Service Information
5-10
Storing constants
Calibration constants can be stored from the front panel or
over the bus, as explained below. The nominal (ideal)
value for each of these constants is 1.0. The allowable
range is from 0.5 to 1.5; if you attempt to enter a value
outside this range, a “Parameter data out of range” error
will occur.
■
Storing Constants from the Front Panel
To enter gain constants from the front panel, select
ENTER-CAL-CONSTANTS in the PREAMP CAL
MENU, then press ENTER. Select the desired constant to
display, then press ENTER. The unit will then display the
present value of the constant. Use the range and cursor
keys to enter the constant value, then press ENTER to
complete the process. Repeat the procedure for the other
two constants.
NOTE
Constants entered from the front panel
are only temporary and will be lost when
power is cycled. In order to save con-
stants permanently, you must use the bus
:CAL:UNPR:PRE:SAVE command, as
explained below.
■
Storing Constants Over the IEEE-488 Bus
To write constants over the bus, send the appropriate
command along with the corresponding numeric constant
value:
:CAL:UNPR:PRE:G1K <value>
(
×
1,000 gain constant)
:CAL:UNPR:PRE:G10K <value>
(
×
10,000 gain constant)
:CAL:UNPR:PRE:G100K <value>
(
×
100,000 gain constant)
Here <value> represents the constant value in floating-
point numeric format.
To permanently save new gain constants, send the follow-
ing command after storing new constants:
:CAL:UNPR:PRE:SAVE
NOTE
Programmed constants are only tempo-
rary and will be lost when power is
cycled unless the :CAL:UNPR:PRE:
SAVE command is sent to permanently
save them.
5.3
Principles of operation
The following paragraphs discuss the basic operating
principles for the Model 1801, and can be used as an aid
in troubleshooting the preamplifier. Refer to drawing
number 1801-106, located at the end of Section 6, for a
schematic diagram of the power supply card.
5.3.1 Block diagram
Figure 5-4 shows a simplified block diagram of the Model
1801. The unit includes a remote preamplifier, a power
supply connecting cable, and a power supply card. Each
of these components is briefly discussed below.
5.3.2 Preamplifier module
The remote preamplifier uses a chopper demodulator sys-
tem. With this topology, the input signal is chopped at a
frequency of 288Hz, and it is then magnetically amplified.
A narrow-band AC amplifier follows the magnetic com-
ponents. The signal is then demodulated and amplified,
and the resulting DC signal is then fed back to the input to
buck the input signal to maintain high input impedance
and low input bias current.
5.3.3 Power supply cable
The power supply cable carries the ±9V DC supplies and
gain/filter control lines from the power supply card to the
preamp module. It also carries the amplified output signal
back from the preamp module to the power supply card
where it is routed to the Model 2001 Multimeter.
5.3.4 Power supply card
The power supply card is made up of three basic sections:
the preamplifier power supply, the common-mode rejec-
tion circuit and its power supply, and the control and
memory circuits. Each of these sections is briefly dis-
cussed below.