Large transients at Start andStop of RF transmission

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joki_2146851
Level 3
Level 3
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I'm have a very noise sensitive high gain amplifier for which I'm communicating with via BCM20732s. As I've been designing and debugging trying to keep the RF out of my amplifier, I've found that the biggest problem are transients when the RF transmission power is switched on and switched off. Oddly during the time the RF is on, I don't see disruption. The question I have is what is causing this and how can it be minimized? I believe it must be a very sharp voltage dip/rise on the supply when the RF kicks on/off causing radiation. It is very consistent, always inducing a negative detected impulse when the RF starts up and positive impulse when it turns off. Perhaps someone who works on the PA can comment.  Is there any way to limit the slew-rate of the RF power on transition? I had originally looked for a software solution (i.e. using a gating signal from the chip when the RF is on) unfortunately there is not such signal that can be routed out on the 20732 to indicate the RF is on.  Any help with understanding these transients and minimizing them would be greatly appreciated.

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1 Solution

Just so you know that Bluetooth utilized Adaptive Frequency Hopping to avoid busy channels. The below thread is somewhat similar to yours....

the strange current pules when connecting a slave device if 20736 as a master

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BoonT_56
Employee
Employee
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By default the chip or module should transmit at a max of 4dBm and the PA is internal to the chip. Do you have captures of this transient like its frequency?

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I do not have equipment that captures 3Ghz and the transient. I just see the transient on my device.

Today I used a stronger antenna with a RF pulse generator and swept across the Bluetooth frequency. Interestingly I say nothing with the straight frequency sweep, but when I chopped the RF (like is done with immunity testing 1Khz) I was able to reproduce the effect -- more strongly with the stronger antenna than with the 20732s itself. Therefore this leads me to believe that it my front-end (transimpedance amp 1500mV/fC with 5th order pulse shaping filter with peaking time of 12us), is seeing the incident pulse train coming and going, essentially AC coupling the pulse train and rectifying the RF so that I get a rising pulse and falling pulse when the RF comes and goes.

All if this tells me the RF is coupling into my board and I need better immunity.

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Just so you know that Bluetooth utilized Adaptive Frequency Hopping to avoid busy channels. The below thread is somewhat similar to yours....

the strange current pules when connecting a slave device if 20736 as a master

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jkindem

Any new findings from your side?

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