Further Frequency Limitations - Agilent Technologies PSA Series Manual

Getting Started
Option 215 Limitations

Further Frequency Limitations

The frequency range of your source/spectrum analyzer combination is
limited by the frequency range limits of both instruments, so you have
to look at both instruments to determine this.
In addition, the digital nature of the signal sources and the PSA Series
spectrum analyzers means that signals and traces are read and
recorded as a series of data points. Each data point on a trace or a
measurement corresponds to a frequency 'bucket', which is a finite
frequency range into which data falls.
1
For example, assume we have a ten-point trace over a frequency range
of 3.0 GHz to 4.0 GHz. It will take 10 buckets to cover the 1.0 GHz
range. So, bucket 1 might collect data for all frequencies from
3.0000 GHz to 3.0999 GHz, bucket 2 might collect from 3.1000 GHz to
3.1999 GHz, and so on. This operation is normally not visible because
you have so many trace buckets. However, it does have an effect on the
behavior of Option 215 External Source Control at the limits of its
frequency ranges.
Option 215 External Source Control requires a 'spare' bucket at the
beginning and at the end of every sweep on the signal source. In other
words, to make the 1.0 GHz sweep from 3.0 GHz to 4.0 GHz in the
example above, one 'spare' bucket is required at the start to 'catch' the
data between 2.9000 GHz and 2.9999 GHz, and similarly, another
'spare' bucket is required at the end of the sweep.
This means that the number of sweep points on the signal source is
always two higher than that specified on the PSA Series analyzer to
allow for these 'spare' buckets.
You need to remember this frequency range limitation when setting up
your Option 215 measurements. Continuing with the 3.0 GHz to
4.0 GHz example sweep above, a problem arises if your signal source
has a maximum frequency of 4.0 GHz. The source can not sweep beyond
4.0 GHz, so the frequency range of the 'spare' bucket at the end of the
sweep must fall completely below the 4.0 GHz upper limit of the signal
source. This 'spare' bucket therefore 'catches' data from 3.9001 GHz to
4.0000 GHz. The previous bucket (3.8001 GHz to 3.9000 GHz)
represents the last bucket actually used in the sweep, so in this
example, the maximum frequency that your 4.0 GHz signal source
could sweep would actually be 3.9 GHz.
In other words, you can never quite reach the full frequency range of
your signal source. How close to the limit you can get depends on the
size of your buckets. This is determined by the number of sweep points
and the range over which you are sweeping. The more sweep points you
use, and the smaller the range of the sweep, the smaller will be the size
of the buckets. Therefore, the closer you will be able to get to the
1. Although a ten-point sweep has been chosen for clarity in this example, the min-
imum number of sweep points on a PSA is 101
10 Chapter 1