Close

The Varicap Diode

A project log for Yet Another MW Radio - The Audion

Another super simple medium wave radio. This time a Franklin Audion (synchronous detector).

agp.cooperagp.cooper 01/07/2017 at 03:440 Comments

The Varicap Diode

I have not used one before but my reading suggests that they work pretty well as a "single" in receiver projects (refer to "B") but for (high powered) oscillators a "twin serial" set up (refer to "C") is recommended:

Image result for varicap diode circuit

(source: www.jestineyong.com/wp-content/uploads/2014/09/varactor-diode-checking.jpg)

In each case value of "R" is not critical but 1M ohm is common. Usually a capacitor at "Vc" is added to minimise unwanted RF in the control voltage (Vc) circuity as shown in the following:

Image result for varicap diode circuit

(source: www.oddmix.com/tech/px/cr_varactor_crystalset_f1.gif)

The 1SV149 (=1/2 BB212) Varicap

I purchased a couple of these some time ago off ebay. There is the datasheet (http://www.qsl.net/df7tv/datasheets/1SV149.pdf). At 1 volt the capacitance is about 485 pf and at 8 volts the capacitance is about 25 pf.

Using a 320uH inductor and a 1000 pf series capacitor the tuning range should be about 500 kHz to 1.8 MHz.

Regards AlanX

Discussions