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I'd like to drive the antenna pin(s) of an Si4732 receiver chip with a device that isn't an antenna. I don't know what voltage to scale to, though, and don't want to fry the chip. The datasheet (or, the datasheet of closely related chips - I haven't found a datasheet specifically for the Si4732) doesn't directly say e.g. "max voltage on AMI"; is there some keyword I don't know, or failing that, a good general heuristic? Also - there's presumably a difference between "max non-damaging voltage" and "max voltage you can get a signal out of"?

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  • $\begingroup$ The question becomes: if you are more likely to have the problem of "too high input amplitude" rather than "too low", what would you need a highly integrated FM demodulator that integrates LNAs, mixing, LO synthesizers and demodulators for? This sounds like you're building a measurement instrument of sorts, and you won't be happy with the preemphasis that a broadcast FM receiver will do then. Probably a good idea to also put a concise, but technically accurate, description of what you're trying to do in the overall picture into your question! $\endgroup$ Apr 13 at 8:46

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The datasheet says:

  • Absolute max voltage on RF input pins: 0.4 V
  • Sensitivity (FM) for 26 dB S+N/N: 3.5 uV

So you have a fairly large range to play with. Likely near the bottom of that you'll have strong broadcast stations breaking through, and near the top you'll have distortion problems.

I think anything in the range 10 uV to 10 mV should be fine.
Use a resistor voltage divider to attenuate the signal if it's larger than this. The input impedance is ~ 1k ohm so make the parallel resistor less than this.

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  • $\begingroup$ I'd throw in that the "Absolute Maximum Ratings" are how far you can drive the chip without permanently damaging it, not how far it still works – 0.4 Vpk is insanely loud for an integrated RF receiver (especially one that was clearly meant to be used with low-efficiency antennas); with the datasheet characterizing blocking performance with a blocker power of 100 dBuV, which is 0.1 V, to be at 30 dBuV (so, roughly 31 µV), I'd say you probably be happy with weak signals, as long as your SNR is good enough; I'd stay closer to 100 µV than to 10 mV, but you'll be fine at either end. $\endgroup$ Apr 13 at 8:42
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    $\begingroup$ Yes there will be a voltage at which the internal LNA saturates, even if this does no harm it may stop working. As it happens, yesterday at home I measured about -30 dBm from each FM radio station, with a simple dipole, so this would be over 10 mV. I'm not particularly close to the transmitters. $\endgroup$
    – tomnexus
    Apr 13 at 14:28
  • $\begingroup$ yep, about what one expects. Was that a ~ half wave dipole or something shorter? I mentioned that this device is probably meant to be used with low-efficiency antennas, due to its high input impedance; that makes the most sense if we're talking stub/rod antennas. $\endgroup$ Apr 13 at 14:34
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    $\begingroup$ This was a dipole for 137 MHz, but the FM band overloaded my LNA. Agree the FM antennas are often small, but I'd expect the receiver to work 5a few km from a transmitter with a full size dipole (remember the old DX-Local switch...) $\endgroup$
    – tomnexus
    Apr 13 at 16:11
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    $\begingroup$ In summary, I am speculating about the maximum useable signal level, based on common sense. The datasheet doesn't specify it. OP should be fine with 1 mV or something. Can always change the resistor values if it turns out it's too strong. $\endgroup$
    – tomnexus
    Apr 13 at 16:13

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