Adaptive Frequency Measurement Method for Pulsed Signals

Adaptive Frequency Measurement Method for Pulsed Signals

      Disclosed is a method for adaptively determining the frequency
of a pulsed RF signal, so that wider pulses are measured more
accurately.  This method provides improved accuracy over traditional
methods which are optimized to operate with some minimum width signal
and do not utilize the additional information contained in wider
signal pulse widths to improve the measurement accuracy.

      A typical embodiment of the method is shown in Fig. 1.  Input
is SIGNAL RF and MAX COUNT INTERVAL.  Output is the digital FREQUENCY
word.  A Logarithmic Limiting Amplifier provides a detected LOG VIDEO
used to detect the presence of an RF pulse, and a LIMITED IF signal
which is used to measure frequency.  The actual frequency measurement
is performed by a Frequency Measurement Logic (FML) device, which
measures frequency by counting zero crossings of the SIGNAL IF and a
reference frequency Fr within the given MAX COUNT INTERVAL.

      The threshold crossing circuit compares the input LOG VIDEO
signal with a fixed analog THRESHOLD, and raises its output TRIGGER
when the threshold is crossed.  It stores the peak log video, and
when the log video input falls a fixed amount below the peak, it
lowers TRIGGER.  TRIGGER is input to the FML, causing it to count the
LIMITED IF and Fr inputs when it is high.

      The delay line is used to compensate for the delay through the
threshold crossing detector, so that the TRIGGER output lines up with
the LIMITED IF output of the log/limiting amplifier.  The delay might
be on the order of 10 nanoseconds, but it could be longer depen...