User`s guide

11
Measurement Time vs. Trace Length
1 4 8 16 32 64 128
0
3
6
9
12
15
18
21
24
27
Trace Length
min
Zoom=128
Zoom=64
Zoom=32
Zoom=Off
Zoom=16
Zoom=8
Figure 8: Measurement times versus trace length for various zoom factors.
For N averages, the measurement times will increase by a factor of N.
To change the negative delay
flag, the user must edit the
program. The variable can be
found in the Measure_group
subroutine. With the USER keys
active, type:
EDIT Measure_group
Set Neg_delay_flag to 0 to
correct negative delay values;
set to 1 to display negative
numbers.
Measurement Times
The time required to perform a
group delay measurement
varies considerably, depending
on the accuracy and resolution
needed. Figure 8 shows typical
measurement times versus
trace length for various zoom
values. These measurement
times will increase by a factor
of N when N averages are used.
If the flag is set such that
negative delay values are
corrected, then 1/fmod worth of
delay is added when the direct
demodulation mode is used, and
.5/fmod worth of delay is added
when the FFT zoom mode is
used. This makes any negative
values positive. This operating
mode can correct a
measurement when one phase
wrap has occurred, but can
make a normalized
measurement invalid.
Following the previous example
(using the zoom FFT and 1 MHz
modulation), the normalized
measurement would show -440
ns, which is clearly not correct
(56 ns - (-4 ns + 500 ns)).
Using the
Personality
Obtaining the Program
The HP 71500A group delay
personality is distributed free of
charge on an “as is” basis,
without a formal HP product
number. It is neither a fully
supported nor specified product.
The user is free to modify the
IBASIC code to customize it for
their particular measurement
application. The program can be
obtained by contacting your
local HP sales or application
engineer.
Firmware Upgrades
What if I have an older
instrument?
This personality needs firmware
revision 1.1 or greater. If you
have an HP 71500A or an HP
70820A module with firmware
revision 1.0 or 1.01, then a
firmware upgrade kit must be
ordered (product number HP
70863A). This upgrade sells for
$2000, and adds several new
features besides the capability
to run IBASIC programs.
Additional features include
increased trace memory for
single-shot measurements, limit
lines, and enhanced trace math
operations.
Downloading the
Program
The group delay personality
must be downloaded into the
memory of the instrument each
time the instrument is powered
up (program memory is volatile).
There are two basic ways to
accomplish this. One method