Why doesn't my circuit oscillate?

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saviobezerra

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First, sorry for my bad english.

I'm building a project using a Colpitts Oscillator with Differential Pair, designed for ~117.6MHz. I'm using LTspice for simulation. The oscillator individually oscillates, but I need it to oscillate when I add a sensor model (as seen in the image in the circuit's feedback loop). When I add the sensor it initially wobbles and then stops, any ideas how I can fix this?


 

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Post a schematic of the circuit that DOES oscillate. See Dana's comment above. Also, C3 is 9.53 femptofarads. That is an unreasonable value. I realize that this is a bandpass network, but the impedances won't work. Stray capacitances will be higher than 10ff.
 
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Bias problem 10k resistor from Q1-B to ground.
C2 is too big and eats up much of the AC. 0.05nF.
C3, C6 from fF to pF.
L1,2 to 19uH to slow down the frequency. The transistors are too slow. They have no gain at 150mhz to 300mhz.
I slowed down the frequency to 10mhz so the transistors can work.
Q1, Q3 have a current of about 1mA which limits the bandwidth of the transistor to 100mhz. If the current was increased to 10mA the BW would increase to 200mhz. I am not saying the transistor will oscillate at 200mhz, just that this is where the gain=1.
 
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The circuit of the "sensor", ie the part with femto Farad value capacitors reminds me of the equivalent circuit of a quartz crystal.

The OP does not appear to have been back here since the 13th April.

JimB
 
Sorry for the absence, I wasn't notified by email, I thought it still didn't respond.

I still haven't solved my Oscillator problem, as I said, I believe I need to study more about Crystal Oscillators

This project is for the University, I cannot change the values of the sensor components, only my differential pair, I understand that the gain/phase of the differential pair is not compensating for the attenuation/phase of the sensor, my biggest doubts are:
-If the sensor is positioning correctly
-What I could and where to put components that compensate my phase and my gain

Again, sorry for my english, I'm using google translator in a lot of the sentences
JimB said:
The circuit of the "sensor", ie the part with femto Farad value capacitors reminds me of the equivalent circuit of a quartz crystal.

The OP does not appear to have been back here since the 13th April.

JimB
Click to expand...
 
Good job, you picked a 500mhz low capacitance transistor and add the base resistor.
113mhz.
 
I think post 11 is backwards.

Is this the sensor? When you remove it the "thing" will stop oscillating. That is the way it works. The capacitors and L2 is what sets the frequency. Remove it and noting happens.

Most sensors cause a inductor or a capacitor to change value and thus the frequency changes.
 

Looks like a spice model of a quartz crystal
 
I understand you cannot change the sensor components. The value of those components will change with water. That is how sensors work.

Please send data on the sensor.
 
 
We should use a source follower or emitter as buffer before we load the oscillator, LI suppose

Sarma vu3zmv
 
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