Rainbow Electronics ATAR862-8 - Manual (Page 6)

6
ATAR862-8
4589B–4BMCU–02/03
General Description
The fully-integrated PLL transmitter that allows particularly simple, low-cost RF minia-
ture transmitters to be assembled. The VCO is locked to 64
´ f
X T A L
, thus, a
13.5672 MHz crystal is needed for a 868.3 MHz transmitter nad a 14.2969 MHz crystal
for a 915 MHz transmitter. All other PLL and VCO peripheral elements are integrated.
The XTO is a series resonance oscillator so that only one capacitor together with a
crystal connected in series to GND are needed as external elements.
The crystal oscillator together with the PLL needs maximum < 1 ms until the PLL is
locked and the CLK output is stable. A wait time of
³
4 ms until the CLK is used for the
microcontroller and the PA is switched on.
The power amplifier is an open-collector output delivering a current pulse which is nearly
independent from the load impedance. The delivered output power is controlled via the
connected load impedance.
This output configuration enables a simple matching to any kind of antenna or to 50 W . A
high power efficiency of
h
= P
out
/(I
S,PA
´
V
S
) of 24% for the power amplifier at 868.3 MHz
results when an optimized load impedance of Z
Load
= (166 + j226)
W
is used at 3 V sup-
ply voltage.
Functional
Description
If ENABLE = L and PA_ENABLE = L, the circuit is in standby mode consuming only a
very small amount of current, so that a lithium cell used as power supply can work for
several years.
With ENABLE = H, the XTO, PLL and the CLK driver are switched on. If PA_ENABLE
remains L, only the PLL and the XTO are running and the CLK signal is delivered to the
microcontroller. The VCO locks to 64 times the XTO frequency.
With ENABLE = H and PA_ENABLE = H, the PLL, XTO, CLK driver and the power
amplifier are on. With PA_ENABLE, the power amplifier can be switched on and off,
which is used to perform the ASK modulation.
ASK Transmission The PLL transmitter block is activated by ENABLE = H. PA_ENABLE must remain L for
t
³
4 ms, then the CLK signal can be taken to clock the microcontroller and the output
power can be modulated by means of pin PA_ENABLE. After transmission,
PA_ENABLE is switched to L and the microcontroller switches back to internal clocking.
The PLL transmitter block is switched back to standby mode with ENABLE = L.
FSK Transmission The PLL transmitter block is activated by ENABLE = H. PA_ENABLE must remain L for
t
³
4 ms, then the CLK signal can be taken to clock the microcontroller and the power
amplifier is switched on with PA_ENABLE = H. The chip is then ready for FSK modula-
tion. The microcontroller starts to switch on and off the capacitor between the XTAL load
capacitor and GND with an open-drain output port, thus changing the reference fre-
quency of the PLL. If the switch is closed, the output frequency is lower than if the switch
is open. After transmission PA_ENABLE is switched to L and the microcontroller
switches back to internal clocking. The PLL transmitter block is switched back to
standby mode with ENABLE = L.
The accuracy of the frequency deviation with XTAL pulling method is about ±25% when
the following tolerances are considered.