micrf501 Micrel Semiconductor, micrf501 Datasheet - Page 9

• Manufacturer Part Number: micrf501
• Description: 300mhz To 600mhz Radiowire? Rf Transceiver
• Manufacturer: Micrel Semiconductor

Applications Information
VCO and PLL Section
The frequency synthesizer consists of a VCO, crystal oscilla-
tor, dual-modulus prescaler, programmable frequency divid-
ers, phase-detector, charge pump, lock detector and an
external loop filter. The dual-modulus prescaler divides the
VCO-frequency by 32/33. This mode is controlled by the A-
divider. There are two sets of M, N and A-frequency dividers.
Using both sets in transmit mode, FSK can be implemented
by switching between those two sets. The phase-detector is
a frequency/phase detector with back slash pulses to mini-
mize phase noise. The VCO, crystal oscillator, charge pump,
lock detector and the loop filter will be described in detail
below.
Voltage Controlled Oscillator (VCO)
The circuit schematic of the VCO with external components
is shown in Figure 3. The VCO is basically a Colpitts oscilla-
tor. The oscillator has an external resonator and varactor.
The resonator consists of inductor L1 and the series connec-
tion of capacitor C13, the internal capacitance, the capaci-
tance of the varactor and C35 in parallel with D1. The
capacitance of the varactor (D1) decreases as the input
voltage increases. The VCO frequency will therefore in-
crease as the input voltage increases. The VCO has a
positive gain (MHz/Volt). C35 is added, if necessary, to bring
VCO tuning voltage to its middle range or VCC/2, which is
measured at Pin 9 - CMPOUT.
If the value of capacitor C13 and C14 become too small the
amplitude of the VCO signal decreases, which leads to lower
output power.
The layout of the VCO is very critical. The external compo-
nents should be placed as close to the input pin (Pin 6) as
possible. The anode of the varactor D1 must be placed next
to pins 7 and 8. Ground vias should be next to component
pads.
Crystal Oscillator
The crystal oscillator is the reference for the RF output
frequency as well as for the LO frequency in the receiver. The
crystal oscillator is a very critical block since very good phase
and frequency stability is required. The schematic of the
crystal oscillator with external components for 10MHz is
shown in Figure 4. These components are optimized for a
crystal with 15pF load capacitance.
March 2003
MICRF501
loopfilter_output
MA4ST350
47k
R8
D1
47nH
L1
C35
2.2p
Figure 3. VCO
C13
15p
VDD
3.6k
R7
Pin 5
Pin 6
Pin 7
OSCOUT
9
The crystal oscillator is tuned by varying the trimming capaci-
tor C20. The drift of the RF frequency is the same as the drift
of crystal frequency when measured in ppm. The total differ-
ence in ppm, ∆f(ppm), between the tuned RF frequency and
the drifted frequency is given by:
where:
The demodulator will not be able to decode data when
∆f(Hz) = ∆f(ppm) × f
deviation. For small frequency deviations, the crystal should
be pre-aged, and should have a small temperature coeffi-
cient. The circuit has been tested with a 10MHz crystal, but
other crystal frequencies can be used as well.
The circuit has been tested with a 10MHz crystal, but other
crystal frequencies can be used as well.
Prestart of XCO
The start-up time of a crystal oscillator is typically some
milliseconds. Therefore, to save current consumption, the
MICRF501 circuit has been designed so that 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 two M-counter output
pulses the rest of the circuit will be turned on. The current
consumption during the prestart period is approximately
300µA.
Lock Detector
The MICRF501 circuit has a lock detector feature that indi-
cates whether the PLL is in lock or not. A logic high on Pin 15
(LOCKDET) means that the PLL is in lock.
The phase detector output is converted into a voltage that is
filtered by the external capacitor C23, connected to Pin 14,
LDC. The resulting DC voltage is compared to a reference
window set by bits Ref0 – Ref5. The reference window can be
stepped up/down linearly between 0V, Ref0 – Ref5 =1, and
Ref0 – Ref5 = 0, which gives the highest value (DC voltage)
of the reference window. The size of the window can either be
•
•
•
•
10MHz
∆f(ppm) = S
S
frequency (due to crystal and components) in ppm°C.
∆T is the change in temperature from room
temperature, at which the crystal was tuned.
n is the ageing in ppm/year.
∆t is the time (in years) elapsed since the transceiver
was last tuned.
T
is the total temperature coefficient of the oscillator
2-6p
C20
C36
1n
Figure 4. Crystal Oscillator
T
× ∆T + n × ∆t
5.6p
C22
RF
is larger than the FSK frequency
Pin 12
Pin 13
C21
47p
DIGGND
DIFV
DD
XOSCOUT
MICRF501
Micrel