Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

MW oscillator with interdigital filter

Status
Not open for further replies.

Ivan_Ryger

Junior Member level 1
Joined
Dec 17, 2010
Messages
16
Helped
1
Reputation
2
Reaction score
1
Trophy points
1,283
Activity points
1,439
Dear all, please have you got experience with design of microwave oscillator with interdigital filter?
I am currently building this oscillator for spectrum analyzer project:
http://lea.hamradio.si/~s53mv/spectana/vco.html
Unfortunately the oscillator according Fig. 4 does not work. I used BFP420 transistor instead of BFP183. It has higher transition frequency and therefore lower phase shift at operation frequency. I was advised by Matjaz Vidmar to use original transistor, but i cannot get it.
I used FR4 material of thickness 0.8 mm (according to original text). I measured the fabricated interdigital filter with center line short circuited at the end. It had S21 coefficient in peak at approx. -20dB.
If i removed the short circuit on center line and connected the tuning varicap, the attenuation further increased to level -40 dB, the peak in transmission characteristic could be tuned with external voltage applied on to varractor. The attenuation seems to me quite high (maybe low varicap Q-factor or losses in FR4 at 2GHz). I silver-coated the microstrips by wet chemical process and used colophone to prevent the lines from oxidation.

I would like to design my own oscillator but i have no experience with design of coupled transmission lines.
Have you got any litterature about these oscillators?
I cannot find the s-parameters of the transistors used.
 

I built this oscillator and works fine, but using BFP450. This transistor is the best for designing oscillators, from the entire BFP series from Infineon.
The DC current should be set about 10 to 15mA (I used for bias only resistors at the base), and the spacing between resonant lines I used 0.5mm.
At the time, the only issue with this oscillator was, I cannot get too much bandwidth, probably need smaller line spacing, but then you lose other parameters as phase noise, or increase of spurious.
One thing to remember, whatever metal use building microstrip resonators (copper, silver, gold, etc), their Q-factor is pretty limited by radiation losses and end-effects phenomenons that occur in microstrip design.
 
Your 20dB insertion loss at peak is far too high. Something is wrong with your interdigital filter, probably too wide spacing between the coupled lines. These oscillators in particular were built hundreds of times, mostly worked from the first try. The author optimized the circuit and the layout for the specified transistor, so if you changed something, like replacing the original BFP183 with BFP450, it's no wonder you get a worse performance. In addition, I agree, silver coating will not make any difference. S parameters and the datasheets for these transistors should be more than available...
 
Your 20dB insertion loss at peak is far too high. Something is wrong with your interdigital filter, probably too wide spacing between the coupled lines. These oscillators in particular were built hundreds of times, mostly worked from the first try. The author optimized the circuit and the layout for the specified transistor, so if you changed something, like replacing the original BFP183 with BFP450, it's no wonder you get a worse performance. In addition, I agree, silver coating will not make any difference. S parameters and the datasheets for these transistors should be more than available...

I will try to measure the distances between interdigits. If I remember well, it was 1mm (PCB fabricated from picture on that website). Maybe this is an issue. Another reason is the dielectric loss of FR4. I will try to measure it.
I used BFP420 transistor. Now i wait for BFP182 as long as the BFP 183 is practically unavailable in our country. I will try to modify the microstrip. Please have you got some theory about coupled transmission lines and interdigital filter design?
Sincere thanks,
Ivan
 

it is pretty straightforward. If it does not oscillate, either 1) you amp has insufficient gain, 2) your bandpass has too much loss, or 3) the round trip phase shift is not right for oscillaitons, and you need to add or subtract phase.

Luckily, you can test all of these on a network analyzer to see, one at a time.
 

Today I have tested the performance of new IDT filter fabricated on 0.8mm FR4. It has 0.5mm spacing between fingers. The measured peak resonance insertion loss is about -10dB @2.15GHz and -13dB@1.75GHz.
It seems that the colophone resin has increased the insertion loss. This PCB is without protective resin and the silver coating. Now I am looking for BFP183 transistor. It seems that the problem will be solved in a short time.
Regards,
Ivan
 

I am not sure what your "frequency determining" resonator is in such a circuit. If you have a bandpass filter that (theoretically) is designed to have a flat passband anywhere between 1.75 to 2.15 GHz, how do you expect the oscillator to decide on one specific frequency it will try to run at?
 

The exact frequency is determined by two conditions, the amplitude and the phase one. The interdigital filter has the central finger connected to ground via varicap. This diode tunes the center frequency of IDT filter and also affects the phase of transmission coefficient. The IDT filter is very narrowband, so the amplitude condition for oscillations is fulfilled only at its center frequency. The phase condition for oscillations is affected by phase of transmission coefficient(being modified by variable capacitor-varicap) and the and phase of transistor S21.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top