S ECTI ON I I I
PRINCI PLES O F O PE RATION
3-1 .
I NTRODUCTION
This section describes the basic princi ples of operation of Mode ls 452 and 852
Dual Hi/La Fi l ters . Fig . 3-1 is a B lock D iagram of Model 452 and Fig . 3 -2
is a B l ock Diagram af Model 852 .
3 -2 . MODEL 452
The i nput signal of Channe l l is applied through a 0 dB/20 d B I nput Attenuator,
which controls the two avai lable gain positions, and is fol l owed by a high
i nput i m pedance buffer ampl i fi e r . The Input Attenuator provides a constant
1 Megohm i n put resistance i n either positio n . The output of the Buffer ampl ifier
is applied to a cascade of two " 2 nd -Order Prototypes" . A " 2 nd -Order Prototype"
is an active RC , 2nd-order ( 1 2 dB/o ctave) fi l ter section which i s used as a building
block to synthesize higher-order fi l ters . The gains of these stages at dc and at
cutoff are as fol lows:
Input Attenuator
I n pu t Buffer
1 st Prototype
2nd Prototype
Overal l (OUT/I N )
Gain a t d c
0 . 0
2 0 . 0 d B
+1 1 . 8 d B
+ 1 . 2 d B
+ 7 . 0 d B
+20 . 0 dB/O . O dB
G a i n at fc
0 . 0
2 0 . 0 dB
+1 1 . 8 dB
- 4 . 1 dB
+ 9 .3 dB
+ 1 7 . 0 dB/ - 3 . 0 dB
Cutoff F requency control is obtained by changing the resistors o f the fi lte r stages
i n 8 - 4 - 2 - 1 BCD steps. The capacitors are changed i n decade steps and
Option -01 adds an extra set of capocitors to extend the cutoff frequency coverage .
The output signal is available after the 2nd prototype through a 50-ohm (or,
optiona l l y , 600-ohm ) resistor. The output of the buffer ampl i fier is avai l ab l e at
T est Poi nt T P - 1 ; the output of the 1 st prototype at TP - 2 .
Channel 2 is identical to Channel l . Bath channels are powered fram one main
power suppl y, through separate ± 1 5 V dc dual -tracking regulators .
3-1
