Method and apparatus for measuring a signal, method of testing with a spinstand, a method of writing a test data pattern and a spinstand

Apparatus for measuring a signal, the signal having a frequency component the power of which is to be detected by the measurement, the frequency component having a varying phase, the signal having at least one interruption thereto, the apparatus comprising:

• a measurement module arranged to process the signal so as to detect the power of said frequency component in the signal;
• a stall control module arranged to stall the measurement module before the start of the interruption such that the signal is not processed during the interruption;
• wherein the stall control module is arranged to calculate a stall period such that the end of the stall occurs:
• i) after the end of the interruption, and
• ii) where there is substantially no discontinuity between the phase of the frequency component in the signal at the end of the stall and in the phase of the frequency component in the signal at the beginning of the stall;
• wherein the stall control module is arranged to keep track of the elapsed stall time while the measurement system is stalled;
• and,
• wherein the stall control module is arranged to end the stall of the measurement module when the elapsed stall time is at the end of the calculated stall period so as to resume processing of the signal.