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SAW Sensor - Why is my circuit not oscillating?

saviobezerra

New Member
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.
 

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Dick Cappels

Active Member
It might be that the rest of your circuit does not have enough gain to make up for the 31 db loss.

Can you look at the input and output of the SAW device, driving it with a voltage source at 117 MHz to confirm that the loss is not too high?

The model of the SAW device should be suspect, even if it was suggested by the SAW device manufacturer.
 

saviobezerra

New Member
It might be that the rest of your circuit does not have enough gain to make up for the 31 db loss.

Can you look at the input and output of the SAW device, driving it with a voltage source at 117 MHz to confirm that the loss is not too high?

The model of the SAW device should be suspect, even if it was suggested by the SAW device manufacturer.
So, understand the gain of my differential pair should be controlled by the current, but even if I increase the current, the circuit does not oscillate (with the Sensor) any tips on how I could correct this gain keeping the differential pair?
 

Dick Cappels

Active Member
So, understand the gain of my differential pair should be controlled by the current, but even if I increase the current, the circuit does not oscillate (with the Sensor) any tips on how I could correct this gain keeping the differential pair?

Some methods come to mind that might extend your gain-bandwidth product.

Make the two transistors in the differential pair into cascode amplifiers by inserting a common base stage in series with the collectors of two transistors in the differential pair.

Make sure all of the transistors in the circuit are operating at a collector current that assures high ft and a high enough VCB to assure that base-collector capacitance will a have minimal effect on gain (and phase!).

Insert NPN transistors in series with the emitters of the differential pair so as to introduce negative resistance with which to compensate for re in the amplifier. I am sorry that I could not find a solid example that I can post. As I recall the circuit is two transistors connected to provide negative resistance.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
This is a design example for a SAW sensor of a similar class, complete with oscillator circuit details and all calculations.
It should presumably be fairly simple to adapt to your sensor frequency?

(Their SAW example "internal equivalent" is not as detailed, but it's explanation rather than a simulation model).

 

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