Communications: Using Direct Digital Synthesis
Summary: This module introduces Direct Digital Synthesis with prepared code that transmits arbitrary input as an FM radio signal. The module then explains how to program the DDS hardware and concludes with a simple Frequency Shift Keying exercise.
Introduction
Direct Digital Synthesis (DDS) is a
method for generating a desired waveform (such as a sine wave)
by using the technique described in Figure 1 below.
 Figure 1: Direct digital synthesis (DDS) (Couch) |
Quantized samples of a desired waveform are stored in the memory
of the microprocessor system. This desired waveform can then be
generated by "playing out" the stored words into the
digital-to-analog converter. The frequency of this waveform is
determined simply by how fast the stored words are read from
memory, and is thus programmable. Likewise, the phase and
amplitude of the generated waveform are programmable.
The DDS technique is replacing analog circuits in many
applications. For example, it is used in higher-priced
communication receivers to generate local oscillator signals.
It can also be used to generate sounds in electronic pipe organs
and music synthesizers. Another application is its use by lab
instrument manufacturers to generate output waveforms in
function generators and arbitrary waveform generators (Couch).
In this lab you will familiarize yourself with the capabilities
of the Analog Devices AD9854 DDS. The DDS board is installed
between the 6-channel card and the DSP card at some (not all)
lab stations. You can tell which boxes have them by the way the
6-channel card sits higher inside the metal box.
Frequency Modulation (FM) Radio Exercise
To get your feet wet and see a demonstration of the DDS,
perform the following exercise. Copy the files
FM.asm (downloadable here) and mod.asm from the
v:\ece320\54x\dds\ directory. Assemble and run
the frequency modulation (FM) program FM.asm.
Next, plug an audio source into one of the two DSP input
channels that you've been using all semester. If you have a
CD on you, pop it into the computer and use that. If not, use
a music web site on the Internet as your audio source.
Connect the computer to the DSP by using a male-male audio
cable and an audio-to-BNC converter box (little blue box),
both of which are in the lab. The computer has three audio
outputs on the back; use the middle jack. Ask your TA if you
can't find the cable and/or box or don't see how to make the
connection. Next, connect a dipole antenna to the output of
the DDS (port \#1 on the back of the DDS board). A crude but
effective dipole antenna can be formed by connecting together
a few BNC and banana cables in the shape of a T.
There should be one or two of these concoctions in the
lab. Once the connections are made, turn on the black receiver
in the lab, and tune it to 104.9 MHz (wide band FM). You
should be able to hear your audio source! note: If your audio sounds distorted, it's most likely
due to the volume of your audio source being too loud and
getting clipped by the DSP analog-to-digital converter.
Spectral Copies
Spectral Copies: The digital-to-analog converter on the DDS is
unfiltered, which means that there is no anti-imaging filter
to remove the spectral replicas. To see this, plug the output
of the DDS board directly into the vector signal analyzer
(VSA), and observe the spectrum. Use 104.9 MHz as the center
frequency, and set the span wide enough so that you can see
the spectra of the replicas to the left and right of the 104.9
MHz signal. Use the marker to find the peaks of the other
replicas, and record their frequencies. Once you've done
that, reattach the antenna to the DDS output, and tune the
receiver to the frequencies you just recorded. You should be
able to hear your audio on each of the other frequencies.
7 π 2
7
2
Note: The clock rate of the DDS is 60MHz, which corresponds to
2 π
2
Therefore, the 104.9
MHz signal you just listened to is roughly equivalent to
How to use the DDS
The DDS has several different modes of operation: single-tone,
unramped Frequency Shift Keying
(FSK), ramped FSK, chirp, and Binary Phase
Shift Keying (BPSK). In this lab we will
use the DDS in single-tone mode. Single-tone mode is easy to
use, and is powerful enough to create many different kinds of
waveforms, including FM and FSK.
FM code
The FM code you just ran (also listed here) is fairly
straightforward. The program first calls the
radioinit subroutine. This routine sets the
DDS to single-tone mode and turns off an inverse-sinc filter
to conserve power. Following radioinit, the
setcarrier subroutine is called. This routine
sets the frequency of the DDS output by writing to the two
most significant 8-bit frequency registers of the 48-bit
frequency-tuning word on the DDS1. Although the
frequency-tuning word on the DDS has 48 bits of resolution,
the upper 16 bits provide us with enough resolution for the
purposes of this lab, and so we will only be writing to the
two most significant registers. See page 26 in
the DDS data sheet for a layout of the frequency-tuning
word.
