corresponding scope display shows massive noise.  Finally, when the input signal is eliminated, the AGC loop still maintains the constant level output, but that output now consisists entirely of noise.

 

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