and right audio, and a stereo indication that is sent to the
front panel board display. The left and right audio are
each buffered, and passed through separate 15.5 kHz
low pass filters. Then each signal is amplified by U18
(pin 8 out) and U18 (pin 14 out) to form FM _AUDIO _L
and FM _AUDIO - R.
HF and FM Local Oscillator
The same PLL, U25, is used for HF and FM
operation. For HF, the 45.1-75 MHz LO signal is
generated by oscillator Q10 then amplified Q2 to be sent
to the HF 1
st
mixer. Q3 amplifies the same oscillator
signal to be sent back to the PLL divider. U24 is a direct
digital synthesizer that functions as a fractional
frequency divider to divide the oscillator frequency down
to 400 kHz. It is programmed like the PLL by the front
panel board processor. After the divider, the signal is
returned to the PLL. The PLL provides up and down
correction pulses that are amplified by Q12, Q11, and
Q48, and then filtered by C253 to produce the DC
voltage that sets the oscillator frequency.
For FM, the 86.7-118.7 MHz LO signal is generated
by Q9 and amplified by Q1 before being sent to the FM
mixer. From Q1, it also is fed directly back to the PLL,
without passing through U24. Q69 selects the proper
damping depending on which oscillator (HF or FM) is in
use.
BFO
The BFO is generated by a direct digital synthesizer
that is programmed within U23, which is a
programmable logic gate array. Again, the processor on
the front panel enables and addresses U23, depending
on mode, and PBT setting. The 8 bit digital output is
converted to an analog signal by R376-391, and
bandpass filtered by L68 and C170 before being fed to
the product detector circuitry.
Audio Selection
The audio selection is provided by U12 which also
provides muting. The /MUTE line is generated by the
processor and is fed through Q51. The /SQUELCH line
is generated by U2 depending on the SQUELCH control
setting and is fed through Q52. The SQUELCH_EN line
is generated by the processor and is used to defeat
muting determined by the SQUELCH setting.
Left and right audio from the Satellite Radio
Interface Board are buffered by U2 (pin 14 and 8) before
passing to U12. Left and right audio from the FM
receiver is fed directly in to U12, having been already
buffered. Left and right audio from the LINE IN jack J2 is
buffered by U13 (pin 1 and 7 out) before passing to U12.
Mono audio from the HF receiver (HF_AUDIO) is fed
directly into both channels, being already buffered. After
selection, the left and right audio is connected to U4 (pin
10,11) to be summed in the case of mono selection, then
to U13 (pins 14 and 8 out), before it is sent to the LINE
OUT jack (J3), and to the audio board.
Connections between the EXT.SPEAKER jack (J4),
the headphones jack (J5), and the audio board, are also
provided on the main board (AUDIO 1 and 2). The
headphone jack contains contacts to provide logic
signals to determine how the audio board must operate
(bridged or 2 channel), and to indicate the presence of
headphones for the processor to display.
U22 provides a means for many of the signals that
must return to the processor to do so in a serial data
format. U19 through U21 provide a means for many
control signals from the processor to be sent onto the
main board in a serial data format. U21 also serves as
HF front end bandpass filter selector, OR 8 bits that
when converted to an analog signal, form V_TUNE for
the FM tracking filter. U26 provides amplification of the
V_TUNE voltage. The supply for U26 is 16 volts,
generated on the front panel board. Q900 and Q901
provide gating to the data signal that addresses U21, so
that the data is only present when U21 is being
addressed. Q13 inverts the enable signal for U19
21.
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