To set the carrier frequency, we first need to determine what
frequency word has to be written to the frequency registers on
the DDS. This can be done using Equation 1:
Frequency word = baseband frequency
60
MHz
2 48
Frequency word
baseband frequency
60
MHz
2
48
406D 3A06
D3D4h. But since we only write to the two most
significant registers of the frequency-tuning word, we only
need the first 4 hexadecimal numbers of this result,
i.e. 406Dh. The first two of those,
40h, need to get written to the most
significant 8-bit frequency register, while the second two
hex numbers, 6Dh, need to get written to the
second-most significant 8-bit frequency register. This is
where the 40h and 6Dh in the
setcarrier subroutine of the FM code come from.
Writing to the frequency registers is accomplished using the
portw instruction. To write to the frequency
or phase registers on the DDS, the second operand of the
portw instruction must be
10xxxxxx, where the lower six bits are the
address of the specific register to be written to. The
address of the most significant frequency register on the
DDS is 04h, and the address of the second most
significant frequency register on the DDS is
05h (see page 26 in the data
sheet). It is important to note that the way our DDS boards
were built, you will not be allowed to make two consecutive
writes. To solve this problem, a subroutine called
nullop is called to waste some CPU time between
writes. nullop does this by simply repeating
the nop instruction 128 times.
After the program returns from the
setcarrier
subroutine, it enters an infinite loop in which it waits for
a serial interrupt to occur. The serial interrupt occurs
every time a new sample is acquired from one of the two
input channels and is transmitted to the DSP via the serial
port. When the interrupt occurs, an interrupt service
routine called ANALOG_IFC (see
core_mod.asm executes and calls the
handle_sample subroutine. The
handle_sample subroutine reads in the acquired
sample from the serial port and scales that sample so that
it can be "mapped" to a frequency in the range of ±75
kHz2. The scaled sample therefore
determines the frequency deviation and is added to
6Dh. The last step is to write the result to
the second most significant frequency register so that the
frequency of the DDS output can be updated.
Programming the Phase
The process for changing the phase of the DDS output is the
same as it was for changing the frequency of the DDS output.
To change the phase, you need to write a phase word to a
phase-adjust register on the DDS. The phase-adjust register
is 14 bits wide and is split up into two smaller registers
that you can write to (see page 26 in the data
sheet). The upper 6 bits have address
Phase word = Carrier phase 2 π 2 14
Phase word
Carrier phase
2
2
14
00h, and
the lower 8 bits have address 01h. The phase
word can be calculated using Equation 2:
portw instruction as before.
Just make sure you use the correct address for the phase
register.
Programming the Amplitude
The DDS also allows you to program the amplitude, but this
functionality is not addressed in this lab. You will be
able to implement a digital communication system in ECE320
without having to program the amplitude. Interested readers
are referred to the data sheet.
FSK exercise
Now that you know how to use the DDS in single-tone mode,
implement a simple FSK system that uses 2 frequencies: 120.005
MHz, and 120.011 MHz. You don't need to encode any data for
this exercise. In other words, your DDS output should just
continuously alternate between the two frequency symbols.
Also, the DDS automatically ensures continuous phase, so you
won't have to keep track of it. Use a symbol length of
approximately 725μs (the same length as your lab 5
symbols). Timer interrupts are an elegant way to control the
symbol lengths, but in this lab we will keep things simple and
control the symbol lengths by creating a second (longer)
nullop subroutine and calling it between writes
to the DDS. The second nullop subroutine should
waste approximately 725μs worth of time. Note: Since we're not using input or output from the
DSP, you don't need to use the WaitData or WaitAudio
macros.
Testing
Note that the corresponding baseband frequencies for 120.005
MHz and 120.011 MHz are 5 kHz, and 11 kHz, respectively.
Since these baseband frequencies lie within the 22.05 kHz
bandwidth of the DSP, you will be able to view your FSK
signal in real time on the oscilloscope
without the contribution from the
spectral replicas. Just feed the output of the DDS into a
second DSP (the anti-aliasing filter on the DSP will get rid
of the spectral replicas), and pass it through to the output
and the scope. You should be able to verify that there is
continuous phase between frequency symbols, and that your
symbol length is approximately 725μs. You should also
view the spectrum of your DDS output on the VSA to verify
that your symbols have the correct frequencies.
Appendix
1. Communication between the DSP and the DDS is
done through a parallel bus.
2. In FM radio, the amplitude of the
message signal being transmitted determines the amount of
frequency deviation from the carrier frequency of the
passband signal. ±75 kHz is the largest frequency
deviation allowed.
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Last edited by Adan Galvan on Feb 25, 2004 11:51 am US/Central.