VFO with DDS

VFO with DDS

The oscillator is based on a board with AD9850 from Analog Divice of which are available in the online sales portal www.ebay.com.

The oscillator is designed to be used as RF generator from 0 to 40Mhz or as a VFO for a transceiver taking into account the conversion of IF and offset SSB or CW.

UPDATES

(07/06/2013) .... Firmware version v1.3 translate to English

(12/03/2013) .... Firmware version v1.3. Was added to the setup menu, the calibration function of crystal reference 125Mhz for fine adjustment of the output frequency. Also restricted access to FI offset adjustment when you are in Transmission.

(04/03/2013) .... Was corrected leg separation on the board 

(02/26/2013) .... Firmware Version v1.2

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I want to acknowledge the contributions of friends like Victor (LU7DSM), Daniel (LU5JAU) and many others than with their technical contributions and suggestions helped me to improve this project.

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The following links can find project documentation (schematic, PCB, micro firmware and manual):

• SCHEMATIC
• MANUAL
• PCB
• FIRMWARE OF MICRO

NOTE: To download the  .HEX file in a folder, click with right mouse button on the "FIRMWARE OF MICRO" and when deployed the window, choose "SAVE LINK AS ....". Then the .HEX file shall be saved in the selected folder.


PCB View

View of Keyboard and Rotary encoder 

Components such as the "rotary encoder" may not be well known, so I show you some pictures of various models.

There are mechanical and optical encoders. The first ones are quite expensive and difficult to obtain while the second are easier to find and far more economical. Some of the mechanical push-button are included on its axis (which is used in this project to change the tuning step). The function of this device is to deliver two square wave signals which are in quadrature or phase-shifted + / - 90 ° according to the direction of rotation. These signals enter the chip to change the frequency of the VFO.  When the clockwise rotation frequency decrement, should be reversed terminals A and B. Mechanical encoders have a resolution of 12 to 15 pulses per revolution while opticians are 30 to 50 pulses per revolution.

Opticians usually are amplified, so you have to powered them. In case of using an optical encoder may have to remove the 100nF capacitors are grounded in the legs 6 and 7 of the micro.These devices can be recovered from old scrap of some models of monitors, mini audio components (volume) or obsolete radio equipment.Also found in the old "mouses" or computer mice, of those who came down with a ball, but in this case you have to adapt to a shaft. These are of the optical type.

OPERATION
The frequency can be varied with the encoder with steps selectable of 10Hz, 100Hz, 1kHz or 10kHz. Depending on the mode selected the output frequency matches the read on the LCD (generator mode) or is conversion to FI if used in SSB or CW mode (mode LSB, USB, CW).
The oscillator frequency can jump from band to band for the displacement facilitate.
Another feature is to have two VFO's, that allows to work in duplex, transmitting on one frequency and receive on another.

METHOD OF USE
The circuit is powered by a DC source appropriately filtered from 9 to 12V. The OFV always starts in the last used frequency band. The RF output is 1Vpp. The circuit has four buttons like "touch". Some of the buttons have a dual function depending on how long are maintained under pressure. A short pressure acts on the basic functions while a long on special.
The key of TX / RX or PTT off the VFO in reception  when VFO is in generator mode.


FUNCTION BAND: Pressing button BAND are going cyclically changing bands from 160m to 10m amateur bands including new.
FUNCTION MODE: Pressing the MODE button are going cyclically changing the generator mode, LSB, USB or CW.
FUNCTION VFO AB / SPLIT: Pressing the button in short form AB VFO oscillator is changed to another.
Pressing the button in the form long VFO AB Split mode is entered. In this mode and being in LSB, USB or CW when moving from TX to RX VFO is changed. To exit Split press again the button VFO AB in the form long .
FUNCTION STEP / SETUP: Pressing in short form STEP button will cyclically changing in steps or frequency jumps when rotating the encoder. These steps can be 10Hz, 100Hz, 1kHz or 10kHz and this is marked on the LCD frequency readout emphasizing the number changed.
Pressing the button in long form, is passed to SETUP function. Rotating the encoder searches menu the parameter to be changed. Then click on the STEP button in short form to enter and modify the value.

1) IF OFFSET: To make this adjustment the DDS should be in reception (Rx), then the VFO is placed automatically in the generator and encoder finds the center FI channel. To store the value you must press the button in form long STEP.
2) SSB OFFSET: Here is placed the value at half the bandwidth of the SSB filter. So if the filter is 3000Hz  should be placed 1500Hz . Is calibrated on the upper sideband.
For the case of LSB to frequency LO (local oscillator) is added the value of BLU. For USB subtracts the SSB to LO. To accept press in form long STEP button.
3) CW OFFSET: Similar to the case of SSB but for the bandwidth CW filter. Note that the conversion CW always done on the upper sideband.
4) CONVERSION: Here you select whether the conversion is positive or negative, ie if the output frequency of the VFO is LO = RF + IF or LO = RF - IF. To accept pres in form long STEP button.
5) CAL. XTAL: It help to correct  the crystal reference  of the DDS and then adjust its output frequency. For calibration use a frequency counter or tune whith a receiver a known broadcasting, then place the DDS  in GEN function and search the display the same frequency of broadcasting. They must hear a beat signal, then go to Settings Menu and choose the function 5) XTAL CAL. Vary the encoder to hear the beat to zero and confirm by pressing the button STEP in long form.
6) LICENSE: Here you can edit the presentation of the display when the oscillator turns placing their own license. Look for the letter, number or character to be changed with the encoder and then pressed to accept STEP  in short form . So on until the 6 spaces.


RESTORING VALUES
In the case of malfunction or errors in the internal memory can be reset the microprocessor and reinstall defoult values. To do this, turn on the VFO pressing the STEP / SETUP  switch for a moment, the display will show the words "reset values" and then the boot logo.