Measurement Principles - Racal Instruments 2461 User Manual

Measurement
Principles
General 1-6
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The measurement technique employed is known as the
recipromatic measurement technique. In this technique the signal
and reference are counted simultaneously. This has the advantage
that for signals of less than 10 MHz, accuracy is maintained
because of the large number of counts in the reference counter. In
addition the opening and closing of the gate is synchronized to the
signal which, together with the Timing Error Correction (TEC)
circuit allows higher resolution equivalent to a 1GHz reference.
In this system, the first edge of the incoming frequency to be
measured (after the µP has armed the start of measurement),
opens the gate and simultaneously allows the signal through to the
input 1 counting chain and the frequency reference through to the
reference counting chain. Both signals are now counted for the
duration of the gate time. The µP determines the length of the gate
time using its on-board timer, which is set by the operator.
At the end of the gate time, the µP arms the end of measurement
and the next edge of the input signal closes the gate and the two
counting chains stop counting.
In the above description, there are two points at which resolution is
limited by the 10 MHz reference, the opening of the gate and the
closing. To overcome this limitation, the time between the signal
opening the gate and the next edge of the reference is detected by
logic gates in the front end and a pulse whose width is proportional
to this time is generated. This pulse is then used to charge a
capacitor with a constant current source so the charge on the
capacitor is proportional the time error. The charge is then
discharged at much slower rate and while this is going on the
reference is counted in the start counting chain. When the
capacitor is discharged then the count in the start counting chain
represents the error between the opening of the gate and the next
counting edge of the reference (see Figure 1-7).
This process is repeated at the closure of the gate by a separate
TEC and counting circuit (the stop counting chain).
After the measurement results have been read out of all counters,
calibrated pulses, derived from the reference, are fed to the TEC
circuits. This is done for two pulse widths for both start and stop
circuits. The resultant counts are then used to calibrate the TEC
circuits. Once these are calibrated the value of the frequency of
the input can be calculated to a much greater resolution.
2461Type C User Manual
Racal Instruments  1998