Crystal Set Detector Simulation using the SPICE Computer Program (Ben H. Tongue)

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A crystal set detector may be simulated in Spice by using a voltage source V1 feeding a parallel tuned circuit L1|C1 through a source resistance R1. The parallel tuned circuit may be made to have any Q by placing a parallel resistor across the tuned circuit. In the simulation circuit files enclosed, an infinite Q is assumed (no RF tuned circuit losses). The actual source loaded Q of the tuned circuit is R1/(Reactance of C1 at resonance). The voltage at the hot end of the tuned circuit is connected through a diode D1 to a parallel RC load R2|C2. The detected output voltage is developed across this load. The purpose of doing this is to enable experimentation to determine how the detection sensitivity changes if the diode type, diode source resistance, and/or load resistance are changed. This program enabled me to develop the graphs shown in Article #1 on my home page that show how detector power loss varies as a function of rectified diode current for a HP 5082-2835 diode and also, more importantly, as a function of diode saturation current Is. The input voltage is modeled as an unmodulated 1.0 MHz sine wave consisting of 4002 individual cycles, sampled at eight points per cycle. If one wants to evaluate the result of using an AM modulated wave, three simulations can be made using min., carrier, and max. Voltage levels of the desired modulated wave.

One of the simulations in the enclosed Zip archive 'Crystal Set SPICE Simulations' uses a Spice model of a Schottky diode similar to the HP 5082-2835. This is called simulation XtlSetSim1 and its files are contained in the directory XtlSetSim1. The other simulation uses a Spice model of the 1N34A. This is called simulation XtlSetSim2 and its files are in the directory called XtlSetSim2. Each of these directories contains all the files that were generated by my SPICE simulator when I ran each simulation. The diodes used in each of these models have the value of CJO set to 0.0 pF. This does not effect the simulation and makes it easier to experiment with various values of C2 without the detuning effect of CJO. The input source and output load resistance values are equal and match the diode RF input impedance and audio output impedance values. One would expect this condition to give the lowest loss (highest Xtal Set sensitivity) at very low signal power levels. This is not so because at very low input power levels, the diode detector exhibits a square law relation, not linear relation between output and input power. See Article #15 for an explanation of how a theoretical 2 dB increase in detector output can be obtained by a deliberate RF mismatch.

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