micrf506 Micrel Semiconductor, micrf506 Datasheet - Page 26

• Manufacturer Part Number: micrf506
• Description: 410mhz And 450mhz Ism Band Transceiver
• Manufacturer: Micrel Semiconductor

The modulator filter will not influence on the
frequency deviation as long as the programmed cut-
off frequency is above the actual bit rate.
The frequency deviation must be programmed so
that the modulation index (2 x single sided frequency
deviation/Baudrate [bps]) always is greater than or
equal to 2 including the total frequency offset
between the receiver and the transmitter:
The calculated f
needed receiver bandwidth, see chapter Switched
capacitor filter.
Using the XCO-tune Bits
The RF chip has a built-in mechanism for tuning the
frequency of the crystal oscillator and is often used
in combination with the Frequency Error Estimator
(FEE). The XCO tuning is designed to eliminate or
reduce initial frequency tolerance of the crystal
and/or the frequency stability over temperature.
Pin 24 XT ALOUT
If the value in XCO_tune is increased (adding
capacitance), the frequency will decrease.
The XCO uses two external capacitors. The value of
these will strongly affect the tuning range. With a
16.0 MHz crystal (TN4-26011 from Toyocom), and
external capacitor values of 1.5 pF, the tuning range
will be (almost) equally divided between Òincrease
frequencyÓ and Òdecrease frequencyÓ. That is,
XCO_tune values greater than approx 16 will
decrease frequency, and XCO_tune values less
than approx 16 will increase frequency.
Micrel
September 2004
Figure 24. Crystal oscillator’s external components
f
DEV
DEV
C10
1.5pF
= Baudrate + f
should be used to calculate the
Y1
TSX -10A
OFFSET
C11
1.5pF
Pin 23 XT ALIN
26
A procedure for using the XCOtuning feature in
combination with the FEE is given below. The
MICRF506 measures the frequency offset between
the demodulated signal and the Lo and tune the
XCO so the Lo frequency is equal to received carrier
frequency.
A procedure like this can be called during production
(storing the calibrated XCO_tune value), at regular
intervals or implemented in the communication
protocol when the frequency has changed.
The FEE can count “UP”-pulses and/or “DOWN”-
pulses (pulses out of the demodulator when a logic
“1” or logic “0”, resp.., is received). The FEE can
count pulses for n bits, where n = 8, 16, 32 or 64.
Example: In FEE, count up+down pulses, counting 8
bits:
A perfect case ==> FEE = 0
If FEE > 0: LO is too low, increase LO by decreasing
XCO_tune value
v.v. for FEE < 0
FEE field holds a number in the range -128, … ,
127. However, it keeps counting above/below the
range, which is:
If FEE = -128 and still counting dwn-pulses:
100,0
1) =>-129 = +127
2) 126
3) 125
…
-20,0
-40,0
-60,0
80,0
60,0
40,0
20,0
0,0
0
8
Figure 25. XCO Tuning
XCO bitvalue
16
24
[phone]
M9999-092904
32
MICRF506
2x1.5pF
2x0pF