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Programming
Math Functions
Adding and Subtracting in dBm
Trace-math functions allow easy addition and subtraction of correction
values in dBm units. For example, to correct for 3 dB of loss in trace A
data values, you can add or subtract trace B, which has been preloaded
with +3 dBm or −3 dBm as its data values. The two traces can then be
added or subtracted using APB (trace A plus trace B) or AMB (trace A
minus trace B) and thus eliminate the effects of the loss.
Note that in the example above, the result is an addition or subtraction
of dBm and not an addition or subtraction of power. Consider a
trace-data value of −50 dBm and a second trace-data value of −50 dBm.
When the two values are added using the APB command, the result is
−50 dBm + −50 dBm = −100 dBm. However, if two −50 dBm power
sources at two different frequencies (but close enough in frequency to be
within the resolution bandwidth passband) are physically summed, the
result is a power of −47 dBm. To illustrate this point further, if trace A
is at 3.0 dBm and trace B is at 7.0 dBm, performing APB; moves trace A
to 10.0 dBm (that is, trace A would move up on the screen). On the
other hand, if trace A is at −10 dBm and trace B is at −6.0 dBm,
performing APB; drops trace A data to −16 dB, even though trace B is
4.0 dBm higher in power in both cases. As you can see, the spectrum
analyzer is not adding and subtracting physical values, but rather
providing an efficient method for calculations in dBm units.
Use AMBPL to Correct Data
The AMBPL (trace A minus trace B plus display line) command
provides the most versatile method for applying correction data to a
trace. AMBPL subtracts the contents of trace B from the contents of
trace A and adds the result to the display line. Consider characterizing
the response of a device under test in a swept-measurement system.
Enter the response of the system in trace B. Insert the device into the
system, then enter this response into trace A. Use AMBPL to subtract
the system response from the response with the device under test; the
result is the response of the device under test, which is centered about
the display line. So, to correct data, use trace B to store a copy of the
uncorrected response and subtract this from new data in trace A; the
result is a corrected response.
If the two traces are identical, as in the following example, the result of
subtracting these two traces will equal 0 dBm. Note, however, that if
the reference level is less than 0 dBm, the results will be off the screen,
or even clipped (clipping is described under "Trace Data Limits" below).
The display line is added to return the result to the screen, with no
clipping occurring. Because you can specify the position of the display
line, you can move the corrected data to any on-screen position.
Example 1 illustrates how to correct data remotely. Before running this
example, connect the calibration signal to the INPUT 50Ω.
320
Chapter 5
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