KESRX01
Bit slicer and Peak Detector
To provide maximum flexibility an independent data
comparator is provided. External circuitry must be provided to
obtain the bit slicer threshold level. Two basic approaches are
supported.
1. For coding schemes with no d.c. content (e.g. Manchester
coding or 33% / 66% pulse width encoding) this can be based
on the integrated d.c. level (using a series R and C). See
Application Note AN207.
2. For coding schemes with d.c. content (e.g. low duty cycle
pulse width modulation) an active peak detector is included.
The output at pin PEAK represents the peak level at the data
filter output (as shown in Fig.5). An external RC time constant
at this pin determines the maximum attack and decay times of
the peak detector. Typical values for the leakage and diode
current source capability are shown in the specifications. The
comparator has relatively low drive capability (push/pull
current source output of 20µA) and hence DATOP should not
be excessively loaded. On chip positive feedback around the
comparator provides a nominal hysteresis level of 20mV.
–
+
PEAK
–
+
INTERNAL CIRCUIT
PEAK LEVEL OUTPUT
Fig. 5 Peak detector output
Sensitivity
In digital radio systems, sensitivity is often defined as the
lowest signal level at the receiver input that will achieve a
specified Bit Error Ratio (BER) at the output. The sensitivity of
the KESRX01 receiver, when used in the 434MHz application
shown in Fig. 6, is typically –103dBm average power (ASK
modulated with 2kHz, 50% duty cycle square wave) to achieve
a 0.01 BER. The input was matched for a 50Ω signal source.
At 315MHz, –105dBm average power is typically achievable.
Consult the Applications Notes refered to at the end of this
Datasheet for detailed PCB design issues to secure
perfomance.
The local oscillator frequency is set at 433.65MHz with a
required accuracy of at least
±
100kHz (see section below) i.e
433.55MHz to 433.75MHz.
This guarantees that the IF (70KHz to 470KHz) falls within
the acceptance bandwidth of the IF filter.
The frequency of operation for such products in Europe is
433.05MHz to 434.79MHz. The choice of such a low IF
frequency ensures that any image falls within the regulatory
band. This in turn ensures that the receiver cannot be blocked
by the image response of an unwanted signal outside of this
band.
Choice of IF frequency and IF bandwidth
The IF frequency is selected to be nominally 270KHz with
the low frequency cut-off at 25KHz and the high frequency
cut-off at 550KHz (nominal). For worst case tolerances the
transmitter frequency may be 433.92MHz
±
100KHz. i.e from
433.82MHz to 434.02MHz (see transmitter design
specification application notes)
Frequency Accuracy
The stability of the local oscillator is equal to that of the
crystal reference oscillator. Therefore to obtain a final output
accuracy of
±
100KHz at 433MHz would require a crystal with
a tolerance specification of
±
230ppm. This tolerance should
encompass all causes e.g. initial accuracy, temperature
stability and ageing. Choose a tighter tolerance crystal for
increased frequency accuracy.
5
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
