BPSK Demodulator

Overview

• Done with Opamps and 4000 series CMOS
• Normal design with
  • Multiply by 2 (done with gilbert cell)
  • PLL for carrier recovery
  • Divide by 2 (JK flip-flop)
  • Mixers (XOR gates)
  • Low-pass filter
• Also finds the initial phase

Implementation

Squaring up signals

• There were several points in the circuit where I wanted to take a signal like a sine-wave and turn it into a square wave with matching frequency. This was implemented with the following circuit.

• This circuit has feedback so the duty cycle ends up very close to 50% no matter the input signal
• However this ciruit doesn't do too well with noise as there is no hysteresis
  • This is a bit of a design trade off bc that would have interfeared with the phase
    • The phase would vary with the input amplitude

Frequency doubler

• This was a gilber cell set up as a mixer
• the output is not sinusoidal. Instead it was a "full wave rectified" sine wave
  • This wasn't a problem bc of the function of the "squaring up circuit"

PLL

• Made of an astable multivibrator and XOR gate for the mixer
• Second order with very low frequency pole in the filter stage bc
  • Most important feature is to not lose lock (one cycle lost would ruin data)
  • Doesn't need to change frequency while device is operating

Startup Detection

• When the input signal is applied the phase must remain constant for about 10ms
• This gives time for PLL to lock and for the start up circuity to set the phase of the divide by 2 so that the output will be high for this phase and low for opposite phase.
• This circuity is also blanks the output while the PLL is locking up and such

PCB

• Due to errors I made with startup and phase I had to modify the original circuit and pcb

Phase

• The phase works out quite nicely with this design Here is a matlab plot I made to show the phase as the signal moves throught the circuit

• In order to get the phase right the divide by two has to update on the falling edge. Initially I got this wrong and I had to use a spare XOR gate as an inverter to fix the problem

Testing

• I needed a BPSK signal to Demodulate. Fortunately this was quite easy to generate. I just had an inverting amplifier and a noninverting ampifier with the same input and outputs added with reistors then by changing the gain of one amplifier I could flip the phase
  • The inverting ampifier had fixed gain of 2 V/V
  • The non-Inverting had gain that could be set to 1 or 3 based on the digital input
  • The "mixing" resitors were the same value

Function

• With nice high bandwidth low noise signals it worked very nicely
  • Able to achive 1.6 kBits/sec
• Struggles with noisy signals, and when bandwidth of input has been restricted
• PLL has narrow lock range so only works with small range of carrier frequencies