VHF oscillator circuit w/ large frequency range and change according to capacitance value

Thread Starter

Adrienboub

Joined Jun 15, 2020
11
Hello,

It's my first post. I'm new in electronics fields. I'm french and my english is not very good, excuse me.

I want to make a VHF oscillator(~ 30 MHz) that his frequency can change with the value of the capacitance.
I found in an another post the RF2506 which seems to works but it's difficult to found it and buy it because it is obsolete.
Do you know a circuit that can do the same or near the same ? Where I can buy it ?

Thanks in advance :D
 

Thread Starter

Adrienboub

Joined Jun 15, 2020
11
For general purposes, you can make a Transistor based LC oscillator that can be tuned using a capacitive trimmer.

View attachment 209807
(replace the Varactors D1/D2 with a Trimmer)

You can also buy Voltage Controlled Oscillator IC's.

https://www.minicircuits.com/WebStore/Oscillators.html
______________________________________________________________________________________________

Hello,
Thank you for your reply.
For the first proposition, where I can find the values of the condensators, resistances and self to put in the schematic ?

For the second, if I buy a VCO my frequency up/down with the value of the voltage but if I want to up/down my frequency with the variable capacitance what I must change ?

Thanks a lot !!!!
 

Thread Starter

Adrienboub

Joined Jun 15, 2020
11
Hello Bertus,

What range should the VCO cover?
There are many ready made VCO's that have large tuning ranges.
Box73 sells a whole series, this is the 25 to 50 MHZ version:
https://www.box73.de/product_info.php?products_id=738

Bertus
Hey!
Thanks for your reply !
This is not a VCO, I want an oscillator which varies between 30 Mhz and 40 Mhz because of a variable condensator.
This VCO have a good tuning range but where I can put my variable capacitor ?
I hope that i'm clear... my english is not great !
 

Thread Starter

Adrienboub

Joined Jun 15, 2020
11
Hello,

The shown block IS a VCO.
With a tuning voltage between 4 and 9 Volts the range of 30 to 40 will be covered.

Bertus
The shown block is not my message...
I understand what is a VCO and i don't want to do that
I want a system where the frequency change according to the value of an external capacitance.
Like the RF2506 . But I can't find it beacause it's obsolete.
Thanks for help
 

wa3tfs

Joined Mar 14, 2017
8
Build the recommended transistor oscillator and take out the variable capacitance diodes and put your adjustable capacitor is place there. That will allow you to change the frequency by tuning the capacitor.
The value of inductor must be selected to resonate with whatever capacitor you are using to output the frequency you want.
 

Papabravo

Joined Feb 24, 2006
16,837
Hello spaky 1,

Thanks you
It's exactly what I want but with a condensator.
In your dataheet it's a Resistance between pin 1(V+) and pin 3(SET).
So it's doesn't work with a condo
A varactor diode is a variable capacitor. The kind of variable capacitor used in antique radios has been out of production for decades. What is most important to you: having an oscillator or tuning it with an air variable capacitor?
 

Thread Starter

Adrienboub

Joined Jun 15, 2020
11
A varactor diode is a variable capacitor. The kind of variable capacitor used in antique radios has been out of production for decades. What is most important to you: having an oscillator or tuning it with an air variable capacitor?
The most important is to tuning it with an air variable capacitor
 

Thread Starter

Adrienboub

Joined Jun 15, 2020
11
Because I want to simulate a capacitor sensor.

I found the 74HC4046 where I can put a variable capacitor between pin 6 and 7.
1627569278189.png
But the curve is not steep enough..
 

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