ozQRP DDS VFO 2 Instructions for use

DDS VFO 2 Construction Manual –Issue 1 Page 1
DDS VFO 2
CONSTRUCTION MANUAL

DDS VFO 2 Construction Manual –Issue 1 Page 2
Important
Please read before starting assembly
STATIC PRECAUTION
The DDS VFO kit contains the following components which can be
damaged by static discharge:
The DDS chip which is pre-soldered to the PCB and shipped in
an anti-static bag.
The microcontroller which is pre-programmed and shipped in
an anti-static bag.
Do not remove the devices from their protective anti-static bags until
you have taken precautions against static discharge.
If possible use an anti-static wrist strap and conductive mat.
These can be purchased readily from electronic retailers.
If these are not available then at least ensure you have
discharged yourself by touching an earthed metal surface.
Try not to directly touch the pins of the devices.
Ensure your soldering iron has an earthed tip.

DDS VFO 2 Construction Manual –Issue 1 Page 3
Contents
1Introduction..............................................................................................................................................................5
2Circuit Description .................................................................................................................................................6
3Kit Supplied Parts List........................................................................................................................................ 10
4The DDS VFO and the Superhet...................................................................................................................... 11
5Construction........................................................................................................................................................... 12
5.1 General............................................................................................................................................................ 12
5.2 Construction Steps..................................................................................................................................... 12
6First time power on............................................................................................................................................. 15
7Configuration......................................................................................................................................................... 16
7.1 Understanding options ............................................................................................................................ 16
7.2 Using the rotary encoder ........................................................................................................................ 18
7.3 Configuration Preview............................................................................................................................. 18
7.4 Entering New Values................................................................................................................................. 18
8Installing in a MST............................................................................................................................................... 20
8.1 Front Panel.................................................................................................................................................... 20
8.2 Wiring ............................................................................................................................................................. 21
8.3 Power .............................................................................................................................................................. 21
8.4 Output............................................................................................................................................................. 21
9Operation................................................................................................................................................................. 22
9.1 Power on........................................................................................................................................................ 22
9.2 Tuning............................................................................................................................................................. 22
9.3 Frequency steps.......................................................................................................................................... 22
9.4 RIT.................................................................................................................................................................... 22
9.5 Voltage Display............................................................................................................................................ 22
9.6 Frequency calibration .............................................................................................................................. 23
9.7 Voltage display calibration..................................................................................................................... 23
10 Notes ......................................................................................................................................................................... 24
10.1 Overclocking................................................................................................................................................. 24
10.2 DDS output.................................................................................................................................................... 24
10.3 Frequency range......................................................................................................................................... 24
10.4 LCD................................................................................................................................................................... 24
10.5 Firmware upgrades................................................................................................................................... 24

DDS VFO 2 Construction Manual –Issue 1 Page 4
List of Figures
Figure 1 CPU....................................................................................................................................................................... 7
Figure 2 DDS generator..................................................................................................................................................8
Figure 3 LCD, Rotary Encoder and Power Supply...............................................................................................9
Figure 4 Component overlay .................................................................................................................................... 14
Figure 5 Installing in a MST....................................................................................................................................... 20
Figure 6 MST3 cable..................................................................................................................................................... 21
Figure 7 LCD operating display ............................................................................................................................... 22

DDS VFO 2 Construction Manual –Issue 1 Page 5
1INTRODUCTION
The DDS VFO 2 is designed as a companion for the MST3 transceiver board although it could be
used with many other Superhet or Direct Conversion radios. It is simple to use having only one
control yet offers many advanced features. A kit of parts, with the DDS chip already loaded onto
PCB and blue backlit LCD, is available from www.ozQRP.com.
DDS VFO Features:
1. Stable frequency –no more drift!
2. Simple to operate using a single rotary encoder control with press button switch.
3. Bright 16 character by 2 line LCD display. Can also be used with a non-backlight display
to reduce current consumption if needed.
4. Output frequency range of 10KHz to 10MHz. Frequency range can be extended by
‘overclocking’ (see text).
5. Selectable 10KHz, 1KHz, 100Hz or 10Hz frequency steps.
6. RIT control allows fine adjustment of receive frequency without affecting the transmit
frequency.
7. Display of power supply voltage.
8. Display state of USB/LSB and TX/RX inputs.
9. Programmable IF frequencies. Separate high and low side IF frequencies ensure the
displayed frequency is the same as the transmitted frequency when changing sidebands.
10. Programmable option to save the current frequency and step value into memory. These
are re-loaded at power up.
11. Fine adjustment of frequency in configuration to allow for master clock tolerance.
12. Voltage display can be trimmed in configuration to allow for voltage reference errors.
13. Can be configured without an IF Offset to allow use as a signal generator or with Direct
Conversion transceivers.
14. Draws only around 50mA at 13.8V DC (with LCD backlight).
15. Spurious outputs at least -45dB.
16. Typically 300mV pk-pk sine wave output (200 ohms).
17. High quality double sided PCB with ground plane, plated through holes, solder mask and
silk screen.
18. Simple and easy to build using through hole components apart from one SMD chip. To
assist constructors the PCB comes shipped with the SMD chip already installed.

