UHF oscillator problem.

Hello, the first prototype is a simple common base design. My target frequency is 850 MHz, but the maximum I could tune it was about 650 MHz now the problem is when I further reduce the length of the resonator the frequency drops to around 500 MHz. The resonator is an Ag coated wire in series with a trimmer.

Anything I could do?

maybe you have a lot of parasitics
please check wires lenght (1cm ~10nH)
also change trimmer to small smd capacitor
AN

http://www.alphaind.com/products_dis...m.asp?did=2066
TAke this as reference , for 850 MHz your need increase printed inductor and tank capacitors.
Theory and model are included.

Are you sure that you have measured the right harmonic (if you use a spectrum analyzer to measure the oscillator)?
Is your trimer cap specified for 800 MHz use? Try first with some better SMD capacitors as said before (ATC series 600 or similar). Keep shortening the resonator.

D.

have you included the parasitics of transistor ?

A feedback problem. The resonator length infulences of feedback network and thus decreases negative resistance of input impedance of the transistor and tuning capability is changed by some parameters. Thus tuning range is shortened.

It's my guess.