AD7008
{This section converts the twos complement au-
dio into offset binary scaled formodulating
the AD7008. If twos complement is used, the
modulation scheme will instead bedouble side-
band, suppressed carrier.}
cillator application is with the AD607 Monoceiver(tm). This
unique two chip combination provides a complete receiver sub-
system with digital frequency control, RSSI and demodulated
outputs for AM, FM and complex I/Q (SSB or QAM). (See
Figure 13.)
r5 = 0x80000000;
r6 = r6 xor r5;
r6 = lshift r6 by -1;
r6 = r6 xor r5;
Direct Digital Modulator
In addition to the basic DDS function provided by the AD7008,
the device also offers several modulation capabilities useful in a
wide variety of application. The simplest modulation scheme is
frequency shift keying or FSK. In this application, each of the
two frequency registers is loaded with a different value, one rep-
resenting the space frequency and the other the mark frequency.
The digital data stream is fed to the FSELECT pin causing
the AD7008 to modulate the carrier frequency between the two
values.
r4 = lshift r6 by -6;
{Load parallel assembly register with modula-
tion data. Q portion set to midscale, I
portion with scaled data}
r5 = 0x00000004;
dm(dds_para) = r5;
dm(dds_para) = r4;
F SELECT
{Transfer parallel assembly register to IQMOD
register}
1
1
CLOCK
0
0
0
32
32
FREQ 0
REG
32
r4 = 0xb0000000;
dm(dds_cont) = r4;
rti;
32
MUX
FREQ 1
REG
PHASE
ACCUMULATOR
Many applications require precise control of the output ampli-
tude, such as in local oscillators, signal generators and modula-
tors. There are several methods to control signal amplitude.
The most direct is to program the amplitude using the IQMOD
register on the AD7008. Other methods include selecting the
load resistor value or changing the value of RSET. Another op-
tion is to place a voltage out DAC on the ground side of RSET as
in Figure 16. This allows easy control of the output amplitude
without affecting other functions of the AD7008. Any combina-
tion of these techniques may be used as long as the full-scale
voltage developed across the load does not exceed 1 volt.
AD7008
Figure 14. FSK Modulator
The AD7008 has three registers that can be used for modula-
tion. Besides the example of frequency modulation shown
above, the frequency registers can be updated dynamically as
can the phase register and the IQMOD register. These can be
modulated at rates up to 16.5 MHz. The example shown below
along with code fragment shows how to implement the AD7008
in an amplitude modulation scheme. Other modulation
schemes can be implemented in a similar fashion.
U3
AD7008
C1
19
20
21
22
23
24
25
26
8
6
5
D0
D1
D2
D3
D4
D5
D6
D7
+5V
+5V
DMD24
DMD25
DMD26
DMD27
DMD28
DMD29
DMD30
DMD31
DMD32
DMD33
DMD34
DMD35
DMD36
DMD37
DMD38
DMD39
V
DMDXX–DATA BITS
DMAXX–ADDRESS BITS
REF
DSP:
SCALE
ANALOG
INPUT TO
FULL
0.1µF
C2
I MOD
10
COMP
0.1µF
ADC
R4
2
1
SCALE
49.9
10
IOUT
10
10
SIN
IOUT
10-BIT DAC
IOUT
R3
49.9
D8
D9
IOUT
9
U1
74HC138
6
10
10
11
12
13
14
15
16
27
32
33
34
35
36
41
42
31
30
38
37
SIN/COS
ROM
D10
D11
D12
D13
D14
D15
WR
CS
TC0
TC1
R5
4
+5V
DMS1
DMWR
7
9
G1
10
FSADJUST
Y7
Y6
Y5
Y4
Y3
Y2
Y1
Y0
4
5
G2A
G2B
COS
10
390
10
11
12
13
14
15
10
Q MOD
V
3
AA
+5V
17
28
39
V
DD
3
2
1
0
+5V
+5V
+5V
DMS3
DMD36
DMD37
DMD38
DMD39
DMA02
DMA01
DMA00
C
B
A
AD7008
V
DD
V
DD
TC2
TC3
44
7
18
29
43
Figure 15. Amplitude Modulation
AGND
DGND
DGND
DGND
DGND
LOAD
SCLK
SDATA
FSELECT
CLK
+5V
U2
50MHz
{__________IRQ3 Interrupt Vector__________}
14
{in_audio is a port used to sample the audio
signal. This signal is assumed to be twos
complement. This interrupt should be serviced
at an audio sample rate. This routine assumes
that the AD7008 has been set up with the Ampli-
tude Modulation Enabled.}
V
CC
RESET
SLEEP
RESET
40
TEST
8
OUT
EE
VOLTAGE OUT DAC,
i.e., AD7245A
0 TO +1 VOLTS
V
7
Ifs =
6233 x (V
–V
)
K1115
REF
DAC
R
SET
irq3_asserted:
{Get audio sample}
r6=dm(in_audio);
Figure 16. External Gain Adjustment
REV. B
–11–
ADI [ ADI ]
