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Measuring Devices with Long Electrical Delay
When making a narrowband measurement of a device with long electrical
delay, measured levels can be affected by the rate at which the source is
changing frequency. This sensitivity is related to the time required for
the source signal to travel through cables or devices which are connected
between the RF OUT and RF IN ports. Since the source frequency is changing
rapidly during a sweep, a long distance or delay between RF OUT and RF
IN
will mean that the signal arriving at the RF
lN
port will be of slightly lower
frequency than the RF OUT signal at the same moment in time. This effect is
referred to as “frequency shift.” The amount of frequency shift is given by
the following equation:
frequency shift = transit tune x
fr~~‘~~<i~~an
The narrowband receiver at the RF IN port is tuned to the exact frequency
being emitted at the RF OUT port, with an input bandwidth determined by
the System Bandwidth selection. If the RF IN signal is lower than the RF
OUT signal, the measurement of RF IN will be attenuated by the receiver’s
frequency response. The amount of attenuation increases as the amount of
frequency shift increases. The amount of attenuation also increases as the
System Bandwidth decreases.
The analyzer has been designed to minimize the effect of frequency shift
when a short cable is connected between RF OUT and RF IN. When a long
cable (or a device with long electrical delay) is connected, however, it is
possible for the measurement to be affected, especially at the analyzer’s
fastest sweep rates. If frequency shift is suspected, use the following
techniques to reduce its effect:
0
increase sweep time
l
decrease frequency span
l
select a wider system bandwidth
l
use shorter cables to connect the DUT to the analyzer
l
use broadband detection to completely eliminate the effect of frequency
shift
5-21
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