|Título/s:||Low distortion signal generator based on direct digital synthesis for ADC characterization|
|Autor/es:||Adad, Walter F.; Iuzzolino, Ricardo|
|Institución:||INTI-Física y Metrología. Buenos Aires, AR|
|Editor:||International Measurement Confederation|
|Palabras clave:||Generadores de señales; Técnicas digitales; Mediciones; Frecuencia; Distorsión; Convertidores digital-analógico|
| Ver+/- |
July 2012, Volume 1, Number 1, 59‐64
ACTA IMEKO | www.imeko.org July 2012 | Volume 1 | Number 1 | 59
Low distortion signal generator based on direct digital
synthesis for ADC characterization
Walter F. Adad, Ricardo J. Iuzzolino
Instituto Nacional de Tecnología Industrial, Av. General Paz 5445, Buenos Aires, Argentina
Keywords: Direct digital synthesizer, frequency synthesizers, direct digital synthesis, ADC characterization.
Citation: Walter F. Adad, Ricardo J. Iuzzolino, Low distortion signal generator based on direct digital synthesis for ADC characterization, Acta IMEKO, vol. 1,
no. 1, article 12, July 2012, identifier: IMEKO‐ACTA‐01(2012)‐01‐12
Editor: Pedro Ramos, Instituto de Telecomunicações and Instituto Superior Técnico/Universidade Técnica de Lisboa, Portugal
Received January 10th, 2012; In final form July 10th, 2012; Published July 2012
Copyright: © 2012 IMEKO. This is an open‐access article distributed under the terms of the Creative Commons Attribution 3.0 License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Funding: This work was supported by Instituto Nacional de Tecnología Industrial
Corresponding author: Walter F. Adad, e‐mail: email@example.com
Analog-to-digital converters (ADC) and digital-to-analog
converters (DAC) have to be characterized in static and
dynamic regime .
Static characterization can be made using a solid state DC
voltage standard or a Josephson system. While, for dynamic
characterization, it is necessary to excite the ADC with different
types of signals waveforms, amplitudes and frequencies. To
attack these problems we have designed an arbitrary function
generator based on direct digital synthesis (DDS).
This technique has some advantages in comparison with
others techniques, such as phase-locked loop (PLL). The DDS
technique has higher frequency resolution, usually generators
based on PLL have a limited frequency resolution in the order
of 1:106, although some advanced PLL achieve much higher
resolutions, while DDS technique can achieve values of
frequency resolution in the order of 1:1014. Additionally, the
total harmonic distortion (THD) of PLL generators typically
has values of -40 dB in comparison with a THD better than
-70 dB achievable by DDS devices.
Moreover, in DDS devices the stability in frequency
depends on the reference external oscillator, multiple devices
can be synchronized, facility of great importance in multi-tone
applications. Finally, the DDS devices are software
programmable and easy to use.
This function generator can be used in other applications
such as impedance measurements (as described in ) and any
measurement schemes which require alternating signals as
2. THEORY OF OPERATIONS OF DIRECT DIGITAL SYNTHESIS
DDS technique consists in digital processing to generate
signals at different frequencies and phases selectable by
software, from a reference clock.
As the DDS technique consists in dividing the reference
clock frequency from a tuning word selectable by software, the
relationship between the tuning word, the clock reference, the
number of bits of the DDS and the desired output signal
frequency is given by 
where δt is the duration of a DDS time step (1/fclock) and δφ is the phase angle changing in one time interval δt. Considering
that the tuning word (M) is the amount by which the phase
accumulator increments on each DDS time step and that 2N is
the capacity of the phase accumulator, then δφ = M / 2N (with
N equal to the number of bits of the phase accumulator).
