Identifying Interfering Signals - Agilent Technologies E4404B Operation Manual

GSM Use Model

Identifying Interfering Signals

Identifying Interfering Signals
To identify interfering signals, you must first locate them in the GSM
frequency band. This is best done by using the Monitor Band
measurement. Sensitivity should be optimized to locate and view small
interfering signals.
To optimize the spectrum analyzer for best sensitivity when identifying
interference signals, three main parameters need to be understood:
resolution bandwidth, internal attenuation, and internal
pre-amplification:
• Resolution bandwidth: Choose the lowest possible resolution
bandwidth filter. The noise floor decreases as resolution bandwidth
decreases. This is because noise is a broadband signal, and as you
reduce resolution bandwidth, less noise reaches the detector. Sweep
speed is inversely proportional to the square of the resolution
bandwidth and increases as resolution bandwidth decreases. To
optimize speed, the smallest span and largest bandwidth possible
should be used that still separates the signals and allows visibility of
all signals of interest. Using monitor channel reduces the span by
focusing on a specific channel instead of an entire band.
• Attenuation: Set the internal input attenuator to the least possible
amount of attenuation, normally 0 dB. However, if the input signal
total power is greater than −10 dBm for 0 dB attenuation, the
analyzer may generate internal distortion. To determine if the
analyzer is internally generating the distorted signals seen on the
display, increase the attenuation and see if the displayed signals
change in amplitude. If no amplitude change is evident, the
distortion is caused by the unit under test and not the analyzer.
• Noise floor: Turn on the internal preamplifier (Option 1DS). This
will drop the noise floor and allow you to view the signals that were
previously below the analyzer noise floor.
Use the procedure shown in
interfering signals to help you identify the source of interfering signals
and achieve the best sensitivity.
1-6
Table 1-2 and the following examples of
Chapter 1