I am having range issues with my design, and I'm trying to determine if the balun/impedance match circuitry between the RF out on the CC2500 and chip antenna may be the cause. I copied the ez430-RF2500 module's circuit for my design. I can get about 30-40 feet (not line of sight, but not through walls either) of range indoors, but it's unreliable. If I5342.Wireless Accelerometer Silkscreen Bottom.pdf want reliable communication, I usually have to stay within about 20 feet (line of sight), although I have not characterized this exactly.
Also, I am not very familiar with RF design; I've only had one intro college level comm circuits course (designed/built an FM receiver). I am more of an embedded programmer than anything else...
First, would someone be able to provided a general explanation of the balun/impedance match circuit? I understand that a balun is required to convert the differential RF signal to a signle-ended signal a0243.Wireless Accelerometer Schematic Pg 2 of 2.pdfnd that the impedance matching network steps up the 50 ohm chip antenna impedance to whatever is required by the balun or CC2500, but that's about it.
Second, I noticed that the CC2500 datasheet and reference designs specify different component values for the balun than are used in the RF2500 module, namely the inductors (1.2 nH vs. 4.7 nH). Are the values for these inductors critical? And why does the RF2500 design deviate from that specified in the CC2500 reference design?
I attached a silkscreen view of the bottom of the board (unfortunately all I have), so the approximate layout of components should be evident. The antenna being used is not the Wurth Electronics antenna on the RF2500 board, but something from Antenna Factor. Hopefully that's not a problem.
http://www.linxtechnologies.com/Products/Antennas/Embeddable/CHP-Series-Chip-Antennas/
Hey,
I have reviewed your schematic and I have found that you have an error in the component values. Your schematic says that you have a 1pF on one leg, but a 100pF on the other leg of the matched lowpass/highpass balun (described in image below). The filters are matched and therefore contain the same component values, but in a different configuration. If you correct this your range should increase.
I have included a image of the reference match for CC2500 below and included what each of the sections tasks are.
I hope it helps.
Thomas
PS: Posting your schematic is very helpful thanks.
Thank you very much for the blue text. That's very helpful for my understanding.
My above post seems to be a bit contorted. To be honest, I'm not sure how the schematic found it's way into the post. I thought I was only able to attach one file at a time... It's been too long for me to recall exactly what happened.
I'll check the designs at work tomorrow, but I think that's an old schematic, meaning we already located and fixed that error (unfortunately). I don't think the devices were able to communicate with the 1 pF cap in there.
Sorry, I meant 100 pF in the last post. We took out the 100 pF and replaced it with 1 pF. If I understand that correctly, it puts the cutoff frequencies for the highpass/lowpass portions at about 2.3 GHz, which I suspect is what we want.
Okay, so BalUn = balanced to unbalanced. Pretty much those four components are taking place of a transformer. The idea is that you have these two outputs that are 180 degrees out of phase, now you want them in phase to give good output, so to do that without a transformer you phase shift one to lead 90 degrees, one to lag 90 degrees, so they are in phase. So that is what is going on there.
Those inductor values are critical, they give the right phase shift with the 1.0pF capacitors. With larger inductors, you will get the wrong phase shift and so your signal out will be a lot weaker as the two signals will not add as well as they should.
As to the decoupling, if you look at the reference design schematic, it shows where they go, in general, you want the decoupling as close to the pins as possible, the .1uF and the 220pF in parallel by each grouping of pins (220pF closer of the two). The two caps are used to give good low impedance across a wide frequency range. Also, several pins are next to each other for power, so those get one set of caps for the two of them.
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