Varactor - Vari-cap diode

[abuhafss]

Hi

I need to tune an LC circuit using a voltage controlled capacitor.

My objective is to achieve capacitance 10pF - 500pF against 0-30V.

Shall appreciate if somebody can suggest me a suitable configuration.

 

[Tunelabguy]

Hi

I need to tune an LC circuit using a voltage controlled capacitor.

My objective is to achieve capacitance 10pF - 500pF against 0-30V.

Shall appreciate if somebody can suggest me a suitable configuration.

You are asking for a 50:1 capacitance ratio. The highest I have seen is 20:1. I don't think you are going to find a device with that wide a range.

 

[abuhafss]

MVAM108 and MVAM125 offers 500pF at 12V and 28V respectively.

http://www.tsf-radio.org/forum/im/133895mvam109.pdf

 

[kripacharya]

the MVAM108, MVAM109 are your closest bets. It won't go down to 10pF though, but they have a high capacitance on the upper end, and go down quite low with a high enough voltage. Check the sheets.

 

[chuckey]

What frequency range do you need? if you have a 10 - 500 PF cap, putting in a few stray capacitors would give you a 30 -520 PF range. i.e. your 50:1 is in practicality a 17.3 :1 range.
Frank

 

[abuhafss]

Here is what I am planning to do. Please correct me where I am wrong, as this would be my first project with varicap.

V1 = Power Supply 6V - 12V
V2 = Frequency Generator 2.5MHz, 10MHz
V3 = Controlled Voltage Supply 0-12V
L1 = Inductor under test

The idea is to tune the circuit electronically such that the LED is illuminated with full intensity. That tuning voltage V3 will vary with the value L1. An analog voltmeter can be calibrated to read the inductance.

 

[chuckey]

It looks to me like C1 and C3 are too low in value. Because D4 rectifies the RF from the signal generator, it generates its own bias which has to be off set by the control voltage. So I think that R2 should be a lot lower in value, 10K? It is likely that the input impedance of Q1 will damp the tuned circuit (Zin ~2K?).
Frank