Measuring Devices With Long Electrical Delay - HP 8712ES User Manual

Optimizing Measurements

Measuring Devices with Long Electrical Delay

Measuring Devices with Long Electrical
Delay
When making narrowband-receiver measurements of devices 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 analyzer's two test ports. Since the source
frequency is changing rapidly during a sweep, a long distance or delay
between the analyzer's two test ports will mean that the signal arriving
at the input port will be of slightly lower frequency than the output
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 span
×
-----------------------------------------
frequency s hift = transit time
sweep time
The narrowband receiver at the analyzer's input port is tuned to the
exact frequency being emitted at the analyzer's output port, with an
input bandwidth determined by the system-bandwidth selection. If the
input signal is lower in frequency than the output signal, the
measurement of the input signal will be attenuated by the frequency
response of the system bandwidth. The amount of attenuation increases
as the amount of frequency shift increases. The amount of attenuation
also increases as system bandwidth decreases.
The analyzer has been designed to minimize the effect of frequency shift
when a short cable is connected between the analyzer's two test ports.
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:
• Increase sweep time.
• Decrease frequency span.
• Select a wider system bandwidth.
• Use shorter cables to connect the DUT to the analyzer.
• Use broadband detection to completely eliminate the effect of
frequency shift.
ES User's Guide 5-21