Also
http://www.hanssummers.com/radio/varicap/index.htm
for G0UPL's testing of several common diodes as varicaps.
Red LED's came out top!
Does anyone else have info on using them as varicaps for tuning?

When using a semiconductor in a transparent package, you should
remember that the ambient light will generate a voltage across a
reverse biased junction.

When using a high resistance potentiometer to generate the tuning
voltage, the voltage generated by the ambient light might upset the
voltage across the diode and hence, the capacitance (and resonant
frequency) varies with the ambient light level.

And, if the ambient light was from a mains-powered bulb (fluorescents
would be worse) there would be a significant amount of 50 or 60 Hz
light-flicker energy. This might be sufficient to inject hum into the
signal... flicking light modulates junction voltage, which modulates
junction capacitance, which modulates the RF oscillator frequency.

So, the LED-varicap-tuned receiver might be both drifty, and
hum-prone, if the LED were exposed to light.

At least keep the ambient light out of any diode with a transparent
package, when used in a high impedance reverse biased circuit.

Yup. A dab of black paint would be a good idea.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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