I am successfully using an AD9915 with a 2.5 Ghz clock to generate a 2.856 GHz signal. The first thing to do is to put a high pass filter directly on the output. Otherwise the low frequency component will overload the amplifiers that follow. I then use a low noise amplifier to make sure I get a good signal to noise ratio. Then I amplify the signal to 20 mW or so. I place a bandpass filter in front of the last amplifier stage. For a bandpass filter I use L-Com WiFi filters. They are tuned to 2.4 GHz or so, but I have had no problem tuning them to 2.856 GHz or even 3.0 GHz. I use the 4-pole filters because I don't want a lot of phaseshift with frequency for my application. They have an 8 pole version as well. This approach is much cheaper than custom filters. I adjust only the tuning screws and not the coupling adjustments. It's easy to disassemble one to see what is what, assuming you have an idea of how BandPass filters work. I always buy the ones for the two highest bands, but I think they are all the same, just tuned differently.
My problem is synchronizing two 9915s. I have succeeded in keeping the phase offset constant during operation, including profile register changes, amplitude, phase, and frequency changes, but I can't zero the phase accumulator registers reliably.
Sorry this has caused you trouble, it's the first I have heard of this text being an issue for anyone. I don't there is any official document outside of this note on Engineer Zone confirming the device works in Super Nyquist. I can confirm that there is no internal filter function on the DDS. The 1.4 GHz max is intended as a guideline for customers. Frequencies up to even 1.75 GHz are possible on the fundamental, but they make for either a very challenging filter design, very high spurs, or both.
Please note that the AD9914 is subject to export restrictions and may require an export license in order for you to acquire it.
Hi Guru ! Could you plese specify the document concerning AD9914 where confirmed this statement ? ( Yes, as with other DDS products, you will likely want to use a bandpass filter if you plan to go Super Nyquist.) We can't find any statements concerning Super Nyquist in the datasheet. The datasheet says - the max frquency is 1.4GHz. (p.3 datasheet) We we would appreciate if you can send a reference of make a document confirmed that. Concerning the real situation - our quality dept. referenced to datasheet and not allowed use 9914 in Super Niqvist. The real conditions ( input signal 900MHz, comparing frequency 2800МHz, output signal = 900+2800=3700 MHz 3-ed Niqist zone) Also , could you please confirm that 9914 has not any internal filters rejecting the the signal higher than 1.4GHz.
-
in reply to ilyin