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Keysight Technologies
Migrating Balanced Measurements from the
HP 8903B to the Keysight U8903A Audio Analyzer
Application Note
02 | Keysight | Migrating Balanced Measurements from the HP 8903B to the U8903A Audio Analyzer - Application Note
Abstract Hewlett-Packard introduced the HP 8903B audio analyzer in 1980. It quickly became the
industry standard because of its unprecedented performance and features. The U8903A
audio analyzer is the next-generation audio analyzer from Keysight Technologies, Inc.
(refer to Figure 1).
This application note explains the HP 8903B and U8903A audio analyzers balanced
connection architectures, and the accessories for migrating connections from the
HP 8903B to U8903A
Figure 1. The HP 8903B (left) and Keysight U8903A (right) audio analyzers
HP 8903B Output and Input Older generation instruments like the HP 8903B provided BNC connector pairs for both
the generator output and the analyzer input. Figure 2 shows a simplified block diagram
of the analyzer input.
The `HIGH' BNC connector is used in isolation when an unbalanced measurement is
made. When a balanced measurement is made both the `HIGH' and `LOW' BNC connectors
are used. The center conductor of the `HIGH' connector carries the signal and the `LOW'
conductor carries the inverted signal. The outer conductors of both connectors are con-
nected to ground. The generator output connections function in a similar way.
BNC
`Float' = Balanced/Unbalanced
HIGH
GND
`Float' 100 k
LOW
GND
GND
Figure 2. Analyzer input connections and block diagram for the HP 8903B
HP 8903B Float Switch A common misunderstanding is that the `FLOAT' switch on the HP 8903B floats the
ground reference of the generator or analyzer. In actuality the `FLOAT' switch shorts the
`LOW' input line to ground when the unbalanced configuration is required (see Figure 2).
When the switch is in the `FLOAT' position the `LOW' input floats to the level of signal
that is applied to it from the source, thereby enabling a balanced measurement.
In the era of the HP 8903B, engineers referred to this as "floating the input", hence the
name float switch.
03 | Keysight | Migrating Balanced Measurements from the HP 8903B to the U8903A Audio Analyzer - Application Note
Keysight U8903A Output The U8903A audio analyzer also uses BNC connectors for unbalanced measurements.
For balanced measurements the instrument uses the industry-standard XLR connectors.
and Input
EIA Standard RS-297-A describes the use of the XLR connector in balanced audio signal
applications.
Figure 3 shows a simplified block diagram of the U8903A input configuration. The same
`FLOAT' switch is used to select balanced/unbalanced configurations. The difference
between the HP 8903B and the Keysight U8903A is that the U8903A uses an internal
relay which is controlled via the user interface.
BNC
XLR
GND
100 k
`Float'
GND
Figure 3. Analyzer input connections and block diagram for the U8903A
04 | Keysight | Migrating Balanced Measurements from the HP 8903B to the U8903A Audio Analyzer - Application Note
Migrating Balanced While Keysight recommends users upgrade to the industry-standard XLR connec-
tors, there are situations, such as in military applications or standards testing, where
Connections from the HP 8903B
this is neither desirable nor possible. As an alternative to re-cabling from the U8903A
to Keysight U8903A to your system, Keysight has made available two accessory cables: U8903A-107 and
U8903A-108 (see Figure 4). The schematic of the analyzer accessory cable alongside the
input block diagram of the U8903A is shown in Figure 5. U8903A-107 is available for the
analyzer input and U8903A-108 is available for the generator output.
Note that the BNC connectors are mounted on insulating spacers. This performs two
functions. First, it sets the connector spacing to match the HP 8903B for compatibility
with test jigs developed for this instrument. Second, the spacers stop the BNCs' ground
connections from touching each other in a random fashion. This eliminates local ground
loops within the cable assembly and ensures the highest measurement integrity. The
plastic spacers can be easily removed for applications where they are not required.
Figure 4. Accessory cables simplify the conversion of balanced measurements from HP 8903B
to U8903A
BNC
BNC
XLR XLR
GND
2 1 1 2
GND 3 3
100 k
`Float'
GND GND GND
Accessory Cable GND
Figure 5. A U8903A-107 schematic alongside the input block diagram of the U8903A
Summary HP 8903B's outputs/inputs employ two BNC connectors (`HIGH' and `LOW') to enable
balanced measurements. There is a manual `FLOAT' switch which shorts the `LOW'
connection to ground where unbalanced measurements are required. The Keysight
U8903A outputs/inputs use BNC (unbalanced) and XLR (balanced) connectors. The
function of the manual `FLOAT' switch on the HP 8903B audio analyzer is replaced by
an internal relay in the Keysight U8903A. U8903A-107 and U8903A-108 accessory
cables allow the migration of balanced measurements from the HP 8903A to the
U8903A without the need to re-cable the whole measurement system.
05 | Keysight | Migrating Balanced Measurements from the HP 8903B to the U8903A Audio Analyzer - Application Note
myKeysight For more information on Keysight
Technologies' products, applications or
www.keysight.com/find/mykeysight
services, please contact your local Keysight
A personalized view into the information most relevant to you.
office. The complete list is available at:
www.axiestandard.org www.keysight.com/find/contactus
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