Transformation from Ideal Lines to MicroStrip
I am building a microwave oscillator. At this moment, I finished the oscillator design with ideal lines, and I want to carry out the next step: Microstrip design. However, when I make the transformation for the different elements, from lines to Microstrip, the HB-Simulation issue a error like: "Circuit is stable. Found loop gain>1 but Nyquist criteria for oscillations not satisfied. This means that the loopgain poin 1+j0 was encircled in acounter clockwise direction..."
I have checked the date several times and it is correct, I think that the problem should be in the structure of the design. I mean, I would be to add more elements or, would be other modification.
IDEAL LINES DESIGN (it works correctly)
MICROSTRIP LINES.
The difference between both designs is: In the Microtrip Design, I added a MTEE and a MLIN (90degrees)(closest from the ports). These elements are not in the Ideal Lines Design, but I think these are totally necessary in Microstrip Design.
Enclosed you can find both designs.
Thank in advance.
Ideal TLs are losless,therefore infinite Q factor of your TL resonator makes your oscillator working.MSs are lossy and therefore Q factor is limited and that's why your oscillator doesn't start to oscillate.
As a rule of thumb, equivalent resistor of your resonator should be 3 times bigger that negative resistance of your oscillator input where your look at.If oscillator doesn't work even you provide this rule, play around some feedback capacitors or increase or decrease slightly OP of the transistor because there is a relationship betwen gm value of your transistor and minimum Q factor of your resonator that can be connected to feedback loop.
Check first parametrically negative resistance looking into active part and choose component values that will give you minimum negative resistance at the frequency of interest.
Question: Why you used 50 Ohm port at left handside ?I couldn't resolve your osc. circuit.Oscport component is faulty inserted, it doesn't work in this configuration.You resonator should be grounded without 50 ohm port element.Check your configuration..
Try to add a small cap (or TL equivalent) from the source to the ground and it will work.