2-8
c.
Gai n Selection
The passband gain of each channe l is selected with the 0 dB GA I N/20 dB GA I N
Switch . The position of this switch determines whether the channel i s operating
with 0 dB Gai n ( OUT), or 20 dB Gai n ( I N ) .
The 2 0 d B Gai n pasition i s useful when fi l tering low-leve l signals . S ince output
noise of the fi lter is independent af the gai n position, a 20 dB effective i mprove
ment in dynam i c range is achieved.
d .
Band-Pass F i l ter
The two channel s af the instrument, one in High -Pass and the other i n Low-Pass
made , may be cascaded as shown i n Fig . 2 -1 1 ta provide Band -Pass aperation .
F l at A m p l i tude (Butterworth) response shou l d be util ized i n both chann e l s .
When channels are cascaded for Band-Pass operatio n , the i nput signal shou l d be
connected to the High-Pass channel ( I N 1 ) , and the output af this channe l ( OUT 1 )
shou l d be connected to the in put of the Low-Pass channel ( I N 2) . The output
signa1 is a vai lable at OUT 2 .
The passband i n this configuration extends from the High-Pass Cutoff F reque n cy to
the Low-Pass Cutoff Frequency setting . Maximum passband gain of 40 d B is
avai lable. If only 20 dB gai n is desired, channe l 1 should be set to 20 d B gain and
channel 2 sho u l d be set to 0 dB gai n .
CAUT I O N
Reversing the gain settings wi l l i ncrease the output noise
of this configuration, because channel 2 wil l ampl i fy the
output noise of channe
I
1 .
The overa l l response of this configuration may be determined by combining the
norma l ized plots g iven in Figures 2 -3 through 2-6 .
The narrowest Band-Pass is obtained by setting High-Pass and Low-Pass C utoff
F requencies to the same val ue fa . I n this case an i nsertion loss of approximate l y
6 d B is produced at fa . For the Mode l 452 , the 3-dB bandwidth extends from
0 . 8 fa to 1 .25 fa and is equal to 0 . 45 fa . This corresponds to a Q of 2 . 2 . For
the Mqd e l 852 , the 3 -dB bandwidth extends from 0 . 9 fa to 1 . 1 2 fa and is equal
to 0 . 22 fa . This corresponds to a Q of 4 . 5 . In this made of operation , both
i nstruments o perate as constant - Q, or constant % - Bandwidth fi lters .
