Why doesn't my circuit oscillate?

[eTech]

It is a model of a sensor to be used in water. The model was not made by me, so I cannot change the values of the sensor components.

What is it supposed to sense?

 

[LvW]

Yes, this is the sensor, as I said it's for a university project, I can't change any sensor parameter, just my differential pair. Do you any way to compensate the sensor gain and phase? Or some material where I can learn about?

I am sorry - but it is not clear to me up to now what the circuit is required to do.
1.) What is the task of the sensor in your circuit? Should it modify the frequency - depending on some environmental changes?
2.) What are the maximum frequency deviations to be expected?
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If the sensor modulates the frequency, it is most important to use an oscillator topology which realizes and keeps the oscillation condition - independent on the changes within the sensor. However, in this case you should know what will happen within the sensor - in detail: How will the sensor transfer function change in response to some environmental changes?
Without this knowledge, a working solution will not be possible.

Note: There are methods to incorporate parts tolerances (and sensor gain/phase values) into the design - however, the values must be known.

 

[rjenkinsgb]

It is a model of a sensor to be used in water. The model was not made by me, so I cannot change the values of the sensor components.

Try this - I thought the sensor may be working as a series-tuned circuit, so blocking the resonant frequency instead of passing it.

It seems to work like this, changing the sensor inductor value slightly does appear to change the frequency.

 

[LvW]

Try this - I thought the sensor may be working as a series-tuned circuit, so blocking the resonant frequency instead of passing it.

It seems to work like this, changing the sensor inductor value slightly does appear to change the frequency.

rjenkinsgb - did you notice that the sensor should be part of the feedback circuit?
In your contribution it acts as a load only. This cannot help at all.

 

[rjenkinsgb]

did you notice that the sensor should be part of the feedback circuit?

It IS part of the feedback circuit! Directly to the left hand transistor base, shunting the feedback signal path & ground.

As I mentioned, fractional changes to the value of the sensor inductor cause changes in the oscillator frequency.

Note where the crystal (the frequency control component) connects in a simple colpitts oscillator - between base and ground, as the sensor is now configured in my modified circuit:

(Though I did say the series/parallel part backwards, otherwise it would not be working.. It has to be in parallel mode, not series, to work in this manner.)

 

[LvW]

It IS part of the feedback circuit! Directly to the left hand transistor base, shunting the feedback signal path & ground.

Did you compare your circuit with the circuit as shown in the first post (task description)?

 

[Ylli]

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