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Why VCO is better than VFO?

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Willen

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Someone is suggesting me to use VCO instead of VFO on my FM Tx. Why varicap is better than trimmer? I am using regulated supply for VFO. Even frequency drift is between 0.4 MHz up and down. I think VCO also unable to solve this drifts, i think it is related to temperature of inductor and junction of transistor. Tell me something please
 
If your VFO is tuned by a mechanical capacitor, how would you modulate it?
 
Someone is suggesting me to use VCO instead of VFO on my FM Tx. Why varicap is better than trimmer? I am using regulated supply for VFO. Even frequency drift is between 0.4 MHz up and down. I think VCO also unable to solve this drifts, i think it is related to temperature of inductor and junction of transistor. Tell me something please

Roff said:
If your VFO is tuned by a mechanical capacitor, how would you modulate it?

Willen said:
Your question makes me confuse!
Can't I say VFO to this type of oscillator?

tuned by mechanical trimmer+inductor (tank)?


because if it is a FM transmitter, then you need to be able to modulate the oscillator to produce your FM transmitted signal
VCO's these days, like the ones from Mini Circuits, are very stable these days, particularly when usd on freq's below ~ 1GHz
If you need stability, then you can incorporate the VCO into a PLL circuit.

Maybe you should do some reading on how to generate a FM signal and how FM modulation is applied to the oscillator :D

Dave
 
I wouldnt even consider a VFO unless I specifically wanted freq coverage over a specific bandwidth..... Do You ?
or are you just wanting a fixed freq ?

Dave
 
Last edited:
Your question makes me confuse!
Can't I say VFO to this type of oscillator?
www.electro-tech-online.com/attachments/fm-transmitter-tuning-png.62593/
tuned by mechanical trimmer+inductor (tank)?
From what I read, a VFO is generally considered to be tunable by a mechanical device such as a variable capacitor or variable inductor.
A VCO can be tuned by a voltage. The controllable device may be a variable-capacitance diode (varicap, varactor), or variable transconductance, etc.
The circuit you posted has a VCO based on Q2. The collector-base capacitance (Ccb) of all BJTs varies as the collector-base voltage varies. In this case, Ccb is part of the resonant circuit that determines the oscillation frequency. It is modulated by applying the audio signal to the base through C3. I would call this a VCO, not a VFO. Technically, I suppose you can say it is a Variable Frequency Oscillator, but, as I said, I believe the industry differentiates between a VFO and a VCO by how it is tuned - mechanically or electrically, respectively.
 
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