Technical Manual Technical Description
© SPECTRA ENGINEERING 2006 Revision 4.2.3
71
4.4.2 TX Signal Processing
TX audio may be sourced from a number of different paths. These include
VF from line, Talk through audio, the microphone, and DC FM/Wideband
input, Tone generator from micro controller, Test TX VF inject from the
Monitor port, CTCSS generator and the internal RF modem.
The TX VF path is readily user configurable with most major functional
blocks being possible to either select or bypass by means of links (refer to
2.2.2.1 for details of link settings). The VF from line enters the board on
SKH-14 and RV4 (TX VF gain) provides gain adjustment to accommodate
different line levels. This is followed by a selectable compressor (IC34A)
with 30dB dynamic range. The output of the compressor apart from being fed
to the main TX audio path is also connected to the input side of the 'Line'
FFSK modem and via a gain control pot the speaker amp (IC60). Following
the main TX audio path, IC36B serves to switch the VF further on the TX
audio path or, under the control of the external input on SKH-13, to loop it
back to the line output via RV7 which provides level adjustment for the
looped back signal. From this switch the VF passes though a second switch
IC32B under the control of the Micro Controller and the external TX talk line
on SKH-10. This switch is used to disable the line TX VF path. From here
the VF passes through a pre-emphasis stage C66, R67 and a 300Hz high pass
filter (IC30B and IC29A) each of which is selectable through links. The
output of this filter is fed to IC30A a summer/limiter amp.
The summing point of this amp also serves to combine the audio from the
talk through path which is level adjusted by RV3, the microphone which is
processed through a similar compression, preemphasis and filtering chain as
the line VF, the tone (Morse code) signal from the microprocessor, the DC
FM input which enters the board on SKH-6 and the Test TX VF injection
from SKE-2. The latter three inputs are all at fixed levels. RV2 on IC30A
provides overall gain adjustment. Following the limiter amp is the TX VF
low pass filter. The standard Bessel filter has a 3.4kHz cut off frequency. A
second summing amplifier IC24B follows which combines the CTCSS,
optional modem (IC52 and IC54) and Wideband audio inputs with the TX
VF. Level adjustment of the CTCSS signal is in three 0.85dB steps either
above or below the nominal level (10% of max dev). These level steps are set
up using the Configure screen of MXTOOLS. The output of this final stage is
fed to two microprocessor-controlled digipots, which serve to adjust the
modulating signal level to the VCO and the reference oscillator. A third
digipot is used to provide an offset null for the reference oscillator centre
frequency. Adjustment of these levels is also by way of MXTOOLS using the
Channel Edit screen.
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