General
The VFO of the JUMA-TX1 has been
implemented using the DDS (Direct Digital Synthesis) principle. Analog
Devices AD9833 DDS IC is controlled by a Microchip PIC microcontroller.
The controller is also responsible for other functions of the TX. The physical
interface of the DDS board consists of a 1x16-character LCD display and
an interface for a rotary encoder that also includes a push button switch.
JUMA-TX1 DDS board is a slightly enhanced version of the JUMA-RX1 DDS board.
JUMA-TX1 DDS board is software compatible with the RX1 board and can be
used for RX applications if needed. JUMA-TX1 software will also run on
JUMA-RX1 DDS board but without the Filter Select output signal.
Mechanical construction
The mechanical size of the DDS-and-control
board is 80 mm x 40 mm. The PCB is designed to be installed with the same
screws fixing the LCD display to the front panel of the box. All components
are located on the bottom side of the PCB.
The functions of the DDS board
XTAL oscillator
The oscillator was implemented
using a standard (low cost) 20 MHz crystal for the microcontroller and
for generating the reference frequency of the DDS IC. Oscillator was implemented
using the osc circuit of the microcontroller and using the output as the
clock signal for the DDS IC as well. Oscillator frequency fine-tuning is
accomplished by a trimmer capacitor connected in parallel with the loading
capacitor of the oscillator.
DDS and low-pass filter
The VFO signal is generated using
the DDS (Direct Digital Synthesis) principle from the 20 MHz reference
frequency. The DDS principle can be used to generate frequencies with very
fine resolution. In this implementation (20 MHz reference clock), the minimum
frequency step is 0.0745 Hz, which is way more than enough. The selected
smallest step is 10 Hz. The maximum usable frequency with the DDS principle
is determined by Nyqvist theory. This means that in theory the max. frequency
would be 10 MHz. In practice, the highest good quality signal is 40 % of
the reference frequency. The DDS principle also generates a series of harmonic
frequencies depending on the fundamental frequency. These are filtered
with a 8 MHz low pass filter. Frequency rage of the DDS board is 0...8
MHz and RF output level is ca. 250 mVp-p with an 200 ohm output impedance.
Voltage regulator
The operating voltage of the DDS
board is +5 V and the current consumption is ca. 20 mA. The required +5
V is derived from the 9 V...15 V supply voltage and it is stabilized with
a 78L05 regulator. The 5 V operating voltage also serves as the reference
voltage for the A/D converter of the PIC16F88.
VFO select
Microcontroller’s MCLR (Reset) pin
can also be programmed as a general purpose input pin. In JUMA-TX1 DDS
board this signal is wired to connector pad J9 and is used as the frequency
memory of VFO select signal. When the signal is high, VFO A is used and
when the signal is low (grounded), VFO B is used. Both frequency memories
can hold any valid JUMA-TX1 frequency. It is the user’s choice, how to
use these two frequencies.
Band select
74HC74 flip flop data inputs are
connected to the LCD display bus signals D2 and D3. Flip-flop is clocked
with DDS data load signal. HC74 Q-outputs are connected to connector
pads J6 and J7. J7 is used for TX board RF LPF filter select signal. When
this signal is low 3,5 MHz filter is selected. High state selects 7MHz
filter. In the software the flip-flop handling is embedded in the
AD9833 DDS chip driver.
TX control signal
The TX board generates the TX state
signal. In the DDS board this signal is connected to the connector pad
J1-1. The TX signal is used to control the DDS output. During RX (TX signal
low) the DDS output is suspended. This is necessary to avoid RX interference
caused by the TX DDS output frequency. When the TX signal is high
(or J1-1 not connected), DDS generates the set frequency. If the DDS board
is started in test mode, the TX signal is ignored and DDS generates continuous
output.
ACK tones
The microcontroller generates the
JUMA-TX1 audible hello message at startup and marker tones for different
frequency tuning steps when the step is changed. A tone signal is available
at connector pad J5. The signal must be DC isolated with an external capacitor.
The signal can be used to drive a miniature speaker. A series resistor
may be added if speaker volume needs to be reduced.
LCD display
The LCD display is a commercial
1x16 character alphanumeric low cost display module. For driving the display
eight data bits and a couple of control signals are needed. The data bus
is driven by the I/O port B of the 16F88 controller. A portion of
this bus is also used for driving the DDS IC and for reading the status
of the rotary encoder. The display module is able to display letters,
numbers, and a selection of other characters. It is also possible to construct
some of your own characters in the RAM of the module. This feature has
been utilized in implementing the graphics needed for the relative power
level-meter. The contrast of the display has been set by resistor R2 on
the DDS PC board.
