SAW Sensor - Why is my circuit not oscillating?

Thread Starter

saviobezerra

Joined Apr 12, 2022
8
First, sorry for the bad english.
It's not the first time I've been looking for help on this circuit on the forum.
I'm from Brazil, I'm doing a university project using which basically consists of using a colpitts oscillator with differential pair, with a SAW sensor in its feedback loop. The SAW sensor has a frequency of 117MHz, and minimal insertion losses of ~-31dB. The challenge lies in designing the oscillator so that the gain compensates for the sensor losses. The SAW sensor model for Spice was used from another work (a master's thesis).

I managed to get my Colpitts Oscillator with differential pair oscillating at ~117MHz, but when I add the SAW the oscillation stops.

I'm not able to identify where exactly is the error of my circuit. Whether it's in the SAW model or my oscillator.
I would like any kind of help, be it articles or basic observations about my circuit.

Attached, I will put the images of the models used along with my SPICE file.

Thanks.

circuito.pngPerdas.pngSensor.png
 

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Danko

Joined Nov 22, 2017
1,368
I'm not able to identify where exactly is the error of my circuit. Whether it's in the SAW model or my oscillator.
I would like any kind of help, be it articles or basic observations about my circuit.
Your Colpitts Oscillator does not work, because sensor shifts phase of signal on 180°.
For example, simple inverting amplifier works just fine:

1655319525792.png
ADDED:
If you want oscillations at given frequency only,
use tank L2C5:
1655324859489.png_____1655324788973.png
 

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Thread Starter

saviobezerra

Joined Apr 12, 2022
8
Your Colpitts Oscillator does not work, because sensor shifts phase of signal on 180°.
For example, simple inverting amplifier works just fine:

View attachment 269550
ADDED:
If you want oscillations at given frequency only,
use tank L2C5:
View attachment 269555_____View attachment 269554
Any ideas on how to do this while keeping the DIFFERENTIAL PAIR? Other works have already been done with other oscillators, mine should be with the differential pair. How could I correct this phase with the pair?
 

Danko

Joined Nov 22, 2017
1,368
Any ideas on how to do this while keeping the DIFFERENTIAL PAIR? Other works have already been done with other oscillators, mine should be with the differential pair. How could I correct this phase with the pair?
@Bordodynov in this post already showed you, how to do it.
1. Put signal from sensor to inverting input of diff. pair (base of Q2).
2. For increasing gain of amplifier, decrease value of R1.
1655421501881.png1655422156902.png
 

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Danko

Joined Nov 22, 2017
1,368
About this last circuit, what is the function of Capacitor C5 and inductor L3?
Capacitor C5 compensates signal delay (phase shift) in amplifier.
L3 is choke, provides big reactance for AC signal,
but has low resistance for DC base current, that eliminates
disbalance of differential pair.
 
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