How Log Resolution Is Measured - HP 3563A Operating Manual

Using The Log Resolution Mode

How Log Resolution Is Measured

How Log Resolution Is Measured
This section gives a brief overview of the log resolution measurement process. While it is not
necessary to read this section, it is important to understand the measurement process when
interpreting overlap processing and real time bandwidth in the log resolution mode.
To understand how the HP 3563A measures with log resolution, start with a one-decade example, at
the 10 Hz to 100 Hz span. The analyzer's first step is to make an 801-point linear resolution
measurement from 0 to 100 Hz. (This 0 Hz start frequency is important for multidecade
measurements.) It then takes the 720 points from 10 Hz to 100 Hz and combines these to
produce 80 logarithmically-spaced lines for the decade. The number of linear points combined to
produce one log point increases logarithmically from log line 1 to log line 80. The shape of the log
resolution window varies from the beginning of the decade to the end; figure 4-1 shows a
representative window,
10
0
-10
-20
- 30
- 4 0
:g
01
0
- 50
:;::
-60
-7 0
-80
- 90
38
When we increase the span to perform multidecade measurements, the log resolution mode
measures decades in parallel. The analyzer's digital filter has two channels for each input channel:
one side for real and another for imaginary. However, if our start frequency is 0 Hz, the data are all
real and we have one side of the filter available for other tasks. (This explains why the initial linear
resolution measurement always starts at 0 Hz.) Log resolution takes advantage of the available
channel by sending the first decade through one side of the filter and all higher decades through the
other side.
4-4
39
4 0
Line Number
Figure 4-1, The Log Resolution Window
4 1
4 2