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VCO from 50MHz to 70MHz

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

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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

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Tam/WB2TT

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
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John Jardine.

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