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Optimal / maximum RF input level

I am not sure if there is an easy answer to this question, but what is the optimal maximum RF input level? Obviously you don't want A/D overload and clipping, but should the signal level be kept even lower than that, perhaps to avoid any nonlinear regions of the front end amp?


  • edited March 2019
    that's a little tricky because you have to consider the sum of all the big signals in the HF range, not just what you see. I use S9+40 dBm as my goal when filtering, notching or nulling the biggies.
  • Right, let's assume I can put an accurate scope on the RF input. What's the largest acceptable peak value? Or even better, what's the maximum A/D count? (With a way to view it on the Admin panel)
  • From here:
    Tests with a signal generator show the ADC overflows when a single-tone exceeding -15 dBm (almost S9+60) appears on the antenna terminals.

    -15dBm at 50 Ohm would be 56mV peak.
  • From the LTC2248 data sheet, "Connecting SENSE to VCM selects the internal reference and a ±0.5V input range. " which according to the schematic is what R408 does.
    20 dB of LTC6401 gain, data sheet again, and 2 dB of LPF loss, an estimate, would produce 18 dB gain ahead of the conversion. This net gain would turn a sine wave of 44.5 mV rms ( -14 dBm in 50 ohms) at the SMA connector into 1 V p-p at the LTC2248 ADC. I'm guessing that the OV indicator comes on when the instantaneous input exceeds this level, which agrees closely with what we all measure.
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