Rev1.0a | 27/50
CMT2300A
4.3.6 Phase Jump Detector
(
PJD
)
PJD is Phase Jump Detector. When the chip is in FSK demodulation, it can automatically observe the phase jump characteristics of
the received signal to determine whether it is a wanted signal or an unwanted noise.
Figure 8. Received signal jump diagram
The PJD mechanism defines that the input signal switching from 0 to 1 or from 1 to 0 is a phase jump. Users can configure the
PJD_WIN_SEL<1:0> to determine the number of detected jumps for the PJD to identify a wanted signal .As shown in the above
figure, in total 8 symbols are received. But the phase jump only appeared 6 times. Therefore, the number of jumpsis not equal to the
number of symbols. Only when a preamble is received they are equal .In general, the more jumps are used to identify the signal, the
more reliable they result is; the less jumps are used, the faster the result is obtained. If the RX time is set to a relatively short period,
it is necessary to reduce the number of jumps to meet the timing requirements. Normally, 4 jumps allow pretty reliable result, e.g.
the chip will not mistakenly treat an incoming noise as a wanted signal, and vice versa will not treat a wanted signal as noise.
Detecting the phase jump of a signal, is identical to detect whether the signal has the expected data rate. In fact, at the same time,
the PJD will also detect the FSK deviation and see if it is legal, as well as to see if the SNR is over 7 dB. With these three
parameters the PJD is able to make a very reliable judgment. If the signal is wanted it outputs logic 1, otherwise outputs logic 0. The
output can be used as a source of the RSSI VLD interrupt, or the receive time extending condition in the super low power (SLP)
mode. In direct data mode, by setting the DOUT_MUTE register bit to 1, the PJD can mute the FSK demodulated data output while
there is not wanted signal received.
The PJD technique is similar to the traditional carrier sense technique, but more reliable. While users combine the RSSI detection
and PJD technique, they can precisely identify the status of the current channel.
4.3.7 Automatic Frequency Control (AFC)
The AFC mechanism allows the receiver to minimize the frequency error between the TX and RX in a very short time once a
wanted signal comes in. This helps the receiver to maintain its highest sensitivity performance. CMT2300A has the most advanced
AFC technology. Compare with the other competitors, within the same bandwidth, CMT2300A can identify larger frequency error,
and remove the error in a much shorter time (8-10 symbols).
Normally the frequency error between the TX and RX is caused by the crystal oscillators used in both sides. CMT2300A allows the
user to fill in the value of crystal tolerance (in PPM) on RFPDK. Based on the crystal tolerance, the RFPDK will calculate the AFC
range while minimizing the receiver bandwidth (to maintain the best performance). Due to the excellent performance of the AFC, it
provides a good solution to the crystal aging problem which would lead to more frequency error as time goes by.
Therefore, compare to other similar transceiver chips, CMT2300A can solve more severe crystal aging problem and effectively
extend the life time of the product. Please refer to
“AN196-CMT2300A-CMT2219B-CMT2218B The Advantages of the Receiver
AFC.” for more details.
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