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Old 26th Mar 2021, 3:42 pm   #152
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Spectrum Analyser TR4172

The impedance presented by the base of the transistor will vary as the different diode-switched networks come into play, and the circuit's a bit easier to design with the base driven from a low impedance source.

The signal amplifier needs to be able to produce some gain. RFery is usually in a relatively low impedance environment so that PIN diode in the emitter needs to run down to fairly low resistance values, so the bias current does need to reach a good few milliamps, so the emitter network of the piecewise-linear transistor is lowish impedance, exacerbating the base loading on the control voltage, and opamps were starting to become cheap commodities.

There's my take on an adjustable piecewise shaper that can both increase and decrease slope in the HP Journal April 1982. I didn't think of it at the time, but I should have patented it. In this case it was linearising a swept VCO and needed to be quite accurate.

David
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