User Manual

SECTION 1: THEORY OF OPERATION
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DIVAUDIO is routed directly to the internal modem as the modem provides internal filtering of received
signals.
For an external device, DIVAUDIO is routed through a 4800 Hz eighth-order Bessel low-pass filter. The
purpose of the filter is to improve the SNR of the recovered signal. Following the filter, the signal is
buffered to provide load isolation and is routed to an external device as FDATA (filtered, diversity
recovered modulation).
Exciter
Transmit Baseband Processing
Functional diagram of the IP1B’s transmit baseband processing section.
The IP1B is capable of being modulated by an external device (console, external modem, etc.) or by the
internal 4-level FSK modem. External modulation and internal modem modulation are, at any point in
time, mutually exclusive.
EXTMOD (external modulation) from an external device is buffered to provide load isolation and is then
routed to an eighth-order Bessel low-pass filter. The purpose of this filter is to remove any high-frequency
components present in the external modulation prior to application to the transmit modulator. Following
the filter, the external modulation is buffered to provide load isolation to the filter. The modulation is
routed though a summer and is applied to a VGA (variable gain amplifier) which provides transmit
modulation deviation control.
The output of the first VGA directly modulates the VCO. This signal is also routed through another VGA,
which provides low-frequency deviation control by modulating the reference oscillator. This configuration
is referred to as two-point modulation (with the VCO and the reference oscillator being the two (2)
modulation points). Two-point modulation prevents the radio's PLL circuitry from counteracting the
modulation process, and provides a clean flat modulation response to the low frequency portion of the
baseband spectrum.
4-level FSK modulation from the internal modem bypasses the Bessel filter as the modem contains built-
in filtering. This modulation is routed through the summer and is similarly sent to the VCO and reference
oscillator in the same manner as the external modulation.