A DDS VFO for the Lydford 40m QRP SSB Transceiver Combining a very cheap Chinese-made DDS module and a low cost microprocessor and LCD display, this VFO delivers band-wide tuning, digital stability and four tuning speeds to the Lydford transceiver. Introduction Temptation is a powerful thing. Browsing across the Internet one day, I stumbled across a reference to (at that time) a recently released Chinese DDS module. It’s shown in Figure 1 below. These modules use one of the more common DDS chips, the Analog Devices AD9850, and were available at an incredibly low price. The Mini-Kits AD9851 DDS VFO can be either used with the DDS-VFO software as an accurate signal generator, or with our Sweep software option. 4 functions of the software VFO frequency is incremented on turning rotary encoder turning. But before sending to DDS, it finds absolute value of VFO. Just a few dollars each, perhaps less than one-tenth what the chip alone used to cost. And these modules came complete with the required reference crystal oscillator and a low pass filter. Temptation became translated to action. Before long, several of these modules had arrived on my bench. Figure 1: Top side and underside view of one of the low cost Chinese AD9850 DDS modules DDS What is direct digital synthesis? DDS chips output digitally generated sine waves at frequencies from near-DC to several GHz depending on the particular device you use. Some DDS chips produce a pair of “quadrature” outputs. These in-phase and quadrature (90 degree phase shifted) outputs are really useful in oscillators used in the latest software-defined receivers and transmitters. One of the earliest and most popular DDS chips was the AD9850. Its output frequency is determined by an internally stored 32-bit “tuning word” usually written to the device by a companion microcontroller, usually via a 3-wire serial interface. A 40-bit word is actually sent because there are a few other “control” bits that are also required. These DDS chips are clocked by a separate reference crystal oscillator. These normally operate at frequencies above 100MHz. The stability of this oscillator determines that of the output. Frequencies generated are Nyquist-limited to half the DDS crystal reference frequency and may be produced at sub-Hertz resolution. If you can add to this list, please post here and I will update this post. Free xbox 360 iso downloads. In practice, filter limitations usually constrain the DDS oscillator output to about 40% of the reference frequency, and most VFO designs seem to focus on 10Hz tuning steps as the minimum resolution used in practice. But it’s not all 'wine and roses' in the DDS world.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |