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?

 

Hi Sav,
Welcome to AAC.
Please post your LTS asc file.
E

 

Do you have a use for this particular frequency?

 

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.

 

The electronic circuitry contains an error. The first transistor is not dc energized - no base current. I suggest adding an inductance from the base to the common.

 

@Bordodynov , good catch!

Are C3 and C6 really 9.53 farads, or am I reading it incorrectly?

 

@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.

 

9.53 femto Farads

Where can you buy those?

 

9.53 femto Farads

Where can you buy those?

You can't. They are only available on a chip or fabricated on the board itself. RF people do crazy stuff sometimes.

 

Use a PCB gimmick capacitor. The are almost free!

 

When I add the sensor it initially wobbles and then stops, any ideas how I can fix this?

Unfortunately it is not oscillate at all.

 

Use a PCB gimmick capacitor. The are almost free!

That would be my vote

 

As noted, Q3 has no DC path for base current so it is cutoff.

 

With those values of C3 and C4 there is no functional path between C5 and C6

 

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

 

It starts oscillate with sensor's capacitance about 50 pF:
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ADDED:
In this circuit take out resistors R2 and R3, then minimal sensor capacitance will be 2.5 pF.

 

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)

 

I cannot change the values of the sensor components, only the differential pair/colpitts.

Well, will change oscillator circuit and frequency:

Can you give me any tips on how to check the Barkhausen conditions of my circuit?

See here: https://forum.allaboutcircuits.com/threads/barkhausen-criterion-using-ltspice.166922/post-1475272

 

Well, will change oscillator circuit and frequency:
View attachment 264981

See here: https://forum.allaboutcircuits.com/threads/barkhausen-criterion-using-ltspice.166922/post-1475272

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)

 

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:
__