Operation

1. The front panel

• the 16x2 character LC display with backlight
• the rotary dial (frequency and power adjustment)
• the red indicator LED (flashes if parameters are stored)
• the ESC (escape) button
• the < (cursor left) button
• the > (cursor right) button
• the ENT (enter) button
• the MEN (menu) button
• the HF output jack of the generator (BNC)

A detailed description of the different modes of oeration can be found in the folowing sections.

Fig. 1: The front panel of the DDS-VFO

2. Setup with defaults

• Turn off the power supply or press and hold the reset button
• Press and hold the ESC and MEN button at the same time
• Release the reset button or turn the power supply on
• Release the ESC and MEN button

The firmware starts with the screen, shown in fig. 2.
These are the default values:

• DDS-VFO set to receiver mode
• Receiving frequency (fr) set to 10,000.0MHz
• IF set to 455kHz
• LO above receiving frequency
• Cursor set to the 100Hz position
• Output power for generator mode set to -40dBm

Fig. 2: Setup screen of the DDS-VFO

3. Startup of the DDS-VFO

If this screen appears, all the parameters are read from the EEPROM. After a delay of one second the program continues with the last saved settings or the default settings after a setup.

Fig. 3: Startup screen of the DDS-VFO

4. Receiver mode

In this mode, the DDS-VFO is used as tuneable local oscillator (LO) for the first mixer stage of the receiver. The internal relay switches the HF from the front panel jack to the internal jack "RX". The output power of the DDS is held constant at +7dBm over the whole frequency range. The DDS output frequency differs from the displayed frequency by the value of the intermediate frequency (IF):

• means, that the output frequency is above the displayed frequency (see fig. 4)
• means, that the output frequency is below the displayed frequency (see fig. 5)

This feature is implemented because of the missing image or mirror frequency (fr +2*IF) rejection of the receiver frontend (untuned input circuit). If a station is transmitting at this mirror frequency, the IF shift can be toggled to get a (maybe) better signal of the wanted DRM-station.

In the lower line of the display, a frequency bar with the coarse frequency information is shown.

The following list shows, what can be done in receiver mode:

• Turn the dial clockwise to increase the frequency
• Turn the dial couterclockwise to decrease the frequency
• Press ESC to switch to generator mode
• Press < to move the cursor one digit to the left
• Press > to move the cursor one digit to the right
• Press ENT to toggle between "LO above fr" and "LO below fr"
• Press MEN to enter the "IF Adjust" menu

In the "IF Adjust" menu (see fig. 6) the IF shift can be set between 1kHz and 460kHz in steps of 1kHz with the dial. By pressing ESC the displayed IF will be applied and the screen is switched back to receiver mode.

The receiving frequency can also be changed by remote from the PC (e.g. by DReaM). The device acts as a Yaesu FT-817, but only for frequency change information. A remote access is indicated by the symbol, as long as data is transferred (see fig. 7).

A change of the mode, frequency, cursor position, IF shift or IF will cause the device to store the actual parameters to the EEPROM after 3 seconds of inactivity. So the write cycles to the EEPROM are reduced.

Fig. 4: DDS-VFO in receiver mode
with LO above fr

Fig. 5: DDS-VFO in receiver mode
with LO below fr

Fig. 6: IF adjust menu in
receiver mode

Fig. 7: Remote frequency control
in receiver mode

5. Generator mode

In this mode, the DDS-VFO is used as high frequency (HF) generator. The internal relay switches the HF from the internal jack "RX" to the front panel BNC jack. The DDS output frequency equals the displayed frequency.

Instead of the frequency bar, the output power is shown. This output power can be changed with the dial in the following steps: -40dBm, -30dBm, -25dBm, -20dBm, -15dBm, -10dBm..+10dBm (in 1dB steps). The choosen power is held constant over the whole frequency range within ± 0,3dBm by a calibrated table (1MHz steps).

The following list shows, what can be done in receiver mode:

• Turn the dial clockwise to increase the frequency or power
• Turn the dial couterclockwise to decrease the frequency or power
• Press ESC to switch to receiver mode
• Press < to move the cursor one digit to the left
• Press > to move the cursor one digit to the right

Remote control is possible like in receiver mode (see fig. 9).

Fig. 8: DDS-VFO in generator mode

Fig. 9: Remote frequency control
in generator mode