Combining these results gives the frequency of the output sine
wave (fo). Replacing M = 1 in equation (1) gives the frequency
resolution of the DDS devices as
ff . (2)
This paper presents a low distortion signal generator with a frequency range from 0 to 10 kHz using the direct digital synthesis (DDS)
method for ADC characterization. The results show that the maximum distortion in the whole frequency range is ‐80.37 dB, the
frequency resolution is 1.421 nHz (with a 48‐bits DDS chip), the stability in frequency is 25 μHz/Hz and the amplitude stability is
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For example, with 48 bits DDS and a reference clock of
400 kHz it is possible to obtain a frequency resolution of
A simplified block diagram of DDS device is depicted in
Figure 1. The block diagram consists of four blocks: a reference
clock, a phase accumulator, a look-up table and a D/A
The phase accumulator sums at each clock pulse the tuning
word. Thus, its output is a digital ramp (binary code), as shown
in Figure 1.
The look-up table converts the phase accumulator output to
a digital sinusoidal waveform.
Because of limitations in the number of bits of the look-up
table, the high resolution output of the phase accumulator (32
or 48 bits) is truncated. This truncation introduces spurious
components in the output signal spectrum that must be filtered
in order to obtain low distortion.
As is explained in , the main spurious component due to
phase accumulator truncation is situated at
( )mod , 2 2N W clockspur N Wff M (3)
where fspur is the frequency of the main spurious component due
to truncation in the phase accumulator, mod() is the modulus
operator, N is the resolution of the phase accumulator and W is
the number of bits entering the look-up table.
Finally, the D/A converter transforms the digital sinusoidal
waveform into an analogue signal. The nonlinearities of the
D/A converter are the major source of harmonic distortion.
3. SYSTEM DESCRIPTION
Figure 2 shows a simplified block diagram of the developed
Internal clock and external clock blocks in the diagram
represent the reference clock. The clock selector allows
choosing between the internal clock (a quartz crystal) and an
external clock. This clock selector is controlled by a
microcontroller. Since the DDS technique does not introduce
frequency instability to the system, the stability in frequency is
dominated by the internal or external clock.
The clock provider block delivers the clock signal to the
system and also provides an external output for synchronization
with other systems.
The system generates single-tone, multi-tone, triangular,
saw-tooth and square waveform signals. To generate dual tone
signals, two AD9852 devices are used, while to generate single-
tone signals, one of them can be used.
Since the AD9852 are only capable of generating sinusoidal
and square waveforms, a third DDS was added, the AD9834, to
generate triangular and saw-tooth waveforms.
To generate the triangular waveform, the AD9834 internally
bypasses the look-up table and directly connects the phase
accumulator output to the DAC.
Figure 2. Simplified block diagram of the developed generator.
Figure 1. Simplified block diagram of a DDS device.
Figure 3. Simplified block diagram of the sawtooth waveform generation.
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On the other hand, to generate the saw-tooth waveform, as
suggested in , pulses generated by the AD9852 switch
between the phase of the two triangular waveforms produced
by the AD9834. Figure 3 illustrates this situation.
As it has been described in last section the digital output
signal must be filtered.
Therefore, Butterworth low pass passive filters of third
order with a cut-off frequency of 20 kHz were placed at the
output of the AD9852. As in , they were implemented with
passive components so that the only noise source is the thermal
noise introduced by resistors. The Butterworth topology was
used because of the flatness in the passband.
To generate different amplitudes of the output signals, low
distortions Programmable Gain Amplifier (PGA) were
employed, the AD8250. The gain can be set to 1, 2, 5 or 10.
Next to the low distortion amplifiers, lowpass filters were
placed (in the case of sinusoidal signal) to remove spurious
frequencies generated by the PGA. They have the same
topologies of the first one, but with a cut-off frequency of
Finally there are an adder and a multiplier circuit to generate
the dual tone signals. AD734AN is used as multiplier because
of its low distortion.
4. SOURCES OF DISTORTION
In order to evaluate the DDS behaviour the main distortion
sources were analyzed, which are:
Phase accumulator truncation.
Internal D/A converter.