DDS VFO 2 Construction Manual –Issue 1 Page 6
2CIRCUIT DESCRIPTION
Integrated circuit U2 is an Analog Devices AD9834 Direct Digital Synthesis (DDS) chip and
forms the heart of the DDS VFO. A 50MHz crystal oscillator module supplies the master clock
and gives usable output frequencies up to a quarter of that value or 12.5MHz.
The DDS is controlled by a 3 wire serial data bus from microcontroller U1.The microcontroller is
an Atmel ATmega328 and runs an internal 8MHz clock which avoids the use and expense of an
external crystal.
A rotary encoder produces quadrature signals when rotated and one line is fed into the
microcontroller interrupt input and becomes the reference input. Once an interrupt is detected
the state of the other input is read and the direction determined. The rotary encoder also
incorporates a press button switch and is used for a number of functions as described later. U1
incorporates pull-up resistors for the encoder inputs so they are normally held high and
capacitors C7 and C8 help filter out switch bounce.
The microcontroller has an internal analog to digital converter (ADC) which uses the 5 volt rail
as a reference. The ADC measures the incoming power supply rail via a resistive attenuator
formed with R5 and R6. The calculated value is displayed on the LCD during use.
The A input is used to monitor the TX/RX state, while the D input is used to detect the state of
the USB/LSB switch. The inputs expect a nominal +12 Volt to 0V state change and are buffered
by resistive attenuators to protect the microprocessor inputs.
The PCB also includes an ISP connector to allow future software upgrades without having to
remove the chip from its socket.
Most of the remaining pins of U1 are dedicated to driving the LCD in 4 bit mode. R9 sets the
backlight current and may be left out if a non-backlight LCD is used. VR1 adjusts the LCD
contrast and normally only needs to be set once at the commissioning stage.
The outputs from the DDS chip are current sources and the output is converted to a voltage by
the 200 ohm resistors R12 and R13. The full scale value of the current and hence the pk-pk
output voltage is set by the 5.6K ohm resistor R11. The output is passed through a simple 5 pole
low pass filter to attenuate unwanted high frequency components.
R14 is used to terminate the filter if the external load is high impedance. In most circumstances
it can be left off the board.
The incoming power supply is regulated to 5V by a 7805 1A regulator which mounts flat on the
PCB to act as a heatsink.

DDS VFO 2 Construction Manual –Issue 1 Page 7
Figure 1 CPU

DDS VFO 2 Construction Manual –Issue 1 Page 8
Figure 2 DDS generator

DDS VFO 2 Construction Manual –Issue 1 Page 9
Figure 3 LCD, Rotary Encoder and Power Supply

DDS VFO 2 Construction Manual –Issue 1 Page 10
3KIT SUPPLIED PARTS LIST
Quantity
Comment
Designator
Capacitors
2
33pF 50V disc ceramic
C15, C17
1
100pF 50V disc ceramic
C16
1
1nF 50V disc ceramic
C11
3
10nF 50V ceramic MLCC
C7, C8, C12
10
100nF 50V ceramic MLCC
C2, C3, C4, C5, C6, C9, C10, C13, C14, C18
1
10uF 25V RB electrolytic
C1
Resistors
2
100R 1/4W 1% resistor
R8, R9
3
200R 1/4W 1% resistor
R12, R13, R14
1
5.6K 1/4W 1% resistor
R11
4
10K 1/4W 1% resistor
R2, R4, R6, R7
3
20K 1/4W 1% resistor
R1, R3, R10
1
30K 1/4W 1% resistor
R5
1
20K horizontal trimpot
VR1
Semiconductors
1
ATmega328 - 28pin DIP
U1
1
AD9834 DDS 20 pin TSSOP
U2
1
7805 5V regulator TO220
VREG1
1
50MHz crystal oscillator module
XO1
1
16 x 2 LCD with backlight.
LCD1
Inductors
2
4.7uH moulded RF choke
L1, L2
Hardware
1
DDS VFO PCB
1
28 pin DIP narrow IC socket
1
3 pin x 2 row 2.54mm pitch header
HDR1 (ISP)
1
Rotary Encoder
RE1
1
4 pin 2.54mm pitch header RA
SK1 (PWR)
1
2 pin 2.54mm pitch header RA
SK2 (OUT)
1
4 pin plug + pins
1
2 pin plug + pins
4
12mm long 3mm nylon spacer
4
2.5mm x 20mm screws
4
2.5mm nuts
1
16 way SIL male pin header
1
16 way SIL female pin header
1
3mm x 10mm screw
1
3mm nut