Pulsar TCF-10B, System Manual, Page: 120 / 249

The sine function is generated from the 13 most
significant bits of the phase accumulator. The
frequency of the NCO is determined by the
number stored in the
D
-phase register, which may
be programmed by two sequential 8-bit inputs.
The frequency programming capability of the
NCO is analogous to sampling a sine wave where
the sampling function is the clock.
If the output frequency is very low with respect to
the clock (less than f
C
/ 8096), then the NCO
output will sequence through each of the 8096
states of the sine function. As the output frequency
is increased with respect to the clock, the sine
function will appear to be more discontinuous,
because there will be fewer samples in each cycle.
At the Nyquist limit, when the output frequency is
exactly half the clock, the output waveform
reduces to a square wave. The practical upper
limit of the NCO output frequency is about 40%
of the clock frequency because spurious compo-
nents created by sampling, which are at a
frequency greater than half-the-clock frequency,
become difficult to remove by filtering.
The 12-bit output of the NCO is applied to the
input of the high-speed Digital-to-Analog
Converter (I5), which converts a digital sine wave
from the NCO to an analog output. The analog
output from I5 is filtered by a 630 kHz Low Pass
Filter (C14, C13, L1, L2, C15), producing a 0.512
Vp-p output at the carrier frequency. The carrier
frequency is applied to Modulator (I7), where it is
modulated by a dc and/or ac signal from a 2 kHz
Low Pass Filter (I10, R24, R25, R26, C30, C31,
C32). The output of I7 drives the Output
Amplifier (I11) and associated components. The
output of I11 is coupled through the Output
Transformer (T1) to provide a 50 ohm balanced
output.
The reference frequency to the NCO is generated
by a Crystal-Controlled Clock Oscillator (CCCO),
consisting of Y1, CMOS inverter (I6A), R3, R4,
C19, C20, and C50, at a frequency of 3.27680
MHz. The CCCO is buffered by I6B, which drives
the Shift and Control Logic (I3) and the NCO
clocks. The modulator (I7) receives its inputs
from the Analog MUX (I9) used for modulation
selection, through the Low Pass Filter whose
functions are described (in paragraphs 11.2.1,
11.2.2 and 11.2.3) below.
11.2.1 Low-Level Operation
When Transmitter key input voltage (pin A10) is
present, it removes the reset from the NCO (I4). If
no other input voltage is present (Transmitter key
signal only), the voltage divider (R12, R10)
supplies the modulating voltage to the modulator
(I7), through the selected analog multiplexer (I9)
channel. The 1 watt low-level operation is
produced when I9 (both A and B) are either “0” or
“1”, causing I9 to connect inputs X0 and Y0, or
X3 and Y3 to the outputs X and Y. Potentiometer
R12 controls the low-level output, which is
between 0 and 1 mW.
11.2.2 High-Level Operation
When the 10W voltage is keyed, it produces a “1”
at the I9 B input, causing channel 2 to be selected.
If no other input voltage is present (10 W key
signal only), the voltage divider (R10, R13)
supplies the modulating voltage to the modulator
(I7) through the multiplexer (I9) channel.
The 10 watt high-level operation is produced
when I9 A input is “0” and I9 B input is “1”,
causing I9 to connect inputs X2 and Y2 to the
output X and Y. Potentiometer R13 controls the
high-level output, which is between 0 and 1 mW.
11.2.3 Voice Operation
When the Voice key input voltage is present, it
produces a “1” at I9A input, causing channel 1 to
be selected. If no other input voltage is present
(Voice key signal only), the voltage divider (R10,
R14) supplies the modulating voltage to the
modulator (I7), through the selected analog multi-
plexer (I9) channel. The Voice operation is
produced when I9 A input is “1” and I9 B input is
“0”, causing I9 to connect X1 and Y1 to the
outputs X and Y. Potentiometer R14 controls the
voice carrier output level of the AM carrier, which
is between 0 and 1 mW. In addition, an ac signal
from AM Voice Input is added to the dc level
(through R8, R11, and C26) to modulate the
carrier. The audio modulating level is adjusted (by
potentiometer R11) to a maximum of 60% modu-
lation.
April 1997 Page 11–3
Chapter 11. Transmitter Module
11