LED’s as Varicap Diodes

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.

 

Volcanic Dust over Northern UK Regions

The Volcanic dust over the UK has certainly effected the the aircraft movement, this is the situation at 08:00 UTC.

The IR Weather Image at 07:00 shows what look like 2 large smoke rings between Iceland and Norway, the weather situation is bring this cloud down across Scotland at 08:00

The visible image at 06:00 shows heavy cloud. I think the eruption occurred during the hours of darkness.

The IR image at 05:00 shows cloud forming on the South East coast of Iceland, maybe this is the location of the Volcano.

A plume of dust seems to be very noticeable on the whole earth IR image in the early hours of the morning.

It will be interesting to know if there any radio effects caused by the dust.

Advertisers of Fake Designer Goods

Purveyors of fake junk goods are posting adverts on this blog now, please be aware that counterfeit goods and drug crime go hand in hand. Personally I would not care for ‘real designer’ goods at cheap prices either.

It is a shame that blog.co.uk can’t do anything to stop these nuisance postings to Comments. Other than restricting comments to members only (not good) I can’t see a way round it. I don’t get or see the problem on blogger, mind you this could be due to the low Google rating of blogger, blogs. I suppose you can’t have everything.