Models 900CT & 900BT

Model 900 Series

Single Channel –

Certified

Initial Setup Procedure

3

1784 Chessie Lane, Ottawa, IL 61350 • Tel: 800/252-7074, 815/434-7800 • FAX: 815/434-8176

e-mail: sales@freqdev.com

• Web Address: http://www.freqdev.com

Initial Setup

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.

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

X

MULT

Msd

B

C

D

Lsd

E

RELATIVE

TUNING ACCURACY

4

0

0

0.1X

GREATEST

0

4

0

1X

LESS

0

0

4

10X

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.

AC POWER SUPPLY

OR

INTERNAL BATTERIES

B

FILTER/

BYPASS

OUTPUT

AMPLIFER

GAIN = K

OUT

COUPLED

POWER LINE

NOISE VOLTAGE

V

B

R

SB

SIGNAL

COMMON

V

P

V

A

V

CM

RSA

INPUT SIGNAL AND

NOISE VOLTAGE SOURCES

A

(+)

(-)

R

CM+

+

R

D

R

CM-

-

DIFF

AMP

20d

B

10d

B

0d

*

Vo = K(VA - VB) + Vcm/CMRR : WHERE

K = 1, 10 AND 10 FOR GAIN SETTINGS

OF 0, 10 AND 20dB RESPECTIVELY.

SEE TEXT FOR REMAINING TERMS.

DENOTES FRONT PANEL ACCESS

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.

B

(

-)

(+)

+Vs

COM

-Vs

DIFFERENTIAL

INPUT AMPLIFIER

SIGNAL

COMMON

*

Circuit model illustrating the relationship between a filter’s differential

input and amplifier and external signal and error sources.