Adding amplifier to RF circuit - Page 1
Forum section: electronics / beginners

author: petergriffin56
author_rank: Contributor
timestamp: May 31, 2019, 07:58:58 pm
content: Hello,I'm designing the RF part of a project but I'm not sure if I'm doing it right or not since RF is pretty complicated (for me at least). Two thing I would like to mention first :- The output for this device is higher than my country's regulations allow (and probably most), but this is for a proof of concept, not to sell commercially or use outside of my school project.- I don't need this circuit to be perfect, I know RF is a complex field. I am not trying to design a perfect transmitter, I would just like for it to (A) not catch fire and (B) be amplified, at least enough to be observable and "succesful".Here are both circuits with their values for my 915MHz, pulled from their respective datasheets. I've also added the relevant comments excerpt from the datasheets, note that the transceiver circuit had both a reference design for a direct tie connection or an RF switch. I chose the RF switch because the design is changed as to match the 50ohm input impedance of the RF switch, which is also the impedance of the amplifier so that helps a lot I think.The amplifier is both 50 ohm in and out, similar to the RF switch, this is what leads me to believe "replacing" it with a few tweaks to the design shouldn't betoocomplicated.If the images are not visible (they don't seem to be on preview) then I didn't add them correctly... I have them as attachements otherwise.And the proposed final circuit, forgive my crude illustrations but I'm just trying to get my thoughts across.Once again this would probably make an RF engineer throw upI know, but I'm just trying to make this work. Some people on other forums have told me this "design" is ok, I would like to know what you guys think.What should I remove, keep ,or increase/decrease value ?For reference this is a CMT2300A transceiver (OOK/FSK) and an ADL5324 amplifier.

author: eb4fbz
author_rank: Regular Contributor
timestamp: June 01, 2019, 06:49:52 pm
content: I would only remove C5 and C9. Also be very careful replicating the ADL5324 reference design layout. I would personally simulate the behaviour of the combined amplifier+filter, but it looks like you have little idea about RF.

author: dmills
author_rank: Super Contributor
timestamp: June 01, 2019, 09:34:33 pm
content: The ADL5324 is NOT a 50R matched gain block, nothing like. Look at the data sheet and see the list of S parameters, that is what you are trying to match into.I would basically use figure 3.2 from the CMT2300 datasheet with the ADL5324 circuit entire placed just before C5 on the CMT2300 circuit (IE just before the switch). You will probably want to add a pmos transistor or something to power off the amplifier when on receive (It might oscillate due to the mismatch).Note that a schematic is less then half a design at these frequencies (Layout is at least as critical), and looking at the amplifier schematic there are two fairly critical transmission line segments that will need to be accurate (and that probably depend on the exact details of your board stackup).Do you have (of have access to) a network analyser? You are pretty much playing blind without this IMHO.