900CT Datasheet, PDF (3/19 Pages) Frequency Devices, Inc. – Tunable Active Filter Instrument

Models 900CT & 900BT
Single Channel – Certified
Model 900 Series
Initial Setup Procedure
Initial Setup
Select desired operating voltage 115 V_ac
or 230 V_ac. See note "Q" on Page 5.
Set the POWER ON/OFF Switch to ON.
A continuously lit POWER lamp indicates
proper internal DC voltages, an essential
requirement for battery-powered models.
Allow the instrument a three-minute
warm-up period to achieve thermal
equilibrium.
To perform initial adjustment and/or
operational testing, set the remaining front
panel controls as follows:
a) The three base CORNER
FREQUENCY switches and the
MULTIPLIER to the desired corner
frequency…
b) The OFFSET control to approximately
mid-range…
c) The GAIN switch to the desired value…
d) The BYPASS switch to OUT…
e) The INPUT switch to ground ( ).
Connect a dc-coupled oscilloscope, of
vertical sensitivity 10mV/CM or better, or
a digital voltmeter (DVM) to the instrument
front panel BNC connector labeled OUT.
Set the OFFSET control for a zero-volt
reading on the scope or DVM.
Subsequent changes of CORNER
FREQUENCY and GAIN control settings
will introduce a small dc output offset,
which should be zeroed for critical
applications.
Leaving all other controls unchanged, set
the Input Switch to (A-B) and apply a
5Vdc signal simultaneously to input BNCs
(A) and (B). The voltage measured at the
OUT BNC should be 5 - 5 = 0 Vdc. This
completes preliminary test and
adjustment.
Corner Frequency Selection
To select a corner frequency, simply set
the CORNER FREQUENCY switches and
the MULTIPLIER switch for the desired
numerical value.
The CORNER FREQUENCY switch
weightings follow standard decimal
positional conventions.
Circuit model illustrating the relationship between a filter’s differential
input and amplifier and external signal and error sources.
INPUT SIGNAL AND
NOISE VOLTAGE SOURCES
RSA
VA
VCM
A
(+)
V_B
SIGNAL
COMMON
(-)
R_SB
B
COUPLED
POWER LINE
NOISE VOLTAGE
V_P
DIFFERENTIAL
INPUT AMPLIFIER
0dB
10dB
20dB
*
GAIN = K
R_CM+
R_D
R_CM-
+
DIFF
AMP
-
FILTER/
BYPASS
OUTPUT
AMPLIFER
OUT
-Vs
(-)
COM
Vo = K(VA - VB) + Vcm/CMRR : WHERE
K = 1, 10 AND 10 FOR GAIN SETTINGS
OF 0, 10 AND 20dB RESPECTIVELY.
+Vs SEE TEXT FOR REMAINING TERMS.
* ^{DENOTES FRONT PANEL ACCESS}
(+)
AC POWER SUPPLY
OR
INTERNAL BATTERIES
SIGNAL
COMMON
The B, C and D switches combined can
select base corner frequency values
ranging from 1 to 499 Hz in 1 Hz steps
with switch weightings as just described.
The accuracy of the corner frequency is
improved by selecting the largest possible
base frequency and down scaling by the
MULTIPLIER. The greatest accuracy is
obtained with the largest base 400, and
the 0.1X MULTIPLIER switch setting.
Relative accuracy of selected 40 Hz
actual corner frequency for different
multiplier switch settings.
BASE
FREQ
Msd
BCD
400
040
004
X
MULT
Lsd
E
0.1X
1X
10X
RELATIVE
TUNING ACCURACY
GREATEST
LESS
LEAST
The differential input
The instrument input utilizes a differential
input amplifier to reject prevalent forms of
electrical interference, while presenting
desirable input characteristics to the
signal source requiring filtering. The
differential input configuration is ideal for
measuring the difference between two
values rather than the values themselves.
Bridge circuits utilizing strain gages,
thermocouples and a variety of other
types of transducers generate differential
full-scale output voltages in the order of
millivolts that are often superimposed
upon volt-level reference and noise
values.
The importance of CMRR
In actual system environments, each
signal and power return conductor can
generate an interference voltage
proportional to the net conductor
resistance and the electrical current level.
Any such interference voltages appear as
common mode signals to the amplifier,
and are rejected as such.
3
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