Encoder
The frequency is set by the rotary
push button encoder. A low cost model producing 30 pulses per revolution
and equipped with a push button switch was selected. The encoder is constructed
as a rotary switch with two contacts. These contacts are phased in order
to produce two signals with 90-degree phase shift. This IQ signal can be
interpreted to indicate encoder movement and direction of rotation. The
signals from the encoder and its pushbutton are connected through separating
resistors to the data bus of the LCD display where their states are read
by the software.
Adjustments
The DDS section doesn’t necessarily
require adjustment. However, the frequency depends on the frequency of
the reference oscillator. For adjusting the reference oscillator, a special
adjustment mode is included in the software. If the pushbutton switch of
the encoder is kept depressed during start-up, it will enter the adjustment
mode. To indicate this mode, the display will show 1 MHz. In this mode,
the setting of the DDS is an exactly calculated number corresponding to
1 MHz output frequency. While in this mode, the frequency generated by
the DDS can be measured with a precision frequency counter and can be
adjusted with the trimmer capacitor C20. Alternatively the frequency can
be adjustet by using the interference method which can be done by listening
the TX1 output and another accurate reference signal with a receiver and
then adjusting the audible intreference ("wow-wow-wow") as slow as possible.
In the adjustment mode DDS generate an output signal regardless of the
state of TX signal.
Due to the resistor tolerances and to the precision of the +5 V regulator, the displayed value of the supply voltage probably isn’t quite accurate. The supply voltage display can be adjusted by adding a suitable resistor in parallel with R23 (910R). A suitable value can be found by temporarily soldering e.g. a 100 k trimmer in parallel with R23. When the voltage display is accurate, the trimmer can be measured and then replaced with the nearest standard value resistor.
LCD display contrast is set by voltage in pin #3 of the display module connector. Resistor R2 (1k8 nominal) is used to set the contrast voltage. The voltage producing ”suitable” contrast depends on the display module and on the temperature of operation. If you use a different type of display than the one in the parts list, or if the contrast is not satisfactory, it can be adjusted by changing the value of R2. Lowering the value will make the display darker and increasing it will make the display lighter. The adjustment range is between zero and a few kilo-ohms. A suitable value can be found by soldering a temporary trimmer (e.g. 5k) on the pads of R2.
DDS board signal description
J1-1 TX signal
When high (5-6V) the DDS generates
the set frequency. When low (0V), the DDS is stopped and no TX LO frequency
is generated. Signal state is indicated in LCD display when voltage display
is selected. The TX board VOX circuit generates this signal.
J1-2 Encoder switch input
Encoder shaft or separate push button
switch to ground. Most of the user interface controls are done with this
switch.
J1-3 and J1-4
Rotary encoder A and B signal inputs.
Rotary encoder switches these signals to ground. Signals are 90 degrees
out of phase. Pulse count is used for tuning and phase information for
up/down direction select.
J1-5 DDS VCC output
DDS board +5V VCC. This is power
feed for an encoder with active logic. Not used in the current design.
J1-6 Ground
Ground connector for the encoder
and the push button switch.
J2-1 LO output
DDS signal output. About 200mV p-p
RF voltage at set frequency.
J2-2 LO ground
Ground connector for LO signal
J3 Bar graph display input
A/D converter input for bar graph
display. Range is from 0 to 240mV. Input is 5V tolerant. All values
above 240mV give full bar display.
J4-1 Power feed input
Power feed to the DDS board. Voltage
can vary between 6,5 and 15V DC. Nominal current is 20mA which varies according
to the encoder and switch states.
J4-2 Power ground
Power feed ground wire connector
J5 Tone output
User interface indicator tone output.
This signal is also used for DDS board internal voltage measurement and
must be DC isolated with a capacitor. A miniature speaker can be connected
between ground and this terminal with a series capacitor. A series resistor
can be added to reduce volume if needed.
J6 Spare
Spare output signal
J7 Band select output
Band filter select signal to the
TX board. 3,5MHz filter is selected when this signal is 0V. A 7MHz
filter is selected when this signal is 5V. DDS software controls the filter
select action. Switch-over from 3,5 to 7MHz filter is done at 4.000.250Hz.
J8 Ground
Ground connector for the VFO select
switch
J9 VFO select input
Switch between J9 and ground,
select the VFO (frequency memory). When open VFO-A is selected. When grounded
VFO-B is selected.