Interference between tracks in the PCB design.
To study the capabilities of the system a simulation program
was developed. Simulation results have shown that the spurious
frequency due to phase accumulator truncation were over the
bandwidth of the system (10 kHz), and can be filtered properly
with the filters mentioned in the previous section. These
reconstruction filters, also reduce the harmonic distortion
caused by the D/A converter.
To reduce the quantization noise effects introduced by the
D/A converter, different measurements were performed in the
system. Some of these measurements are showed in section 5.1
and they consist of varying the relationship between the DDS
output frequency and the reference clock frequency.
The PCB was designed to reduce interference between
tracks, as suggested in  and . The following issues were
taken into account:
Ground plane inclusion. A two layers PCB was
designed. The bottom layer as a ground plane. With its
inclusion the THD has been reduced in 10 dB.
Capacitive crosstalk reduction. To reduce the
interference between tracks, parallel tracks were
avoided, keeping them as short as possible.
DDS’s AD9852 synchronization. The generation of
dual-tone signals employs two AD9852 devices
synchronized. To accomplish that, the tracks from the
reference clock to these devices were routed keeping
both the same length.
Reference clock low impedance return path. Tracks
crosses behind the track of the reference clock
(bottom layer) were avoided in other to prevent the
spread of the return current throughout the circuit.
Figure 4 shows the PCB design of the two-tone sinusoidal
To verify the system performance, different tests were
carried out. Most of these tests were realised in order to achieve
the target THD.
5.1. Spectral analysis without reconstruction filters
In order to evaluate the THD of the system, a simulation
program was developed. It implements the four blocks shown
in Figure 1.
Figure 5, shows the DDS spectrum obtained by simulation
Figure 4. PCB design.
ACTA IMEKO | www.imeko.org July 2012 | Volume 1 | Number 1 | 62
at the output of the D/A converter. A Spurious free dynamic
range, SFDR = -51.67 dBc was achieved.
Then, the output of the system without the reconstruction
filter was measured using the setup showed in Figure 6. In this
scheme, the DDS was programmed through a microcontroller
connected to a PC to generate a frequency of 1 kHz. The
output signal of the DDS was acquired with an acquisition
system developed in the laboratory which has a
THD = -100 dB.
The measured output spectrum is depicted in Figure 7,
where a SFDR = -57.17 dBc can be observed. These results
illustrate the necessity of including reconstruction filters to
obtain a low distorted signal.
It is important to point out that the SFDR achieved is due
to a reconstruction filter was not applied in order to evaluate
the DDS performance.
5.2. Spectral analysis after the application of the reconstruction
Using the measurement setup showed in Figure 6, but
adding the reconstruction filter described in previous section at
the output of the DDS, a THD of -80.37 dB was obtained by
measurement while a THD of -80.62 dB was obtained by
simulation. It is important to mention that in this case the
SFDR is equal to the THD because the amplitudes of the
harmonics three to six is depreciable with respect to the
strongest spurious frequency (situated in the second harmonic).
The results are summarized in Table 1 and showed in Figure 8.
As a conclusion, the introduction of the reconstruction filter
reduced considerably the THD, eliminating harmonics above
7 kHz and reducing the amplitude of the first five.
5.3. Influence of reference clock frequency on the output signal
To study the behaviour of the system, the THD of the DDS
was calculated at different frequencies of the reference clock
using the measurement setup showed in Figure 9.
As in Figure 6, the DDS was programmed through the
microcontroller connected to a PC to generate a determinate
frequency (1 kHz and 0.227 V). Using a programmable signal
generator, the reference clock frequency of the DDS was varied
in order to select the best frequency for it. The filtered output
signal of the DDS was acquired with the same acquisition
system as in the previous test.
Table 2 and Figure 10 show the THD obtained by
measurement at different relationships between the frequencies
of the reference clock and the DDS output signal.
