uhf tunable oscillator - RF Cafe Forums

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fede-rfcafe
 Post subject: uhf tunable oscillator
Posted: Fri Aug 19, 2011 8:17 am 
 
Lieutenant

Joined: Mon Aug 15, 2011 6:41 pm
Posts: 2
Hello


I can´t understand something about this oscillator.

Image

from http://www.seekic.com/circuit_diagram/Power_Supply_Circuit/TUNABLE_UHF_OSCILLATOR.html

It says that f0 can vary from 350 to 500 MHz. Wavelength at those frequencies goes from 86cm to 60cm.
The planar inductance is much smaller than the wavelength so why the feed point is at 15% from ground as it says?
There is no impedance variation along the inductance length, isn´t there?

Thanks
 
   
 
nubbage
 Post subject: Re: uhf tunable oscillator
Posted: Sat Apr 14, 2012 8:17 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi
The resonator is based on a quarter wave-length line. The line length is shorter than a quarter wavelength because of the tuning capacitor and the active device output capacitance.
The impedance along a transmission line does change: it is proportional to the tangent of (2 * PI * length/ wavelength). That is why the 50 ohm output tap needs to be near the ground end.

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Posted  11/12/2012