Measurement of close-to-carrier frequency stability in microwave oscillators
This thesis is concerned with two main lines of work which have followed
parallel and complementary paths. The first one consists in using the
technique of subharmonic sampling to investigate the frequency stabi lity
of microwave oscillators using a convenient low frequency replica. The
second line has been concerned with a detailed study of the Allan variance
and the dependence of this variance on the operation of digital frequency
counters, with special emphasis on the effects of 'dead-time'. This has
led to the concept of an 'extended' variance over a time window NT
obtained by averaging elementary estimates over a window T.
After Chapter 1 which contains an overview of the basic concepts of
frequency stability and summarises current measurement systems,
Chapter 2 discusses in detail the operation of digital frequency counters
and related mathematical modelling of the operation. As indicated above
the chapter concludes with an original method of obtaining an extrapolated
measurement of the two pair Allan variance.
Chapter 3 extends the sampling theory to the down-conversion of a microwave
signal using trapezoidal pulses with the objective of generating a
low frequency replica. Recovery of the sampled signal, optimisation
techniques and noise limitations are discussed in detail. Based on these
considerations Chapter 4 explains the experimental equipment developed;
it allows a microwave signal of up to 3 GHz to be translated down into
the range 5 MHz to 7.5 MHz using a sampling gate driven by a variable
width pulse generator coherent with a highly stable 5 MHz clock.
Chapter 5 discusses the data acquisition and processing together with
the calibration and checks of the experimental apparatus. Verification
of the operation showed good agreement with the theoretical calculations
and with an alternative mixing technique.
The thesis concludes with Chapter 6 where after a detailed overview
of the research work suggestions are given for further work.