ZL1BPU exciter

Description: Direct Digital Synthesis (DDS) frequency generation has been established for some time; however, many homebrew DDS projects tend to be complex and utilize costly chipsets. Murray's exciter requires only three inexpensive integrated circuits (ICs), and the entire assembly can be constructed for under $50. Notably, the exciter can be programmed via a PC to function as a beacon, with software designed for modes that are popular among LF amateur radio operators and experimenters. Murray's article includes links to a comprehensive user guide, which is highly recommended for anyone interested in building this project. A quick check of parts availability in the US revealed sources for all the necessary ICs. To share shipping and handling costs and to encourage more US LowFERs to experiment with the modes supported by the ZL1BPU exciter, an offer was made to purchase up to ten sets of parts at cost, which was quickly fulfilled. This article aims to assist those who acquired parts sets in getting their units operational and to provide ideas for others interested in building this engaging project. Important Notice: As with any high-speed processor, undervoltage conditions or voltage transients can disrupt the memory of the Atmel AVR chip. One solution is to incorporate a voltage detector reset chip as described later. Additionally, Murray suggested placing a 0.01 µF capacitor from the reset line (pin 1) of the microcontroller to ground, which has proven effective in resolving various issues, particularly with loading EEPROM memory. Although circuit diagrams have not yet been updated to reflect this fix, it is strongly advised to add the 0.01 µF bypass capacitor on the reset line. The ZL1BPU exciter closely follows Murray's schematics, with the primary distinction being in the oscillator section. A good-quality crystal within the 10 to 15 MHz range is suitable, but for stable operation and proper frequency spacing in "Jason" mode, a 12.8 MHz temperature-compensated crystal oscillator (TCXO) is recommended. These can often be salvaged from older cellphones, although specific frequencies may not always be available. Suppliers like Digi-Key and Mouser Electronics offer TCXOs for approximately $15 each. Despite being the most expensive component of the exciter, this cost is comparable to that of a high-quality custom crystal. The TCXO's output level is specified at 0.8 volts peak-to-peak, which aligns with measured values. Concerns regarding whether this level would adequately drive the external clock for the AVR microprocessor led to the addition of a 74HC4046 as a buffer amplifier, utilizing only the phase detector function while disabling other chip functions. This approach is similar to that used in Almost All Digital Electronics frequency counter boards, effectively converting a small signal (as low as 50 mV peak-to-peak) into a 5-volt logic-level signal. After constructing the exciter, it was found to operate effectively with the TCXO output driving the AVR microprocessor directly. However, the AVR microprocessor specifications indicate a requirement for a 2-volt peak-to-peak drive signal on the clock input, suggesting that while the buffer amplifier may be optional, it is advisable to allocate space for it on the circuit board. Additionally, a 74HC4024 seven-stage binary counter was incorporated to divide the 12.8 MHz signal down to 100 kHz, providing a reference frequency.

The circuit design for Murray's exciter is both efficient and cost-effective, making it an attractive option for hobbyists and amateur radio operators. The integration of simple components allows for a straightforward assembly process, while the ability to program the exciter via PC enhances its functionality. The inclusion of a voltage detector reset chip and bypass capacitors addresses potential issues with memory integrity, ensuring reliable operation. The choice of a TCXO not only stabilizes frequency output but also ensures compatibility with the desired operational modes. The use of a 74HC4046 and a 74HC4024 enhances the circuit's performance, enabling effective signal processing and frequency division. Overall, this project exemplifies a practical approach to DDS frequency generation, catering to both novice and experienced builders in the amateur radio community.Direct Digital Synthesis (DDS) frequency generation has been around for quite a while, but most of the homebrew DDS projects are rather complex and use expensive chipsets. Murray ’s exciter has only 3 inexpensive IC ’s,and the whole thing can be built for under $50. Perhaps best of all, the exciter can be programmed via a PC to operate as a beacon, and the software is tailored for operation using modes that are popular among LF hams and experimenters. Murray ’s article is at and there are also links to a very comprehensive user ’s guide. His writeups are excellent, and you should view all of the information if you plan to build this project.

I did a quick check of parts availability in the US, and quickly found sources for all of the ICs. To share the shipping/handling charges and to encourage more US LowFERs to experiment with the modes supported by the ZL1BPU exciter, I offered to purchase up to 10 sets of parts and to make them available at my cost. The offer was quickly "sold out". This article is intended to help those who purchased sets of parts to get their units running, and to provide ideas for anyone else who wishes to build this interesting and useful project.

Important Notice: As with any high-speed processor, it`s easy for an undervoltage condition or a voltage transient to scramble the memory on the Atmel AVR chip. One fix for this problem is to add a voltage detector reset chip as described later in this article. Some time ago, Murray also suggested adding a 0. 01 uF capacitor from the reset line (pin 1) of the microcontroller to ground. I found that this fixed a lot of problems, especially the apparent inability to load the EEPROM memory correctly.

Unfortunately I never got around to updating the circuit diagrams or including a mention of this fix in my article. Maybe someday I will make the addition to the diagrams. In the meantime, I strongly suggest adding the 0. 01 uF bypass capacitor on the reset line! My version of the ZL1BPU exciter follows Murray ’s schematics very closely. The primary difference is in the oscillator section. Any good-quality crystal in the 10 to 15 MHz range can be used, but for stable operation and for proper frequency spacing in "Jason" mode (, Murray suggests a 12.

8 MHz temperature compensated crystal oscillator (TCXO). These can sometimes be scavenged from old cellphones, but my junkbox doesn ’t have any of those (not with the proper frequency, anyway). Digi-Key and Mouser Electronics both have TCXOs in their catalogs for about $15 in single unit quantities.

Although this is by far the most expensive component of the exciter, it ’s no more than the price of a good custom-made crystal. According to the manufacturer ’s specifications for the TCXO, the output level is 0. 8 volts p-p, and this agrees with what I have measured. I was concerned that this would not provide an adequate drive level to serve as the external clock for the AVR microprocessor, so I added a 74HC4046 to the circuit as a buffer amplifier.

Only the phase detector portion of the 74HC4046 is used; all other chip functions are disabled. This idea is used in the Almost All Digital Electronics frequency counter boards, and is very handy when you want to take a small signal (as low as 50 mV p-p) and turn it into a 5 volt logic-level signal. After building the exciter, I found that it seemed to work just fine with the TCXO output driving the AVR microprocessor directly.

However, the AVR microprocessor specifications call for a 2 volt p-p drive signal on the clock input, so although the buffer amplifier may be optional, I ’d leave room for it on the circuit board just in case. I also added a 74HC4024 seven-stage binary counter to the circuit, which divides the 12. 8 MHz signal down to 100 kHz. This provides a reference frequency th