rf amplifier AD9850 - Page 1
Forum section: electronics / projects

author: xargon
author_rank: Newbie
timestamp: January 06, 2018, 11:34:08 am
content: Hi guys I'm working on a project using a AD9850 DDS module to transmit signals to devices running on at least three separate frequencies (lets say 27Mhz, 38Mhz & 49Mhz). The AD9850 is being driven by a micro-controller and so far everything is working as expected. The only issue I now have is I only get about 5-10 meters of working signal range using a piece of wire as an antenna straight from the AD9850 output. I studied basic transistor amplifier circuits many years ago but I'm not sure where to start in designing an amplifier that can work at those three separate frequencies or where to start designing three separate amplifiers for each frequency range (I can be flexible here).Basically if someone told me something along the lines of 'common emitter stage for some gain feeding into a common collector stage for more output power' I should be able to come up with something, but not sure what the simplest option is, or what type of setup to even begin looking at, software is more my thing these days.....My other issue was how do I treat the load being a simple piece of wire for an antenna, I don't understand how to quantify it for design, as in it's not a 50ohm load or similar.Thanks for any help/ideas.

author: danadak
author_rank: Super Contributor
timestamp: January 06, 2018, 12:24:36 pm
content: Some ref material -http://www.qsl.net/va3iul/RF%20Power%20Amplifiers/RF_Power_Amplifiers.pdfhttp://www.analog.com/en/products/rf-microwave/rf-amplifiers/power-amplifiers.htmlhttps://www.nxp.com/products/rf/rf-amplifiers-low-medium-power:MC_50961The QRP manuals -http://www.arrl.org/shop/search.php?mode=search&page=1Antenna simulator -http://eznec.com/demoinfo.htmhttps://www.eevblog.com/forum/rf-microwave/freeopen-source-antenna-simulation-tools/Regards, Dana.

author: orolo
author_rank: Frequent Contributor
timestamp: January 06, 2018, 07:22:13 pm
content: In the typical 985x module, there are two sinewave outputs: one goes through a lowpass elliptic filter, with a terminal load of 200 ohms. The other is unfiltered, with a load of 100 ohms. About the filter, take a look at thedocumentation of the module, keep in mind there is an errata, L1 is obviously 470nH, not uH. I get a 3dB frequency of 75MHz; it should keep away major aliases up to 49MHz, so you should use this output to avoid harmonics in your transmission.Since the AD9850 uses a current source at the output, the waveform is a 1V amplitude sinewave going from ground to a peak of 2V. The output of the module is low impedance (100-200 Ohm), so you should probably use a buffer (a single FET common drain would work, or a AC coupled emitter follower-- remeber the output goes to ground) to avoid distorting the output of the DSS module. Then you can amplfify the voltage if you need it (remember it's already a 1V amplitude singal) and/or amplify power. How much power do you need anyway? The low power bands don't allow much more than a few mW, if I remember correctly.About the antenna, take a look atthis calculator. The wire diameter matters. Anyway, you'll find a very low radiation resistance , and a big capacitive reactance. I think the usual cure is a series variable inductor that is manually tuned at the required frequency for maximum performance.