Scope of the Problem
The wireless sensor network receiver described in Chapter 3 becomes
desensitized when an interfering RF signal reaches a level of approximately Pdesense = −90 dBm
(10−12 Watts), or edesense = 7.07 μVrms (17
dBμV) in a 50-Ω system.
This incredibly small signal is both the magic of radio and the bane, for it
means that the surrounding electrical environment must be very quiet to get
maximum performance.
9.4.4.1 Digital-RF
Isolation Needed
If a receiver requires an interference "floor" of −90 dBm to avoid desensitization, and it is working with a
microcomputer with switching 2 V signals, the needed isolation may be
computed:
or almost 11 orders of magnitude.
Fortunately, one gets several tens of dB due to the relative
amplitudes of the digital signal's Fourier coefficients at the fundamental
frequency and the harmonic causing the interference, but it can still be a
challenge to meet this isolation requirement. For example, it was demonstrated
previously that if the rise and fall times of a 1-V, 1-MHz digital waveform are
50 ns, harmonics near 150 MHz have a peak amplitude of approximately 45 dBμV. The level from the 2-V signal will be 6 dB higher, or
approximately 51 dBμV. The root-mean-square (rms)
value of this signal is 3 dB less than this, or 48 dBμV. Because the interfering signal, as detected by the
receiver, must be less than 17 dBμV to avoid desense,
there must be 48 − (17) = 31 dB of isolation between
the digital signal and the receiver.
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