DL9SEC DDS-VFO

with DRM receiver frontend

Introduction


This project started in the middle of 2003, when i read some articles about DRM (Digital radio mondiale) and found the website of Christoph Petermann (DF9CY), where he published some schematics for a DRM demonstration receiver.


Because of my interests as radio amateur and engineer, i decided to build my own microcontrolled direct conversion receiver with a DDS-VFO. The first version of this receiver (downmixing the RF to a 12kHz IF) includes the microcontroler (with LCD, rotary encoder and keypad), the DDS-unit, the mixer stage and the 12kHz-amplifier. The software for the MCU was very simple with a fix table of DRM station frequencies. It works fine for a first attempt, but there was a lot room for improvements...


View of the DDS-VFO
Fig. 1: View of the DDS-VFO


Overview


1. The DDS-VFO


Click for a large view of the DDS-VFO block diagram
Fig. 1: Block diagram of the DDS-VFO


Features of the DDS-VFO:


2. The DRM receiver frontend


Click for a large view of the DRM-RX block diagram
Fig. 2: Block diagram of the DRM receiver frontend


Features of the DRM receiver frontend:


Operation


1. The front panel


Fig. 1 shows the front panel of the device which contains:


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


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


2. Setup with defaults


After the first programming of the microcontroller, the content of the EEPROM has to be filled with reasonable values. The firmware includes a setup routine, that can be executed by the following steps:


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



Setup screen of the DDS-VFO
Fig. 2: Setup screen of the DDS-VFO


3. Startup of the DDS-VFO


The DDS-VFO starts with the screen, shown in fig. 3. The backlight is turned on smoothly and a double beep sounds.


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.


Startup screen of the DDS-VFO
Fig. 3: Startup screen of the DDS-VFO


4. Receiver mode


After performing a setup, the device starts in receiver mode. This mode is indicated by an antenna symbol (Symbol "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):



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.


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


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


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



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 "Remote control" 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.


IF adjust menu in receiver mode
Fig. 6: IF adjust menu in
receiver mode


Remote frequency control in receiver mode
Fig. 7: Remote frequency control
in receiver mode


5. Generator mode


The generator mode is indicated by an double sine symbol (Symbol "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:



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


DDS-VFO in generator mode
Fig. 8: DDS-VFO in generator mode


Remote frequency control in generator mode
Fig. 9: Remote frequency control
in generator mode