Hello everyone!
I'm designing a near field transmitter composed of a Pierce oscillator, a BPSK modulator, a power amplifier and a transmitting coil.
I'm having problems with the oscillator. The schematic is attached. The design was made by another student, who also fabricated a PCB for the oscillator and tested it.
The problem with the oscillator is that the output sine wave is asymmetric. This becomes a problem in the following stage (BPSK modulator), because when changing phases (from 0° to 180°) the sinewave is inverted (multiplied by -1). The result is a BPSK signal with an irregular envelope.
The schematic I have is the one used by the previous student who also fabricated and tested the PCB. I haven't tested the PCB myself, but in the results from the previous student the output signal from the PCB of the oscillator was centered in 0 V and was symmetrical.
I don't know how to solve this problem. I changed some parameters of the Pierce oscillator, but the problem persists.
I also attached the screenshot from the oscilloscope (from when the PCB was tested), and the datasheet of the quartz crystal used in the oscillator.

 

Lower the vertical position on the scope until the symmetry at zero is even, or close.
Use the DC offset to design a Schmitt trigger.

 

Hello everyone!
I'm designing a near field transmitter composed of a Pierce oscillator, a BPSK modulator, a power amplifier and a transmitting coil.
I'm having problems with the oscillator. The schematic is attached. The design was made by another student, who also fabricated a PCB for the oscillator and tested it.
The problem with the oscillator is that the output sine wave is asymmetric. This becomes a problem in the following stage (BPSK modulator), because when changing phases (from 0° to 180°) the sinewave is inverted (multiplied by -1). The result is a BPSK signal with an irregular envelope.
The schematic I have is the one used by the previous student who also fabricated and tested the PCB. I haven't tested the PCB myself, but in the results from the previous student the output signal from the PCB of the oscillator was centered in 0 V and was symmetrical.
I don't know how to solve this problem. I changed some parameters of the Pierce oscillator, but the problem persists.
I also attached the screenshot from the oscilloscope (from when the PCB was tested), and the datasheet of the quartz crystal used in the oscillator.

Hello there,

This kind of problem is usually caused by improper transistor bias which means it could be over or under driven. The output may be swinging too far up and down which would be caused by the input changing too high or too low or both.
Bipolar transistors can only be driven so much or else the amplification will not be linear. It has to be biased in such a way that it is linear through most of the range used.

For example, if the base current goes too high it is going to cause the collector voltage to fall too fast and may even bottom out and that would cause clipping. If the base current is too low, the output may rise too fast and may even top out which would clip the top of the output wave. You can identify this kind of problem by viewing the base current (and possibly voltage) and the collector voltage. Even if the base current or voltage is a perfect sinusoid, the output voltage may be very distorted if the transistor is not biased properly or is under or over driven. Bipolar transistors can act extremely nonlinear under those conditions.

I assume you want a nice clean sine wave output. If you are not getting that then the design has to be improved with attention to the above details.