Measuring Devices With Long Electrical Delay - HP 8712ET 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 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
IN 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 span
×
-----------------------------------------
frequency s hift = transit time
sweep time
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 in frequency than the RF OUT signal, the measurement of RF IN
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 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:
• 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.
ET User's Guide 5-19