Sequerra 1 Owner's Manual - page 24
3. F M L I M I T E R
The F M L i m i t e r provides a means o f removing all o f the A M noise p r o d u c e d f r o m the selected
signal, leaving o n l y the zero axis crossing i n f o r m a t i o n t o be fed to the F M Detector. T h e F M
L i m i t e r also supplies the first f i v e decades o f vertical i n f o r m a t i o n f r o m the T u n i n g Display.
4 . F M D E T E C T O R
T h e F M Travis-Smith D e t e c t o r takes the i n f o r m a t i o n contained on the zero axis crossing f r o m
the L i m i t e r and converts t h a t t i m e displacement i n t o an audio signal. Its o u t p u t drives the
M u l t i p l e x Decoder, and the h o r i z o n t a l axis o f the oscilloscope in the T u n i n g Display.
5. M U L T I P L E X D E C O D E R
T h e i n p u t c i r c u i t to the M u l t i p l e x Decoder is used t o separate the 19 K H z p i l o t and t o remove
the 6 7 K H z (SCA) signal f r o m the composite stereo signal. T h i s is a u n i q u e l y designed f i l t e r
system w h i c h precisely c o n t r o l s t h e phase a m p l i t u d e relationships o f b o t h L + R signal, and the
L—R signal. T h i s system can be t e r m e d a quasi-vestigial side band processing c i r c u i t . A t this
p o i n t , the separate 19 K H z signal is fed i n t o a frequency d o u b l e r w h i c h is used t o phase lock
t h e free running 3 8 K H z constant a m p l i t u d e oscillator.
F r o m the 3 8 k H z phase synchronized oscillator, the m u l t i p l e x peak decoder switching system
is d r i v e n . T h e peak d e t e c t i o n system is the one t h a t p e r m i t s the least possible phase error in
the d e m o d u l a t i o n process and can be p r o p e r l y called a push-pull peak detector.
The o u t p u t of the m u l t i p l e x system is fed i n t o the o u t p u t f i l t e r system w h i c h performs the
f o l l o w i n g f u n c t i o n s : First it inserts the n o r m a l 75 usee. (50 usee, available on special order)
de-emphasis curve; then it adds t o it the r o l l o f f whose i n f l e c t i o n p o i n t is around 2 3 k H z ; an
a d d i t i o n a l n o t c h i n g f i l t e r is inserted after this t o remove the remaining components o f the 3 8
k H z oscillator. This system is capable, w i t h a saturating signal, o f a signal t o noise ratio o f
greater than 7 0 d B b e l o w peak levels.
6. D O L B Y
T h e D o l b y d e m o d u l a t o r performs the f u n c t i o n o f the a u t o m a t i c frequency c o r r e c t i o n o f a
D o l b y encoded signal. It is o n l y used w h e n the station is broadcasting in a D o l b y encoded
mode.
7. F R E Q U E N C Y C O U N T E R
T h e Frequency C o u n t e r displays t h e frequency o f the station that the set is tuned t o . It
operates in the f o l l o w i n g manner: This c o u n t e r has been designed t o c o u n t to 200. I n its reset
a r i t h m e t i c it is always set t o 1 8 9 . 3 . A signal f r o m the local oscillator in the F r o n t End is fed
i n t o the i n p u t o f the counter. Since the local oscillator's frequency is 10.7 M H z higher than
t h e stations frequency, the c o u n t e r in its scaling w i l l read the stations frequency. T h i s counter
uses a 1 M H z crystal as its time base, and has an accuracy o f ± 100 Hz reference to the stations
frequency.
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