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3 - Installation
The overvoltage protection circuit senses voltage near the output terminals, not at the load. Therefore the
signal sensed by the OVP circuit can be significantly higher than the actual voltage at the load. When
using remote sensing, you must program the OVP trip voltage high enough to compensate for the voltage
drop between the output terminals and the load. Also, if the sum of the programmed voltage and the load-
lead drop exceeds the maximum voltage rating of the dc source, this may also trip the OV protection
circuit. Refer to OVP considerations for more information.
Maintaining Stability while Remote Sensing
The remote sense bandwidth and slew rate of standard dc power sources are adequate for compensating
for load lead voltage drop for slow to moderate rates of load changes. However, the high pulsed current
draw of digital cellular phones presents a challenge to standard dc power sources operating in remote
sense mode. Their bandwidth and slew rate are not adequate for dealing with the 0.05 to 0.2 amp/µs slew
rates imposed by these devices. A large voltage transient occurs at the load, due to the inability of the dc
source to keep up with the rate of load change.
In remote sense mode, the HP 66311A effectively compensates for load lead voltage drops resulting from
very high slew rate load current transitions and thus keeps the remotely sensed output voltage at a
constant level. For 0.05 amp/µs to 0.2 amp/µs slew rate loading in typical test applications, the transient
voltage is reduced more than an order of magnitude over that of a standard dc source.
Output Compensation (High Mode/Low Mode)
High bandwidth performance and stability are achieved by using a software-switchable output
compensation circuit. This compensation circuit sets the remote sensing response of the dc source for the
capacitance of the cellular phones. The compensation function is set using either the front panel
TYPE:CAP command located in the Output menu (see chapter 5), or the OUTput:TYPE[:CAPacitance]
SCPI command as explained in chapter 8. The circuit covers the following capacitance ranges:
Low Mode: 0 to 12,000 µF
♦
High Mode: 5 µF to 12,000 µF
♦
The HP 66311A is shipped from the factory with the output compensation set to Low Mode. If you do
not know the input capacitance of the phone that you are testing, leave the input capacitance set to Low
Mode initially. This is because in Low Mode, the output of the dc source will be stable when testing
cellular phones that have virtually any input capacitance (from 0 µF to 12,000 µF). Low mode however,
has a slower transient response (see appendix A).
The High Mode output compensation setting provides faster transient response performance for most
phones. (Most phones have input capacitances greater than 5 µF.) In High Mode however, the operation
of the dc source may be momentarily unstable with phones that have input capacitances less than 5 µF.
If you are testing phones in High Mode and want to determine if the input capacitance of your phone is
less than 5 µF, perform the following test.
NOTE:
It is important that this test is done with the dc source installed in the test system where it
will be used, since system stability is also dependent on wiring and the phone
impedance.
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