Idle Speed Lambda Sensor Output 
Rover 214SEi MEMS MPI & CAT 
   29/3/99

The  distinct hi-lo pattern is caused by the ECU operating in closed loop mode and continually switching from lean to rich fuel maps  and back again. The voltage output on this sensor is a little low perhaps due to a poor earth connection.
Lambda sensor signal always exhibit some background noise in the signal due to operational characteristics of the sensor.