# Why doesn't my circuit oscillate?

#### 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?

#### ericgibbs

Joined Jan 29, 2010
15,525
Hi Sav,
Welcome to AAC.
E

#### DickCappels

Joined Aug 21, 2008
8,710
Do you have a use for this particular frequency?

#### 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.

#### Bordodynov

Joined May 20, 2015
2,993
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.

#### DickCappels

Joined Aug 21, 2008
8,710
@Bordodynov , good catch!

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

#### 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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#### MrChips

Joined Oct 2, 2009
26,099

#### Papabravo

Joined Feb 24, 2006
18,422

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

#### DickCappels

Joined Aug 21, 2008
8,710
Use a PCB gimmick capacitor. The are almost free!

#### Danko

Joined Nov 22, 2017
1,302
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.

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#### Papabravo

Joined Feb 24, 2006
18,422
Use a PCB gimmick capacitor. The are almost free!
That would be my vote

#### crutschow

Joined Mar 14, 2008
29,792
As noted, Q3 has no DC path for base current so it is cutoff.

#### MrChips

Joined Oct 2, 2009
26,099
With those values of C3 and C4 there is no functional path between C5 and C6

#### 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

#### Danko

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

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#### saviobezerra

Joined Apr 12, 2022
8
It starts oscillate with sensor's capacitance about 50 pF:
View attachment 264920__View attachment 264928
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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#### 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)

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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:
__

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