Instructions

Page 48
7.4 Measuring Relative Amplitude
When both measurement markers are turned on, a readout is provided of the difference
between them in terms of both frequency and level.
A common requirement is to measure the signal amplitude at a specific frequency under
different conditions. The PSAxx05 provides a convenient method of doing this.
Having measured the amplitude in the initial condition using marker M1, the value can be fixed
from the Marker Setup sub-group menu (Traces/Markers > Marker Setup > Fix/Unfix M1). The
M1 readout shows Fx at the left hand side. With the new condition applied, the measurement
can be made again by moving M2 to the same frequency using the key Move M2 to M1. The
difference in amplitude level can then be read directly.
To return M1 to normal operation, press Fix/Unfix M1 again. See section 4.6.1.
7.5 Sweep Modes & Signal Image Rejection
In any spectrum analyzer an input signal creates a potential image at a distance away of twice
the IF frequency. The PSA Series 5 uses a combination of analog and digital techniques to
suppress these images.
Image Rejection Ratio (IRR)
The image rejection ratio (IRR) is expressed in dBs and is the ratio between the amplitude of
the real signal and the amplitude of the visible image. Thus if the IRR is 60dB, a signal with a
level of -15dBm would create an image with a level of -75dBm.
7.5.1 Default Sweep Mode – Normal (Auto)
In the PSA Series 5 default sweep mode, image suppression is achieved in real time as the
sweep progresses. In order to ensure that the IRR is sufficient to bring the image down to the
noise floor, an alignment procedure has to be run.
The alignment is run when the instrument is first switched on and is repeated from time to time
thereafter as temperature changes and other factors alter the analog parameters. During
alignment the sweep is halted for a brief period and an “Alignment in Progress” message is
displayed.
7.5.2 Advanced Options
A number of alternative image rejection methods are offered. These are selected using the
Advanced Options key on the Sweep Modes sub-menu (see section 4.3.5).
Normal (Manual)
If the user wishes to avoid the occasional interruption of the sweep that can occur in Normal
(Auto) mode, alignment can be set to Manual instead of Auto. In this mode the IRR may
gradually deteriorate resulting in images being visible above the noise floor. When this occurs
a warning message of “Alignment Recommended” will appear, and alignment can be run at any
time by pressing the Status hard key or Manual Align key (see section 4.3.5).
Threshold
The IRR after alignment is typically >60dB. Whether an image appears above the noise floor is
dependent upon the IRR, the noise floor (which is related to RBW/VBW) and the level of the
real signal.
The frequentness with which auto alignment takes place in Normal (Auto) mode, or the
“Alignment Recommended” message appears in Normal (Manual) mode, can be reduced by
Normal
(Auto)
Normal
Manual
Threshold
DC
Mode 1
DC
Mode 2