Specifications

23
Minimum duty cycle
The duty cycle of the pulsed carrier has a direct
impact on the system noise floor. This is seen as
a scaled degradation of the noise floor by 20 log
(duty cycle) in a residual measurement. The rec-
ommended minimum duty cycle of 5 percent was
selected to provide a usable PLL signal that is
necessary to phase-lock the reference source see
Table 3-2. It is possible to reduce the duty cycle to
less than 5 percent, providing there is sufficient
signal for the PLL circuitry to achieve phase-lock.
Table 3-2. Noise Floor Degradation with Duty Cycle
Duty Cycle Test Set Noise Floor Degradation
50% 6 dB
10% 20 dB
5% 26 dB
Peak tuning range
The peak tuning range (PTR) must be <PRF/2;
otherwise, when the phase detector constant is
measured, the beat note will be confused with
the PRF lines. This also guarantees that the PLL
bandwidth will be less than the frequency of the
PRF, which is necessary for stability. If the PLL
bandwidth approaches PRF/2, excessive phase
shift (due to the sampling process that the puls-
ing effectively represents) will cause the loop
to peak or become unstable.
VCO tune constant
The VCO tuning sensitivity cannot be measured
under pulsed conditions because the system can-
not differentiate between a beat note and a PRF
line. Consequently, the VCO tune constant must
be entered manually. It can be determined from
a CW measurement and entered for the pulsed
measurement, or it can be known or calculated
(as in the case of a signal generator) and entered
into the software.
Zero-beat of sources
Under pulsed carrier conditions, the system can-
not zero-beat the reference source automatically,
because it cannot differentiate between a beat
note and a PRF line. The operator must manually
zero-beat the two sources to within 5 percent
of the PTR.
Ignore-out-of-lock
For pulsed carrier measurements, the ignore-
out-of-lock condition must be selected because
of the PRF feedthrough. The out-of-lock circuitry
will detect the PRF lines and interpret them as
beat notes. The presence of a beat note normally
indicates that the loop is not phase-locked. When
ignore-out-of-lock is selected, the operator is respon-
sible for ensuring that phase-lock is established
and maintained during the measurement. This is
accomplished by connecting a wide-band oscillo-
scope to the monitor port of the 70420A test set to
verify the absence of a beat note and to monitor
the DC output level during the measurement.