04-02-2019, 11:54 AM
The AD9833 board has a 25MHz xtal so the theoretical maximum frequency (nyquist limit) it can synthesise is 12.5MHz. With a really good reconstruction filter you can go up to about 80% of the nyquist limit and still have decent looking waveforms. With a simple low pass filter you can reasonably go up to about 40%, so about 5MHz in this case.
Even if the output has visible steps it doesn't mean it's going to work badly. It means that you have images of the wanted specturm repeated at higher frequencies. This may not matter. For example the modulators used in Aurora and Hedghog give something close to a square wave in Band I. But unless you are using mutliple channels or (illegally) transmitting then the presence of 3rd and higher harmonics is of little importance.
A spectrum analyser will show the out of band signals. Then you can decide how bad they are for your application.
Frank has been experimenting with DDS for Band I video and audio. I think he's using a clock around 200MHz, an FPGA to do all the modulation arithmetic and a high speed DAC. It ought to work well for Channel B1 but without a good filter will look pretty horrible at B5. But it will probably work OK, despite looking horrible.
Even if the output has visible steps it doesn't mean it's going to work badly. It means that you have images of the wanted specturm repeated at higher frequencies. This may not matter. For example the modulators used in Aurora and Hedghog give something close to a square wave in Band I. But unless you are using mutliple channels or (illegally) transmitting then the presence of 3rd and higher harmonics is of little importance.
A spectrum analyser will show the out of band signals. Then you can decide how bad they are for your application.
Frank has been experimenting with DDS for Band I video and audio. I think he's using a clock around 200MHz, an FPGA to do all the modulation arithmetic and a high speed DAC. It ought to work well for Channel B1 but without a good filter will look pretty horrible at B5. But it will probably work OK, despite looking horrible.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
