A Direct Digital Synthesis DDS AudioRF Signal Generator 1Hz to 30MHz

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A Direct Digital Synthesis (DDS) Audio/RF Signal
Generator(1Hz to 30MHz)

• Christo Pelster, ZS6AHQ
• M.Eng (Stelllenbosch)

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Older Versions
They are very useful, but have some shortfalls

• Variable FrequencyOscillators (VFO)
• Free running
• Harmonics are usedfor higher bands
• Frequency not accurate
• Low resolution
• Unknown level/impedance
• Stability and drift problems
• Availability where to buy

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Newer Versions
Professional Equipment is VERY expensive You get
what you pay for, but few amateurs can afford the
luxury.
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Main Design Objectives

• Wide frequency coverage
• Constant and known output levels
• Known output impedance
• Ease of calibration without special test
  equipment
• Good Frequency accuracy
• Fine tuning resolution
• Good Frequency Stability
• Intermediate construction skills needed to build
• Affordable
• Mixed Analog (Audio, RF) and Digital Design
• Provide a useful tool to the home constructor

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Main Features

• Output Frequency 1Hz to 30MHz
• Frequency Resolution 1Hz
• Audio Section with 600O impedance from 1Hz to
  100kHz
• RF Section with 50O impedance from 100kHz to
  30MHz
• Output level RF0dBm 1.5dB, Audio 0dBm
• Calibrated by using a standard Digital Volt Meter
  (DVM)
• Easy one button, one knob operation
• Selectable frequency step size
• Large, easily readable 8 digit display
• Pure Sine wave generated by Direct Digital
  Synthesis (DDS)

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Applications

• RF
• Home construction of RF front ends,mixers, IF
  amplifiers.
• Check frequency readouts andradios with VFO
  dials
• Frequency reference for VFO transceivers
• S-meter characterization and comparison
• Antenna return loss measurements(requires return
  loss bridge)

• Audio
• Audio amplifier frequency response
• Test Sound card frequency responsefor Software
  Defined Radios (SDRs)

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Block Diagram
4 Digit LCD
4 Digit LCD
RotaryEncoder
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32
LCD_LOAD
AY-043832 segmentLCD Driver
AY-043832 segmentLCD Driver
LCD_DATA
PIC16F628A
LCD_CLK
STEP
SPARE
DDS_CLOCK
DS_FSYNC
DDS_DATA
30MHz
OPA843
OPA843

X-TalRef Osc
50O
RF Out
75MHz
LPF
RF OP Amp
RF OP Amp
5V
-5V
Vref
600O
Power Supply
Audio Out
MC33172Audio OP Amp
230VAC
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Functional Description (1)

• Display Function
• Man-Machine Interface
• 4-digit LCDs with 7-segment displays
• Displays the generated frequency with 1Hz
  resolution

• Frequency Control Function
• Rotary Encoder that can be turned
• Push button to select step size
• Blinked digit indicate which step size is selected

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Functional Description (2)

• Control Function (what the PIC does)
• Check frequency limits and set limits when
  reached
• Check Step Change input and change the step size
  if the push button switch has been pressed
• Check for changes in the Rotary Encoder
  (increment or decrement)
• Calculate and look up the 7 segment values of the
  digits and send them to the display
• Calculate the DDS word and send it to the DDS IC

(Not multi-tasking, but all these tasks are done
between every detentof the rotary encoder, so
fast that the user does not even notice)
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Functional Description (3)

• Analogue Audio Section
• 1st Audio Op Amp buffers DDS reference signal.
• 2nd Audio Op Amp inverts, amplifies and
  subtract VREF.
• Drives 600O output (0dBm and -60dBm)

• Analogue RF Section
• 1st RF Op Amp buffers DDS output and drives Low
  Pass Filter (LPF). Sets gain DC coupled.
• LPF removes DDS harmonics
• 2nd RF Op Amp AC coupled output stage (0dBm)
  drives 50O (0dBm)
• -73dBm attenuator for S-9 measurements

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Analogue Section Circuit Diagram
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Theory of Operation (1)
DDS How does a Direct Digital Synthesizer
generate a signal
The phase angle (?) rotates througha fixed angle
for each unit of time
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Theory of Operation (2)
The AD9834 builds the output based on this
equation.
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Theory of Operation (2)
The angular rate depends on the Frequency of the
signal
? 2pf
Knowing that the phase of a sine wave is linear
and given a reference interval (clock period),
the phase rotation for that period can be
determined.
?Phase ? ?t
? ?Phase / ?t 2pf
Solving for f and substituting the reference
clock frequency for the reference period (1/fMCLK
?t)
f ?Phase fMCLK/2p
The AD9834 builds the output based on this
equation.
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Theory of Operation (3)
AD9834 Block Diagram (from Analog Devices data
sheet).
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Theory of Operation (4)
Audio Section Op Amp Differential
Configuration.
FromDDS output(VDDS)
Output 0VDC 4.4Vp-p
-Vss
FromDDS VREF
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Theory of Operation (5)

