请教一个关于RFID阅读器系统的问题

guj1987

请教一个问题，下面是《The RF in RFID Passive UHF RFID》书中的一个章节


文中下划线这句话是什么意思，(−95+45) = −50 dBc公式具体是什么意思

4.4.3 Phase and Amplitude Noise
Because an RFID reader for passive tags is transmitting at the same time the receiver listens
for the tag response, there is always some leakage from transmitter to receiver. In a monostatic
configuration (Figure 4.1), this leakage is dominated by the signal reflected from the antenna
back into the receiver, and is typically around 15–20 dB below the transmitted signal. In a
bistatic configuration, TX-RX isolation of 30–40 dB is achievable.
There are a couple of basic sources of noise in the transmitted signal. The first is phase
noise from the oscillator (Section 4.3.3 above). To get an idea of the magnitude of the phase
noise contribution, let’s use the example shown in Figure 4.29. The frequency range of
interest is that corresponding to the tag signal; for a tag data rate of 100 kbps we might want
to look at frequencies from around 50 kHz to 100 kHz from the carrier, where the phase
noise is typically about −95 to −105 dBc/Hz. Very roughly speaking, the bandwidth is
around 50 kHz or 45 dB from 1 Hz, so the total noise will be about (−95+45) = −50 dBc
in this band. Let us imagine that the transmitted signal is 1 W (30 dBm), of which 15 dBm
is reflected from the antenna and sneaks into the receiver. The received phase noise is then
approximately (15−50) = −35 dBm. If on the other hand a bistatic arrangement is used,
the injected signal from the transmitter starts at around 0 dBm (or better) and the phase
noise is then −50 dBm. (In practice, as we will discuss in the next section, receivers are
typically arranged with I and Q channels, and the absolute phase of the leakage signal
determines how much of this noise ends up in each channel.)