These results also confirmed the DAC SFDR specification
of the AD9852 device increasing the SFDR as the reference
clock frequency and output frequency ratio change . As a
conclusion, a THD of -80 dB (objective of this work) was
achieved when the relationship between frequencies of the
reference clock and the DDS output signal was set to 80 and
Figure 5. DDS Output spectrum obtained by simulation. Figure 7. DDS Output spectrum obtained by measurement.
Figure 6. Measurement setup employed to evaluate the output spectrum of
the system without the reconstruction filters.
Figure 8. DDS Output spectrum obtained by measurement after the
application of the reconstruction filter.
ACTA IMEKO | www.imeko.org July 2012 | Volume 1 | Number 1 | 63
This behaviour is taken into account when selecting the
clock frequency, and for that reason we use as upper limit to
select the clock frequency 400 kHz. As indicated previously
with this range is possible to filter the output signal of the DDS
by a 3rd order low pass filter.
5.4. Influence of set the DDS output frequency to an exact
submultiple of the reference clock frequency
Using the measurement setup shown in Figure 9, the DDS
was programmed to generate an output frequency which is
multiple of the reference clock. After that, the DDS was
programmed to generate an output frequency which is not
multiple of the reference clock.
The results are shown in Table 3. They show a difference of
0.2 dB in the THD when the output signal frequency was not
an integer multiple of the reference clock frequency.
5.5. Internal frequency multiplier
The AD9852 has an internal frequency multiplier with which
is possible to increase the frequency of the device internal
To evaluate the influence of this frequency multiplier on the
THD, from the measurement setup showed in Figure 9, the
DDS was programmed to generate the same output frequency
using different multiplier settings. Then the THD was obtained.
The results are depicted in Table 4.
It is important to point out that the DDS internal frequency
was the same at both cases (400 kHz). As can be seen in
Table 4, no changes were observed in the THD at different
values of the DDS internal multiplier, therefore there is not
variation of the THD as a function of the DDS internal
5.6. Frequency and amplitude stability
In order to measure the frequency and amplitude stability of
the signal generator designed, the DDS was programmed
through a microcontroller to generate 62.5 Hz sinusoidal wave
(0.227 V) using the internal clock of the system.
To analyse the frequency stability, the output signal of the
generator was connected to a time interval counter, which was
programmed to collect data through a PC during ten hours.
To measure the amplitude stability, the output of the
generator was measured employing a high accuracy multimeter.
The measurement results of the frequency and amplitude
stability are shown in Table 5.
Figure 11 depicts the Allan deviation, σy(τ), of the DDS output signal frequency when the internal clock was used.
The standard deviation is a statistic tool employed to
quantify the dispersion of a number of samples. The variance is
a numeric measure of the deviation of these samples with
respect to the mean value. However, the classic variance only
can be calculated from stationary data. It implies that the data is
independent of time. When data is not stationary (such as in
frequency and amplitude measurement), classic variance does
not converge and another tool is required. So, the Allan
Figure 9. Measurement setup.
Frequency of the reference clock / DDS output frequency
Figure 10. THD as a function of the frequency of the reference clock.
Table 1. Maximum Total Harmonic Distortion of the system.
THD simulated THD measured
‐80.62 dB ‐80.37 dB
Table 2. THD at different fREFCLK, f0 ratios.
1 kHz 20 MHz 20000 ‐77.87 dB
1 kHz 100 kHz 100 ‐80.37 dB
1 kHz 80 kHz 80 ‐80.57 dB
Table 3. THD as function of DDS output frequency.
DDS output frequency Clock reference output
1000 Hz 80 kHz ‐80.57 dB
1001.457 Hz 80 kHz ‐80.77 dB
Table 4. Influence of multiplier in the THD.
1 kHz 80 kHz x5 ‐80.16 dB
1 kHz 100 kHz x4 ‐80.16 dB
Table 5. Frequency and amplitude stability of the DDS output.
