Micrel MICRF505BML/YML
August 2006 16
M9999-092904
+1 408-944-0800
The lengths of the N, M, and A registers are 12, 12
and 6 respectively The values can be calculated
from the following formula:
()()
A N 16
f
A N 16
2
M
f
f
RF
XCO
PhD
+×
=
+×
==
VCO
f
M
≠0
1 ≤ A < N
where
f
PhD
: Phase detector comparison frequency
f
XCO
: Crystal oscillator frequency
f
VCO
: Voltage controlled oscillator frequency
f
RF
: RF carrier frequency
There are two sets of each of the divide factors (i.e.
A0 and A1). If modulation by using the dividers is
selected (that is Modulation1=1, Modulation0=0), the
two sets should be programmed to give two RF
frequencies, separated by two times the specified
frequency deviation. For all other modulation
methods, and also in receive mode, the 0-set will be
used.
Crystal Oscillator (XCO)
Adr D7 D6 D5 D4 D3 D2 D1 D0
0001001 ‘0’ ‘0’ ‘1’ XCOtune4 XCOtune3 XCOtune2 XCOtune1 XCOtune0
The crystal oscillator is a very critical block. As the
crystal oscillator is a reference for the RF output
frequency and also for the LO frequency in the
receiver, very good phase and frequency stability is
required. The schematic of the crystal oscillator’s
external components for 16MHz are shown in Figure
5.
C11
5.6pF
C10
5.6pF
Y1
TSX-10A
Pin 24
TALOUT
Pin 2
XTALI
Figure 5. Crystal Oscillator Circuit
The crystal should be connected between pins
XTALIN and XTALOUT (pin 23 and 24). In addition,
loading capacitors for the crystal are required. The
loading capacitor values depend on the total load
capacitance, C
L
, specified for the crystal. The load
capacitance seen between the crystal terminals
should be equal to C
L
for the crystal to oscillate at
the specified frequency.
C
L
=
1
1
C
10
+
1
C
11
+ C
parasitic
The parasitic capacitance is the pin input
capacitance and PCB stray capacitance. Typically,
the total parasitic capacitance is around 6pF. For
instance, for a 9pF load crystal the recommended
values of the external load capacitors are 5.6pF.
It is also possible to tune the crystal oscillator
internally by switching in internal capacitance using
5 tune bits XCOtune4 – XCOtun0. When XCOtune4
– XCOtune0 = 0 no internal capacitors are
connected to the crystal pins. When XCOtune4 –
XCOtune0 = 1 all of the internal capacitors are
connected to the crystal pins. Figure 6 shows the
tuning range for two different capacitor values, 1.5pF
and no capacitors.
The crystal used is a TN4-26011 from Toyocom.
Specification: Package TSX-10A, Nominal frequency
16.000000 MHz, frequency tolerance ±10ppm,
frequency stability ±9ppm, load capacitance 9pF,
pulling sensitivity 15ppm/pF. When the external
capacitors are set to 1.5pF and the XCOtune=16,
the total capacitance will normally be ~9pF.
-60,0
-40,0
-20,0
0,0
20,0
40,0
60,0
80,0
100,0
0 8 16 24 32
XCO bitvalue
[ppm]
2x1.5pF
2x0pF
Figure 6. XCO Tuning
The start up time is given in Table 7. As can be
seen, more capacitance will slow down the start up
time.
The start-up time of a crystal oscillator is typically
around a millisecond. Therefore, to save current
consumption, the XCO is turned on before any other
circuit block. During start-up the XCO amplitude will
eventually reach a sufficient level to trigger the M-
counter. After counting 2 M-counter output pulses
the rest of the circuit will be turned on. The current
consumption during the prestart period is
approximately 280µA.
