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# Oscillators : Feedback or -ve Resistance

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#### anisnn

##### Newbie level 6
Hi all,

I read somewhere that oscillators can be classified either as a feedback or a -ve resistance. What is the difference between the two?

Thanks,
Anis

negative resistance will be always there in oscillator.
The way for generating -ve resistance is either by series feed back or Parallel feed back.
without positive feed back no oscillations are possible.

### anisnn

Points: 2
Thank you both. But the article Compensating for Ideality Factor and Series Resistance Differences between Thermal Sense Diodes.. how is it related to -ve resistance? perhaps you can explain to me a bit?

Thank you abshikebabs! So are resonator can be both feedback and have -ve resistance. It is just up to me to choose which method for simulation. Are there any rules of thumb on how to choose which method is better?

Thanks!

oscillator produces output without input and this is related to energy generation and hence means negative resistance...... and oscillator is supposed have a feedback with a multiple of phase shift of 0° or multiples of 360°....

For lower frequency up to 4GHz you can go for paralle feedback since you are beginer.
design of series feed back oscilltor is comparitively simpler.

In parallel feed back, simulation take lot of time,
Also If you are using resonators in feedback path then coupling gap is seperate issue.
Go for series feedback.
Only thing is you have to do lot of experimentation
it will never work according to requirement for 1st attempt.
Best of luck

### anisnn

Points: 2
What do you mean by adjusting feedback length?

See the picture attached
It is "Interdigital-filter microstrip oscillator" of frequency 2GHz.
An interdigital band-pass filter is used as part of the feedback network. In order to bring the total phase shift to an integer multiple of 2*PI radians, additional delay lines may be required to obtain the correct feedback phase.
The total amplifier plus feedback phase shift still has to be an integer multiple "m" of 2*PI radians, however "m" is not restricted to unity and may become very large at light-wave frequencies.
Although the described oscillator may look complicated, it includes some advantages when compared to conventional low-frequency designs. Although the Q of microstrip resonators is not very high (in the range of 50-100), the phase slope may be made high thanks to the large total phase shift (increasing the multiple of 2*PI radians). On the other hand, oscillation at unwanted multiples of 2*PI radians can be suppressed by tailoring(adjust or trimming length of spacing between coupled lines ect.) the amplitude response of the feedback network

### anisnn

Points: 2
Hi Abshikebabs!

Thank you for your help & explanations. As you suggested I am learning how to use series feedback. I would like to use the topology shown below.

I am also reading how to perform the open loop simulation based on Alechno's paper which can be found at

However I am confused how to use s-parameters to load and match the circuit when we break the loop. Can you explain this to me? As far as I understood, we need to load the output of the resonator with the input load of the amplifier. However, what does matching the input mean? If I create this circuit - amplifier - resonator - input load and verify that the S21 gain > 1 at zero phase shift, will the oscillator oscillate when I close the loop?

thanks!!
Anis

go to
www.rfic.co.uk
there is tutorial on feed back oscillator. read it first.
There are lot of ways of designing an oscillator.
First make good stable amplifier & provide feed back to make circuit oscillate. in feed back it self you add resonator.
If you design an amplifier then only you need matching other wise you dont need.
I have designed series feedback or reflection oscillator & i have not used matching networks.
I have just terminated my circuit to 50Ohm.

Are you designing CMOS Osillator?
which software are you using?
I can only guide you in microstrip circuits ,I have not done silicon design.
In your matching problem I suggest make stable amplifier for the frequency you want. your I/p & O/p impedances will be 50 ohm.
so make feedback line such that its zo is 50 ohm.
Try out in simulation.
I am also trying feedback oscillator. it requires lot of simulation. If you dont have software then mathematical calculation take very long time.

yes.. I have read the tutorials. I am using Microwave Office. I am not designing the amplifier. I am using a commercial amplifier, MAR 6+ with a MEMS resonator. I have made the phase shift =0 and gain > 1 at resonant frequency for open loop simulation. But I did not put the input impedance of the amplifier at the end of the resonator. How can I obtain the input impedance of the amplifier if I just have the S-parameters? Is it ok if I just put 50 Ohms to terminate the circuit? What other criteria in the simulation should I look for?

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