Frequency stability Amplitude stability
25 μHz/Hz 13 μV/V
ACTA IMEKO | www.imeko.org July 2012 | Volume 1 | Number 1 | 64
variance was used because it converges for different types of
noise present in electronic circuit .
Figure 11 shows that an averaging time in the range of 20 to
16000 s can be employed because of the predominant noise in
the whole range is white noise.
It is important to note that the DDS technique does not
introduce frequency instability to the system, so it is the
frequency stability of the reference clock.
Figure 12 shows the Allan deviation of the output signal
amplitude. In this case, to achieve the optimal σy(τ), an averaging time (τ) of 15.25 s has to be used because this τ is the minimum
σy(τ) in the range of white noise.
We designed a low distortion (-80.37 dB) arbitrary function
generator of a resolution equal to 1.41 nHz and frequency
stability of 25 μHz/Hz.
The total harmonic distortion reached by the proposed
design is suitable for the application, but one possible
modification can be performed in order to improve this feature.
It consists in the use of an external digital to analogue converter
(DAC) with more bits and low distortion at the output of
AD9852. It is important to note that this device can be
programmed to employ an external or internal DAC.
The proposed signal generator was designed to generate
multi-tone signals in a relatively simple way. For example, to
generate four tones, as the device can be synchronized to an
external clock, adding a second board and connecting an
external clock to both PCBs, the system will be capable of
generating four tones. In comparison with commercial signal
generators most of them can generate two tone signals.
During these research other DDS commercial chip was
analyzed, the AD9959. This device has four DDS cores. The
advantage of employing it is that four tone signals can be
generated with one PCB (in comparison with the system
presented in this paper). Despite this, the design with this
device was discarded because it was analysed that while a good
PCB design could ensure a feature of – 100 dB SFDR, the
limitation is given by the isolation between channels of the
device specified as better than 65 dB . Other disadvantages
are lower frequency resolution (32 bits in comparison with 48
bits of the AD9852) and internal DAC of 10-bits instead of
12-bits converters of the implemented design.
Other important conclusion obtained from the results is that
as the direct digital synthesis technique is based on digital signal
processing it does not introduce instability to the frequency.
Therefore, the only contribution to the frequency uncertainty is
the relative uncertainty of the reference crystal. As the system
has the possibility to use an external reference clock, the
achieved stability can be improved.
This function generator may be used in electrical metrology
and in schemes of measurements that require AC signals as a
stimulus, for example, in Time and Frequency domain, where a
Cesium clock can be used to synchronize the system proposed
(so the frequency stability of the system is the frequency
stability of the Cesium) and the DDS can be programmed to
generate different frequencies to test frequency meters.
 IEEE, Tech. Rep., “IEEE Std 1241.2001, IEEE Standard for
Terminology and Test Methods for Analog-to-Digital
 Fu, Y.; Li, Z.; Zhang, Z.; Sun, J.; Chen, L.; , "DDS sources for
precise measurement", Precision Electromagnetic Measurements
(CPEM), 2010 Conference on , vol., no., pp.402-403, 13-18 June
 Analog Devices, “A Technical Tutorial on Digital Signal
 Collins, N. & Limerick, “DDS devices produces sawtooth
 Lacanette, K., Application Note, AN-779, “A basic introduction
to Filters – Active, Passive, and Switched Capacitor NI”, 1991.
 Montrose, M. I. “Printed Circuit Board Design Techniques for
EMC Compliance”, 2000.
 Brandon, D. “Synchronizing Multiple AD9852 DDS-Based
 Analog Devices, “CMOS 300 MSPS Complete DDS AD9852”,
 López, J. M., “La varianza de Allan en la metrología del tiempo y
la frecuencia”, 2005.
 Analog Devices, “4-Channel, 500 MSPS DDS with 10-Bit DACs
Figure 11. Allan deviation of the DDS frequency output signal. Figure 12. Allan deviation of the DDS amplitude output signal.