AussieSusan wrote:cmr77 wrote:I'm tempted to get one of the ADF4351 board and have a play with some filtering as it's a cheap device.
Agreed that it is a cheap device but how do you intend to go about the filtering.
The 'simplistic' approach is a LPF that would cut out the higher frequency harmonics of the square wave. But the issue I see is that the LPF would need to be tuneable.
At the low end of the frequency range - lets say 50MHz for an example, - the harmonics are at 150Mhz, 250MHz,350MHz etc. All of these are perfectly valid frequencies that the device can create as the fundamental frequency. Therefore any LPF that would work at the low end of the frequency range would remove all higher fundamentals as well.
Trying to make a tuneable LPF or BPF that covers such a wide range of frequencies would be an engineering challenge I suspect.
The data sheet for the device really only talks about how to use the device for 2.2GHz and up frequency output and you need to use the internal 'divide by x' register for the lower frequencies which is why the signal is 'squared up'.
I think that this chip would be very useful for the range it is intended to be used, but that other DDS chips would be better at the lower frequencies.
I have ordered a test board from this guy, https://www.sv1afn.com/adf4351m.html
currently on back order but his plot seems fine anyway on small bandwidth tho.
But I will test it myself on my old HP8566B spectrum analyzer, my goal is to have some tracking possibility up to 500 MHz
over that it is more difficult to make filters anyway, and I think if I cover the most used HAM bands could be a practical use .
I have used in another project the AD9910 with is little more expensive but have a fine sinewave up to 400 MHz it is based up on a 14 Bit DAC
I get back with test result when i have them, meanwhile Steve have to enable Spectrum analyzer software do accept DDS tracking for frequency over 90 MHz