Larcan FM-250, Техническое руководство

Страница: 46 / 86

FM-250 FM BROADCASTING EXCITER
Issue Date: 21 Jan 2003
output has a pulse repetition frequency of 76kHz with a 1.879uS pulse. The outputs of IC9
are routed to a Quad bilateral switch (IC11) which with precision weighting resistors (R53
to R60) , a 14 step replica of a 38kHz sinusoid is produced. This has a distinct advantage
over direct switching at 38kHz, as any spurious products produced appear at frequencies
532kHz and above and are easy to filter.
The output of IC8 pin 11 is routed via IC13/a , which produces a 38kHz square wave at its
outputs. These outputs are applied to another Quad bilateral switch IC12 which alternately
switches the Left and Right audio signals at the inputs of this switch at 38kHz These signals
are applied to the weighting resistors above.
The output of IC13/a pin 5 is routed via resistors R51 and R52 . (R51 is used to vary the
phase). This 38kHz signal is divided by 2 (via IC7/b) , whose output is at 19kHz . This signal
is then routed to the 19kHz bandpass filters.
The stereo coded signal produced has virtually no commutation products due to the use of High
speed CMOS switching and the final output from the weighting resistors is routed to IC17
via R73 .
h)
19kHz Bandpass filters
The 19kHz square wave above is routed to IC16/a and IC16/b , which are configured as
bandpass filters centered at 19kHz. These filters convert the 19kHz square wave to a
sinusoid with
≤
0.1% distortion. R63 and R67 adjust the filters for maximum amplitude at
19kHz and the output from IC16/b is routed to a SMB connector SK1 . This is for external
use and isolation is provided via R69 .
The signal is also routed to IC17 to provide the 19kHz pilot signal in the final Composite MPX
output. The pilot level is adjusted via R70 .
i) MPX Filter
The output from IC17 buffer is routed to a fifth order Linear phase filter ( IC18/a and
IC18/b) which provides
≥≥
50dB attenuation above 532kHz . The required filter Passband
(53kHz) has a flatness characteristic of
≤≤
0.02dB with a phase variance of
≤≤
0.1
°°
. Allpass
filter IC19/a provides a Fine phase adjustment of the stereo multiplex signal at 15kHz. The
total combination exhibits a group delay variance of
≤≤
10nS.
j) MPX output buffer
R91 (MPX Level) and R96 (Mono Level) provide adjustment for their respective functions
and RL1 provides the switching mechanism for Mono or Stereo . The final signal is routed
via buffer IC19/b to the output at SK2 (SMB connector).