We are to design a VCO for an fm band.. I chose 91.7MHz (200kHz bandwidth)..
I have a working design, a Clapp oscillator. The oscillator is modulated by a varactor (MV104) centered at 2V (1V to 3V), so the capacitance range is ~59pF to 39pF.
I think no matter what topology (split capacitance) I use, the main equation for resonant freq won't change: f=1/(2pi*L*C), where C is the series equivalent resistance of the 2 feedback capacitors and the modulating capacitor (Cd).
I made an excel file so I can see the values. I have a 220nH inductor (air core, I coiled it) so I would like to work with it.
With analysis, I also found that:
high L/C ratio is better = higher L = automatic lower Ctotal (for a given freq) = lower capacitance change *needed* for a given BANDWIDTH = lower Cd(varactor*fixed* + parallel cap)
The problem is that the Cd needed for 91.6MHz and 91.8Mhz have a difference in the order of tens of femtofarad for L=~1000nH... is this OK? Will the varactor be ok if it is paralleled with another capacitor to achieve the lower capacitance swing? I feel like it will be swamped or something...
Please help!!
I have a working design, a Clapp oscillator. The oscillator is modulated by a varactor (MV104) centered at 2V (1V to 3V), so the capacitance range is ~59pF to 39pF.
I think no matter what topology (split capacitance) I use, the main equation for resonant freq won't change: f=1/(2pi*L*C), where C is the series equivalent resistance of the 2 feedback capacitors and the modulating capacitor (Cd).
I made an excel file so I can see the values. I have a 220nH inductor (air core, I coiled it) so I would like to work with it.
With analysis, I also found that:
high L/C ratio is better = higher L = automatic lower Ctotal (for a given freq) = lower capacitance change *needed* for a given BANDWIDTH = lower Cd(varactor*fixed* + parallel cap)
The problem is that the Cd needed for 91.6MHz and 91.8Mhz have a difference in the order of tens of femtofarad for L=~1000nH... is this OK? Will the varactor be ok if it is paralleled with another capacitor to achieve the lower capacitance swing? I feel like it will be swamped or something...
Please help!!