LED’s are so cheap and plentiful these days that it is hardly necessary for the Ham to source Varicap Diodes for Frequency Modulation.
Varicaps or Varactors are a reverse biased diodes that exhibit a variable capacitance characteristic. Almost all diodes exhibit this characteristic, Varactors are manufactured to produce specific a Voltage/Capacitance curve. An increase in Voltage produces a decrease in Capacitance.
An LED is used forward biased to produce light. It is therefore logical to use them reversed biased as Varicaps. The following diagrams on Wilkipedia show the construction and Anode/Cathode of an LED this is the same as all other diodes. The longer lead is the Cathode/Positive. If the body has a flat it is the Cathode/Negative lead that is the adjacent to it. Beware of the physical construction diagram not ALL LED’s have a physically larger Cathode. I fitted a clear case Green emitter LED today which is opposite.
One major connection confusion can arise due to the fact that the Cathode of a diode used as a rectifier will be the one that produces the Positive DC voltage when AC is applied to the Anode.
So normally when we utilise an LED as a Varicap we invert the LED in the diagram above for reverse bias.
It has been noted that even forward biased LED’s can be used for FM modulation. Are we switching on the LED and so adding the parallel capacitor in or out or does the LED still act as a Varicap.
I was at the right stage today in some construction to carry out a test. If we apply a 4 level DC signal to the LED and it merely produces a 2 level change then it is a switch, if it produces a 4 level change then it is acting as a Varicap.
First I inserted a Green LED, reverse biased in an 8.19Mhz crystal Oscillator circuit and applied the 4 level signal from a PIC. Clearly we have 4 reasonably linear steps. For clarity I have increased the deviation, when correctly adjusted the WSPR decoded perfectly.
Next I reversed the LED so that it was forward biased. No other settings were changed. Clearly the LED is still acting as a Varicap the capacitance increases with increased applied voltage so producing upside down modulation. the shift is again reasonably linear.
As can be seen the capacitance swing is also greater with forward bias.