VCO from 50MHz to 70MHz

[Sönke Klinkhammer]

Hi NG,

I'm trying to design a VCO with an external tank circuit and a varactor
diode. The output frequency should range from 50MHz to 70MHz with the
varactor diode having a range of about 50pF. The tuning voltage is up to
5V. I have been trying to find the right components for the Max2620 as
VCO-chip, but have had trouble to fulfill the loss-criteria while still
getting the desired bandwidth.

I haven't decided yet which VCO-chip to take, so I would really
appreciate any kind of advice.

Regards, Soenke

 

[Tam/WB2TT]

Hi NG,

I'm trying to design a VCO with an external tank circuit and a varactor
diode. The output frequency should range from 50MHz to 70MHz with the
varactor diode having a range of about 50pF. The tuning voltage is up to
5V. I have been trying to find the right components for the Max2620 as
VCO-chip, but have had trouble to fulfill the loss-criteria while still
getting the desired bandwidth.

I haven't decided yet which VCO-chip to take, so I would really appreciate
any kind of advice.

Regards, Soenke

50 PF strikes me as too large a value to use at VHF. You will need about a
2:1 ratio of Cmax/Cmin; so, try for a total capacitor range, including
stray, of 10 - 20 PF. I am assuming the LC is hung across a high impedance
load. To keep losses down, you need the inductor to have as high an unloaded
Q as possible. Something in the 100 - 200 range is easily doable.

Tam

 

[John Jardine.]

Hi NG,

I'm trying to design a VCO with an external tank circuit and a varactor
diode. The output frequency should range from 50MHz to 70MHz with the
varactor diode having a range of about 50pF. The tuning voltage is up to
5V. I have been trying to find the right components for the Max2620 as
VCO-chip, but have had trouble to fulfill the loss-criteria while still
getting the desired bandwidth.

I haven't decided yet which VCO-chip to take, so I would really
appreciate any kind of advice.

Regards, Soenke

It's only a single transistor Colpitts oscillator. Those things resonate
using a bit of wet string. 50-70megs would be easy. Just divide the example
10MHz LC components by 5 and set C17 to suit your 50p Varicap.
I.e C3,C4 47p:C5 33p:C17 15p.(lose C6).
The S parameters start biting way beyond where you are working and if
Maxim's "negative resistance" pretension is frightened of normal inductor
losses then you wouldn't want to be using the chip anyway.
john