More features for DdsModTerm software

🔘I have added some important new functionality to my DDS programming software, "DdsModTerm", ( see 📅31 December 2024 ); updated to version 1.0.0.rev16. It is now possible to generate audio 👂frequencies, as well as program the AD9850 and AD9851 DDS chips, using one application. For example, the 'Phasing Exciter' ( see 'featured post' in side-bar, 📅 02.11.2017 & 23.12.2024 ) requires both audio frequencies (👂AF) and radio frequencies (📻RF). So having the same software able to generate both is very convenient while carrying out testing, alignment, and repair tasks. This has required some changes to the lower half of the program window, (see image below, click to enlarge, compare with 📅 31.12.2024).

Audio tones can be generated from the lower-right panel 

The audio 👂 tones are produced entirely independently of the DDS. Free downloads of audio .wav files are available online. I chose files of single frequency sine-waves ∿; 1200Hz, 1400Hz, 1500Hz & 1600Hz being best suited to the data transmission modes I use on the 472-479KHz amateur band. I imported these files  as resources into the DdsModTerm software, and wrote some code so that the tones are played back through the 💻 pc sound-card 🔊by clicking the appropriate button. The individual .wav files are quite small ( about 1.3Mb ) and playback duration ⏳is only 15 seconds. I added a second progress bar to track the playback, and a button to play back in a continuous loop, if preferred.

While the software was being revised, this opportunity was also used to calculate another pair of alias frequencies  ( for 2 x 🕒clock ), and make some minor cosmetic changes to the window's appearance.🔘

Free .wav files downloaded from 🔗 OnLineSound  with thanks.

DDS = Direct Digital Synthesiser

AD9850 & AD9851 are Analog Devices Inc. parts.

Phasing exciter for the MF 630m band

DDS interface dongle left, antenna c/o top, main board below

antenna change-over circuit removed to reveal main circuit board

So that I can join in the fun of using weak signal data modes such as JT9, JT65, FT8 and WSPR on the 475KHz 630m band I have built a phasing exciter for a low power single-sideband transmitter. Audio input signal is generated by pc data mode software and sound card. The rf signal source is a cheap AD9851 DDS module bought at an online auction site. Its frequency of operation is changed using the usb dongle and interface software obtained from www.spectecs.com. The rf phase shifter is a dual J-K flip-flop chip, af phase shifter comprises two quad opamps, and the mixer is a dual 1to4 mux/demux fet bus switch.
To give the project a smart appearance I chose a Hammond type 1455N1201BK box for it. Even before the prototype was finished, I couldn't resist the temptation of connecting it to my inverted 'L' antenna and trying it out. The exciter on its own produces only 1mW ( 1 milli watt, 0dBm ); but this was sufficient for my WSPR beacon signal on 474.2KHz to be received with SNR -23dB at a distance of 11kms by a monitoring station.
The prototype was completed with the addition of a class A rf driver amplifier to increase the output power to 100mW, and an automatic rf sensed antenna change-over switching circuit. Since then my signal has been received in Norway, distance 1071kms, Estonia, distance 816kms and Germany, distance 700kms, which surprised me as the antenna's directionality favours the east. Sadly there don't seem to be any foreign receiving stations in the east at the moment.

Chirp-Hell on longwave

On 14th November 2012 I reported success with the tests on the work-bench of the improved ssb phasing exciter for my 2190m longwave transmitter. Soon after that I installed it inside the transmitter enclosure. Since then I had been waiting for an opportunity to test it using full transmitter power into my antenna in a real 'on-air' situation, with a more distant receiver. 

So early this morning at about 1.00am I carried out transmission tests with Jacek, SQ5BPF, in Warsaw. My signal was quite readable on his grabber; screen captures above. The noisy band conditions at his location are also very evident.

I was transmitting on 136.9KHz upper sideband, modulating with 800-810Hz chirp-hellschreiber audio tones. Transmission speed was either 5 or 10 secs/character; the vertical markers are 1min apart. We then completed a chirp-hell to qrss1 cross-mode contact; quite obscure, so it's probably the first time ever it has been done !

Improved phasing exciter

I have completed some improvements to the single-sideband phasing exciter, ( post 25 January 2010 ), for my longwave transmitter, just in time for use during the good propagation conditions on the 2190m band over the winter.
The passive phase shift network was not producing accurate 0 and 90 degree phase shifted AF. In fact it was well outside specification; perhaps not surprising as I bought it in 1978. So I replaced it with an active circuit based on a dual op-amp. I optimised the unwanted ( lower ) sideband suppression at 800Hz. There is little point anyway in achieving exact quadrature audio channels over the entire speech band as the modes in use on longwave are extremely narrow-band.
I added a tuned class-A post-mixer transistor amplifier stage.
Upper sideband is now selected automatically as I have disabled the sideband switching facility; until such time when I see a lower sideband signal on the band.
The top picture is my signal, received on 137.7KHz, from just the exciter sitting on the workbench, ( lower pic ), when transmitting using chirped multi-tone Hellschreiber mode. As can be seen, the signal occupies only about 5Hz of band !

Single side band generator for the longwave transmitter

Uses the phasing method for ssb generation. CMOS logic integrated circuits ( D type and J-K type flip-flops ) produce the rf phase shift; a passive R-C phase shift network takes care of the audio phase shift. Plessey SL640 mixers are used. The prototype is shown undergoing tests. It is now fitted inside the transmitter case at the rear left. When it's connected to the PC soundcard, data, encoded as audio tones, can be transmitted.