VCO control input
Output power control
Power amplifier positive supply
Power amplifier ground
Transmit enable/stand by
Power amplifier output/antenna interface
Power amplifier output/antenna interface
VCO test control input
Phase detector output
Output stage (PA)
The input signal at pin DATA produces amplitude shift key
When the IC is enabled (TXEN high) a phase locked loop
locks the output of the VCO to a multiple of a crystal defined
reference input. The output of the VCO operates at the final
output frequency and is the input to a power amplifier stage.
The power amplifier directly drives the antenna.
(ASK) modulation of the VCO output. This is achieved by
on–off keying of the bias current in the output power amplifier
stage. The output of the PA is a balanced output (pin OUT and
OUTB) and is current source driven (open collector outputs).
The outputs of which should be D.C. referenced to a positive
supply voltage (anticipated to be VCC in most applications).
The current source outputs can drive a PCB antenna directly
(Figure 6) or if a higher output power is required on limited
supply headroom via a simple impedance transforming net-
work. A balanced output stage is used as it automatically
suppresses the even order harmonics of the fundamental. In
order to obtain the benefits of this output stage it is essential
to use a balanced antenna.
Phase locked loop
A divide by 64 prescaler is present in the PLL feedback
loop. The final output frequency is then Fo = 64xFref.
The phase detector used is a phase frequency detector
(PFD) with a current (charge pump) output. This phase
in the range –2π <θe < 2π. The charge pump provides an
output current in the range ± 50µA and hence gives a phase
detector gain of (50/2π ) µA/rad (≈8µA/rad).
The advantage of the PFD over a pure phase detector is
that it is also a frequency discriminator and will always lock the
loop irrespective of the initial frequency offset. The PLL loop
characteristics such as lock–up time, capture range, loop
bandwidth and VCO reference sideband suppression are
controlled by the external loop filter.
In the intended application, it is expected that the
transmitter will spend a large proportion of time in ‘‘stand by”
not transmitting data. To maximise battery life it is important
that very little quiescent current is taken in this mode.
The ‘‘stand by mode” is selected by setting pin TXEN low
and similarly the transmitter is enabled by setting TXEN high.
To minimize stand–by current TXEN is used to bias an on–
chip npn transistor connected in a common collector
configuration (Figure 3 below). This transistor is used to
provide the supply to large portions of the IC. Collapsing the
supply when TXEN is set low results in a very low stand by
current. The voltage on TXEN should not exceed VCC by more
For certain applications spurious sidebands at the
reference frequency must be adequately suppressed and a
3rd order loop is recommended.
From an application standpoint the TXEN pin must be able
to source the bias current for the input transistor and should
also be decoupled if possible to prevent high frequency noise
directly coupling into the IC power supply. The value of the
decoupling capacitors and the drive capability of the TXEN
source will affect power up delay. Since TXEN enables the
PLL it is therefore essential that it is set high prior to any data
transmission and that it remains high during the
To minimize external component cost,s the VCO is fully
integrated. The frequency of the VCO is controlled by the
voltage on pin LF.
Reference crystal oscillator
A single transistor Collpits crystal oscillator provides a
reference clock for the PLL. The oscillator is configured for
parallel resonant operation in the fundamental mode (typical
operating frequency of 3–7MHz). The crystal is connected
between pins XTAL2 and VEE1 with external components as
shown in Figure 6.
(i) Stand by (TXEN low, DATA low)
Alternatively, a reference clock can be provided by an
external source connected to pin XTAL2 Figure 7.
(ii) PLL Mode/Transmit SPACE (TXEN high, DATA low)
(iii) Transmit MARK (TXEN high, DATA high)