# VCO design problem.

#### PavleHRIBAR

Joined Jan 18, 2022
2
Hello,

I have to design VCO for frequencies from 85-108MHz using varactor(s). I checked the internet for some designs, and I think this one will be fine, although I'm not using very wide band.

Do you think this design is fine, or would something else be better?

However, the problem is that I don't know where to start. I believe that the transistor must be biased in such a way as to have around half of the Vcc voltage drop on the R2 resistor (Common Collector A-class amplifier). But I don't know what values of resistors should I use. I also don't know why R3 on the collector is needed. And I don't understand why it has R7 when it is already biased with R1 and R5 resistors.
About frequency: If my thinking is correct the frequency could be calculated with 1/(2*pi*sqroot(LC)) where C is the serial capacitance of C3, C4, D2, D1 together. Right?
I also have some trouble understanding the working of the Common Collector Colpitts oscillator. I don't understand where does the phase shift come from. The phase shift of the CC amplifier is 0 degrees, but the phase shift of the tank circuit is 180 degrees, so there must be something wrong here.

#### Papabravo

Joined Feb 24, 2006
19,569
There may be some problems with your approach.
1. None of the component values are likely to be applicable to your situation
2. Designing a Colpitts VCO from scratch would probably be a better option
3. The varactor diodes you choose must provide an adequate change in capacitance to cover the frequencies of interest
Notice in the example, that the bandwidth of 2200 MHz - 950 MHz = 1250 MHz vs your requirement of 108 MHz -85 MHz = 23 MHz. Your range is much narrower.
You need to make sure the active device that you use has the required gain at the frequencies of interest.

I know that Youtube videos generally have a bad reputation, but this one might be an exception for understanding the Colpitts oscillator:

#### drjohsmith

Joined Dec 13, 2021
541
Looks like you want to make an FM radio "jammer" / "transmitter"
You need to check this is legal in your part of the world,

Laying out / building any high frequency circuit , especially one involving feedback such as this , is difficult to get right without much experience,

If this is your aim to make a transmitter,
I'd suggest that you purchase an off the shelf FM transmitter from any far east supplier,

#### DickCappels

Joined Aug 21, 2008
9,508
This one tunes the FM band among others. Band 4 tunes from 70 to 135 MHz. The tuned circuits for the other bands and the band switch may be omitted.

Q2 may be omitted if you don't need a harmonic rich output for a frequency counter or to trigger a scope.

#### PavleHRIBAR

Joined Jan 18, 2022
2
Looks like you want to make an FM radio "jammer" / "transmitter"
You need to check this is legal in your part of the world,

Laying out / building any high frequency circuit , especially one involving feedback such as this , is difficult to get right without much experience,

If this is your aim to make a transmitter,
I'd suggest that you purchase an off the shelf FM transmitter from any far east supplier,
Yes, it's connected with FM radio, but not a transmitter. I want to build local oscillator for superhet receiver.

#### DickCappels

Joined Aug 21, 2008
9,508
Here is almost a solution:

The range can be increased by increasing the range of tuning voltages. You can shift the oscillator range by adjusting the inductance in the tank circuit.

Alternatively, have you looked at FM receiver chips and https://www.minicircuits.com ?