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Model 8902A
Opera tion
FM De-emphasis
FUNCTIONS
Pre-Display,
25ps, 50ps, 7 5 ~ s . 750ps
DESCRIPTION
The de-emphasis networks equalize the pre-emphasis often encoded o n broadcast FM signals. This
type
of equalization provides a simple means of reducing audible noise on FM communication
systems. Pre-emphasis gives a
6
dB per octave, high-frequency boost to the baseband audio before
modulating the carrier. De-emphasis reverses the boost after demodulation at the receiver. The 3
dB corner o f the pre-emphasis o r de-emphasis network is usually expressed as a time constant. The
relationship between the time constant,
rO,
and the 3dB corner frequency,
fo,
is
f0
=
3
5
~
~
.
The 3dB corner frequencies of the de-emphasis equalizers (typically accurate to f3%)are listed in
the following table:
I
FM De-emphasis
Time
Constant
(PSI
3
dB
Frequency
(Ha
25
50
75
750
6366
31
83
21 22
212.2
PROCEDURE
To add de-emphasis to the demodulated FM, select the
key corresponding to the desired time
constant. (Unless AUDIO INPUT has been selected, the de-emphasized FM is available at the
MODULATION OUTPUT/AUDIO INPUT.) To also display the de-emphasized FM, select PRE
DISPLAY.
HP-IB
In local operation, to turn off one of the de-emphasis functions, press the lighted key a second time.
In remote operation, turn the equalizers o f f with code PO. (HP-IB code PO also turns off the PRE
DISPLAY function.)
3- I87
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