Why doesn't my circuit oscillate?

Thread Starter

saviobezerra

Joined Apr 12, 2022
8
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?

circuit.jpg
 

LvW

Joined Jun 13, 2013
1,438
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?
How can the oscillator "individually" oscillate without the feedback loop which contains the sensor model?
In any case, you must analyze the loop gain and verify the oscillation condition.
 

Papabravo

Joined Feb 24, 2006
18,422
@Bordodynov , good catch!

Are C3 and C6 really 9.53 farads, or am I reading it incorrectly?
f stands for femto

\( 9.53\text{f}\;=\;9.53\times\;10^{-15}\text { Farads} \)

That frequency of 117.6 MHz. is in the band of Aviation Radio Navigation aids called VOR (VHF Omnidirectional Range). Causing interference in this band can result in some serious attention from the men in black.
 
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Thread Starter

saviobezerra

Joined Apr 12, 2022
8
How can the oscillator "individually" oscillate without the feedback loop which contains the sensor model?
In any case, you must analyze the loop gain and verify the oscillation condition.
The oscillator works normally at 117MHz without the sensor. My problem really is when I insert it

model.png
 

Danko

Joined Nov 22, 2017
1,302
It starts oscillate with sensor's capacitance about 50 pF:
1649778221491.png__1649778598237.png
ADDED:
In this circuit take out resistors R2 and R3, then minimal sensor capacitance will be 2.5 pF.
 

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Thread Starter

saviobezerra

Joined Apr 12, 2022
8
It starts oscillate with sensor's capacitance about 50 pF:
View attachment 264920__View attachment 264928
ADDED:
In this circuit take out resistors R2 and R3, then minimal sensor capacitance will be 2.5 pF.

Why did you use 51pF? I cannot change the values of the sensor components, only the differential pair/colpitts. Can you give me any tips on how to check the Barkhausen conditions of my circuit? I don't know if I'm doing it correctly in LTspice (I'm adding a 1V AC source and using the .ac simulation)
 

Danko

Joined Nov 22, 2017
1,302

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Thread Starter

saviobezerra

Joined Apr 12, 2022
8
Sorry, but I'm still confused. I believe that I am using LTspice correctly to visualize the gain (dB) and phase (°), and as you can see in the image my circuit gain is positive and the phase is very close to 0°.
This circuit is for a university project, I must design a colpitts oscillator with differential pair that is robust enough to oscillate when I insert the sensor. I can't change the value of any sensor component, and it has minimal insertion losses at ~117.6MHz, so my circuit must also oscillate at that frequency. I am attaching my .ac simulation file and a table in Excel that I used to calculate the component values (I used Clarke-Hess's book as a source)
Sem título.png
 

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Danko

Joined Nov 22, 2017
1,302
Sorry, but I'm still confused. I believe that I am using LTspice correctly to visualize the gain (dB) and phase (°),
and as you can see in the image my circuit gain is positive and the phase is very close to 0°.
See gain and phase shift:
1649812107847.png__1649813053260.png
 
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