OPERATION
2-10
where T
s
= settling time, N
i
= number of test frequency cycles used in the
measurement, f = test frequency, T
di
= deintegration time, T
rs
= resync time, T
d
= delay time, N
m
= number of submeasurements per measurement, and T calc =
calculation time. T
di
, T
d
and T
calc
are constants, T
rs
and f are set by the
measurement frequency. N
i
and N
m
are determined by the measurement rate
and T
s
set by the user. See the table below for the appropriate values.
N
m
Slow 8
Medium 8
Fast 8
100, 120 Hz
1kHz 10 kHz 100 kHz
N
i
Slow 20 40 400 4000
Medium 4 4 40 400
Fast 2 2 20 200
T
rs
1/f
T
di
2 ms
T
d
2 ms
T
calc
3 ms
T
s
2-99 ms
In addition to these factors, the measurement time is increased when using
autoranging, binning or the GPIB or RS232 interfaces. Binning adds about
2.5 mS to the total measurement time. Autoranging adds n•( T
meas
- 1mS)
where n=number of range changes required and T
meas
is calculated above.
It is difficult to determine the exact amount of time added when using GPIB or
RS232 interfaces, since it is dependent on baud rate, the speed of the computer
and the software. It takes about 5 mS for the unit to respond to a command after
it is received and about 5 mS to format a response to send back over the
interface. In general, simple commands and responses, like setting the frequency
or checking what range the unit is on, can be returned in about 10 mS. Longer
responses, like XALL?, can take as long as 2 seconds over RS232. If
communications speed is critical, the binary data format, which reduces the
number of bytes transferred, can be used.
SETTLING TIME Occasionally it is advantageous to delay making a measurement after the unit is
triggered. This allows bias voltages on capacitors to stabilize or contacts on a
handler to debounce. The settling time is set in 1 ms intervals from 2 to 99 ms. To
set the settling time, press the [Calibrate] key until the 'SEttL' message appears in
the left display and the present settling time value in the right display. Enter the
new settling time value, from 2 to 99, and press the [ENTER] key. If an illegal
value is entered the unit will beep and display 'rAngE Error'.
TRIGGERING The SR715/720 can make measurements continuously or in response to a
trigger. To change the trigger mode, press the [Mode] key until the desired mode
LED is on, either CONT or TRIGGERED. In the continuous mode the unit will
trigger itself at its maximum measurement rate. In triggered mode, the trigger can
be from the [Trigger] key, from the RS232 or GPIB interfaces, or from the Handler
interface. While making a measurement, the unit will ignore any triggers it
receives until the current measurement is complete. The TRIG LED will flash
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