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HAMRADIOINDIA Direct Digital Synthesizer for Amateur Bands 1 HAMRADIOINDIA DDS VFO for Amateur Bands HAMRADIOINDIA contact@hamradioindia.org www.hamradioindia.org HAMRADIOINDIA
Chapter Direct Digital Synthesizer VFO for Amateur Bands
A stable multi band HF transceiver is a dream of every ham. The first step to this is to make a multi band stable VFO. But it is very difficult to get stability in ordinary LC VFO at higher frequencies. The Advanced DDS technology helps you to become your dream true. This project used a PIC microcontroller – A range of microcontroller produced by Microchip Inc. in this application, PIC16F628 and AD9851 “a complete DDS synthesizer chip” from Analog Devices. PIC
The PIC16F628 is an 18-Pin microcontroller with 2K byte program memory, it is a FLASH-based and a member of the versatile PIC16CXX family of low-cost, high-performance, CMOS, fully-static, 8-bit microcontrollers. PIC16F62X devices have special features to reduce external components, thus reducing system cost, enhancing system reliability and reducing power consumption. DDS
The AD9851 is a highly integrated device that uses advanced DDS technology to form a digitally programmable frequency synthesizer. AD9851 generates a stable frequency and phase-programmable digitized analog output sine wave. This sine wave can be used directly as a frequency source with an output tuning resolution of approximately 0.04 Hz with a 180 MHz system clock. The AD9851 contains 6 X REFCLK Multiplier circuit that eliminates the need for a high frequency reference oscillator.
Figure 1. Basic DDS Block Diagram 2
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The basic block diagram is shown in Figure 1. The frequency out put can be calculated using the following formulae
Fout = (? Phase X System Clock)/ 2 ^ 32 Where:
? Phase = decimal value of 32 bit tuning word
System Clock = REFCLK frequency in MHz or 6 X REFCLK frequency (in MHz) if the 6 X REFCLK Multiplier is enabled Fout = Output frequency of DDS in MHz
The DDS circuitry is basically a digital frequency divider function whose incremental resolution is determined by the frequency of the system clock, and N (number of bits in the tuning word). The phase accumulator is a variable-modulus counter that increments the number stored in it each time it receives a clock pulse. When the counter reaches full-scale it wraps around, making the phase accumulator’s output phase-continuous. The frequency tuning word sets the modulus of the counter, which effectively
determines the size of the increment (?Phase) that will be added to the value in the phase accumulator on the next clock pulse. The larger the added increment, the faster the accumulator wraps around, which results in a higher output frequency. Figure 2. Output Spectrum of DDS
In the example shown in Figure 2, the system clock is 100 MHz and the output
frequency set to 20 MHz. A low pass filter is essential to remove unwanted images. A good rule-of-thumb is limiting the output frequency to 1/3rd of System Clock frequency. One can use the other images also with help of a good band pass filter. Note that these images are not harmonics and it keeps a 1:1 relation with the fundamental frequency, i.e. if fundamental frequency increased 1 Hz then the image frequency also shifted 1 Hz. 3
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1. Dual VFO, i.e. VFO A and VFO B facilities. 2. Up to 20 RX frequency storage on memory 3. The Software allows 2 modes of operation, a. VFO mode b. Memory mode
4. Keypad frequency entry
5. Step size can be changed from 1Hz to MHz range.
6. RIT facility.
7. Save to any MEM and copy to any VFO from MEM is possible. 8. LSB/USB/CW modes
9. Allows -Ve and +Ve IF OFFESTS
10. Calibration screen for changing all the setup screens including Chip type and System Clock Frequency. 11. Signal Generator mode.
12. Time out function for ignoring any wrong entry 13. Low cost mechanical encoder is used
14. A 2 x 16 line LCD display used for display freq and other messages
Construction notes.
The opto-coupler 4N35 is isolating the PTT from the circuit. To change frequency of the DDS, Keypad and Mechanical Rotary encoder are provided. Another six push
button switches are used for managing other functions of the VFO. An ordinary keypad used in telephones, which is easily available from the local market is used. If the Mechanical encoder is not available one can use the encoder found in the “Scroll Mouse”. The scroll mouse contains two optical sensors and one mechanical encoder inside it. You can easily remove the mechanical encoder from the mouse and fit it to the front panel of your radio.
DDS Chip AD9851BRS/AD9850BRS comes with SSOP package, it is somewhat difficult to solder for a home brewer. Seek the help of a service people to solder this chip to board is an alternative. May be one can try the method Peter Rhodes, G3XJP, in his article AN557 (). He solders this chip to a 24DIP socket using a short wire leads. But this method is not supported by the PCB design shown here. Or one can contact the local mobile phone servicing people to get the chip soldered to PCB. 4
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Figure 3. Dimensions of AD9851BRS
A 2 line x 16 LCD with or without R/W pin (software use delays rather than busy
checks) is used to display VFO frequency and other messages. The LCD contrast can be adjusted using R7 (10K pot). Commonly available LCD display (using Hitachi HD44780 controller) is used in this circuit
After assembling double check all wiring and check for any dry solder,
loose connections etc. PCB may be cleaned with isopropyl. Note that higher than 5 V will damage both DDS and PIC chips. Always use a good IC socket for the PIC16F628, this will helps if you need to re-program the chip.
The MODE SELECT output (from 74LS139) are used to switch crystals
and other operations while changing the modes (i.e. CW/LSB/USB etc.). One of the out put will be low and all other are high of 74LS139. one of the Band select out put will be low in the following conditions.
1. Frequency less than 10 MHz – used for selecting 40 M 2. Frequency less than 17 MHz – used for selecting 20 M 3. Frequency less than 25 MHz – used for selecting 15 M 4. Frequency greater than 25 MHz – used for selecting 10 M
Figure 4. LCD display on TX mode. 5
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Calibration and setup screens
By pressing down the CAL button for one second, you can enter the calibration screen; here you can change the followings.
1. Enable 6 X REFCLK Enable or Disable internal multiplier
2. DDS_SYSTEM_CLK. Range 30 MHz to 190 MHz for AD9851
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