Microwave oscillator problem

[E-goe]

Hi

I designed and tested some microwave oscillators using the negative resistance method ( as described by Gonzalez and Vendelin ).

I first designed the load and terminating networks using a shorted stub and transmisson line the get the oscillator oscillating. But my problem was that the resonating frequency deviated from the wanted one.

Because I'm just in the experimenting fase the shorted stubs are fabricated by drilling a hole through the substrate and soldering a little wire that connects the capacitor and the ground plane of the plate ( using microstrip ) to get a short at RF.

The first thing that came up to me was designing the load and terminating networks with open stubs in stead off shorted stubs. So I made a microstrip version of this and tested the oscillator. The oscillating frequency shifted towards the wanted one ( net yet exact but its better than the previous with shorted stubs ).

But I find something strange in my measurements. The output power of the fundamental frequency is now 11dB lower than when inplemented with shorted stubs.

Can anybody explain this?????????

Greetz E-goe

 

[FANT]

On the first case is because you didn't take in the schematic diagram the lenght of the via.
Usually it is about 1 nH, to reduce it is possible to drill two holes close one to the other, but you will always have about 0,6 nH.
Wit the open stubs I don't understand why; have you coupled properly the output ?.
A scematic is appreciated.

Mandi

 

[vagarwal]

did u happen to check the negative resistance magnitude on smith chart...i mean on the simulator that u have used for the design...
if u could attach the schematic, maybe some help could be gathered...