• Audio Section
• Two Equations with 4 unknowns..! (VREF 1.2V)
• Solve iteratively by first assuming RF 10 x RG
• Calculate R1, R2 for 0V DC offset (assume R12k2)
• Adjust gain to 10.5, re-calc R2 for 0V DC offset
  etc
• R12k2, R23k9, RF18.8k (12k6k8), RG1k8
• Try to keep R1R2 RFRG (approx 1.4 to 1.6k)

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Theory of Operation (6)
Output Voltage from DDS 0.42Vp-p with DC offset
FSD Value 18 x (VREF/RSET) x RDDS
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Theory of Operation (7)
Inverting Amplifier m -RF/RG 10.5 for 0dBm
in 600O
-4.4V is equal to 4.4Vp-p
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Theory of Operation (8)
Add Offset b2.1 DC offset is now 0V
4.4Vp-p0VDC
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Theory of Operation (9)
2.1Vp-p(loaded)
4.4Vp-p(unloaded)
PL (dBm) 10 x LOG(PL x 1000) 0dBm
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Theory of Operation (10)
RF Section RF Op Amps Inverting Configuration.
FromDDS output(IFSD)
Output VDDS 0.421Vp-p
IFSD3.18mAR1(eff) 133O
Output 0.655Vp-p
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Theory of Operation (11)
RF Section Low Pass Filter 5th order
Elliptical 0.1dB Passband Ripple Standard
Component Values L1, L2 11 turns, 24AWG on 4mm
drill bit tightly wound
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Theory of Operation (12)
Cut off at 30MHz, Stop Band at 45MHz
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Theory of Operation (13)
RF Section Final Gain Stage DC Block, 0dBm
210mVRMS(0.6Vp-p)
FromLPF output
Output 1.28Vp-p(Unloaded)
Compensation for LPF gain roll-off
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Theory of Operation (14)
0.64Vp-p(loaded)
1.28Vp-p(unloaded)
PL (dBm) 10 x LOG(PL x 1000) 0dBm
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Results (1) RF Output Level
Amplitude dBm
Frequency MHz
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Results (2) 30MHz
Amplitude dBm
Frequency MHz
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Results (3) Audio M0KGK SDR
Amplitude dBm
Frequency Hz
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Construction

• 4 PCB Boards
• Display Board
• Analogue Board
• Power Supply Board
• -73dBm Attenuator Board

• 3 Components only available in Surface Mount
• 1 x DDS IC AD9834CRUZ
• 2 x RF OP Amps OPA843ID
• 1 x 75MHz Clock Oscillator

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Display Board

• 2 x 4-digit displays placed side-by-side in
  sockets
• 2 x AY-0438 32 segment shift registers LCD
  drivers
• Easy to read, 12.7mm high digits
• Fast response vs. 2 x 16 display controllers
• Good contrast
• Rotary Encoder 24 pulses/rev. 4 pulses/detent
• Double sided PCB

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Analogue Board

• PIC Micro Controller 16F628A - Microchip
• DDS IC AD9834CRUZ Analog Devices
• 75MHz Clock Oscillator - Crystek
• RF OP Amps OPA843IP Texas Instruments
• Audio OP Amp MC33172P On-Semi
• 3.3V Regulator MCP1702 - Microchip
• Discrete Resistors, Inductors, Capacitors
• Double Sided PCB

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Power Supply

• Power Supply 5V (500mA) and -5V (100mA)
• 6-0-6V 1.5VA PCB mount transformer
• Single Sided PCB

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-73dBm Attenuator

• 3 x 20dB 1 x 13dB Stages
• Zin 50R with -38dB RL
• Zout 50R with -29dB RL
• Double Sided PCB

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Assembled Unit front view
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Assembled Unit rear view
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Availability

• Double Sided Through-hole plated PCB with Solder
  Mask and Silk Screen in progress.
• Depending on the interest, kits (excl. enclosure)
  with SMD components already mounted can be
  compiled.
• Provide Name, Call sign and e-mail address on
  inquiry form.

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The Audio/RF DDS Signal Generatoris a handy
addition to the shack of any home constructor of
radio equipment

• Thank you
• Christo Pelster, ZS6AHQ