L. F. Transmitter using Direct Digital Synthesis
I've recently changed my LF driver from one which covers the band in 100 Hz steps, to
the unit pictured below. This covers the band in increments of 0.25Hz and can
operate "between the lines" of the Loran interference which is present
throughout most of Europe. The design is based on the MiniDDS
project by Johan Bodin SM6LKM.
Full details of the DDS section together with firmware listings and photo's etc. can be found on Johan's web site:
This driver uses the Analog Devices AD 9832 DDS chip which is controlled by a PIC16F84.
0 - 6 MHz sine wave.
LF 136kHz band 272kHz square wave. (For class D tx with onboard /2.)
LF 136 to 200 kHz LF bands Sine wave output. Including CW keying circuit, output level control and max output preset. - For use with audio and linear amplifiers.
Variable step size. 1Hz 10Hz 100Hz 1k 10k (min 0.25Hz in /4 mode)
LCD readout of DDS frequency, or DDS freq / 4.
Input to the DDS is via a rotary encoder. (I've used 265-1723 from RS components. It's easier than building your own -£1.89).
Prototype with 2 x 16 line display. Photo:G0WMD.
Front panel.. Top left 136kHz Output level (power)
control. Top right - rotary encoder. Underneath the encoder is a biased
toggle to select the DDS step size.
Display shows MHz. Kilohertz. Hertz. (0.137.725) Selected Step Size= 100Hz
PCB Layout Here's the prototype board - It's 150 x 46 mm
The latest Mk 2 version - December 1999 - Includes a linear BPSK input and full 0.25Hz coverage of the LF allocations in The USA, ZL and VK. i.e. 135k to 200kHz.
In the Mk 2, the Toko filter has been replaced with a higher performance trapped low pass. The two high speed op-amps following the DDS have been replaced by a dual version.