Interference from SDRAM
Interference from SDRAM
(OP)
I am debuging the low sensitivity of the WiFi from our product. There is a SDRAM inside the product and the SW is running inside the SDRAM. The DSP and SDRAM are put inside the shielding can, it worked very well. Recently, we changed the SDRAM from brand A to brand B, the sensitivity of WiFi is degraded by 10dB. Provided that the SW is same, the clock rate is same. When I swap the old SDRAM to it, the sensitivty is as good as before.
According to the data sheet, they are nearly the same. The current consumption from brand B(poorer sens) SDRAM is 20% higher by measurement.
Does anyone know what make it so different for interference?
According to the data sheet, they are nearly the same. The current consumption from brand B(poorer sens) SDRAM is 20% higher by measurement.
Does anyone know what make it so different for interference?
RE: Interference from SDRAM
(Just yesterday, my coworker told me that Windows XP caused less television interference than Windows ME. He lives in a rural area and has very weak over the air TV signals.)
RE: Interference from SDRAM
RE: Interference from SDRAM
Maybe the first step would be to use a spectrum analyzer to sweep units with brand A and with brand B. Then line-up the spikes with the known clock frequencies, look for the change, and see it an explanation falls out.
Another option would be to simply strike brand B from the parts list and call it a day. This is not an uncommon solution.
The 'net is full of stories about bugs that cropped up due to subtle hardware differences. The March 20, 2008 edition of EDN (just arrived) has an article at the back called "The case of the bad memory chip" where the start-up uninitialized contents of DRAM chips varied and uncovered a subtle SW bug.
Stand-by to see if anyone else has some other suggestions.
RE: Interference from SDRAM
I know that having a transmitter antenna and a receiver antenna inside the same shielded can makes energy couple much more to each other. Orientation of the two antennas can really change the coupling.
From an antenna coupling point of view, if two antennas are aligned (!!) or misaligned (!-), there is a great difference in RF coupling (20-40 dB). Maybe one of the SDRams has a different direction of wires/current flow than the other.
Having them in separate shielded can's may solve the problem, or adding some microwave absorber between them inside the shielded can will help enough.
kch
RE: Interference from SDRAM
RE: Interference from SDRAM
You say that the design is the same except for the new SDRAM, but is it really? Did your layout guy change ANYTHING? A cut in the ground plane, re-routing some traces, etc, could make a huge difference. You are trying to avoid something called "ground loops".
You might be able to slow down the edges of the digital signals flowing between the two chips, which would dramatically change the ammount of power floating around at microwave frequencies. Maybe some 20 ohm series resistors in the digital path.