MAX7032
Low-Cost, Crystal-Based, Programmable,
ASK/FSK Transceiver with Fractional-N PLL
______________________________________________________________________________________ 27
On Timer (t
ON
)
The on timer, t
ON
(see Figure 10), is a 16-bit timer that
is configured through register 0x0B for the upper byte,
register 0x0C for the lower byte (Table 15). The infor-
mation stored in this timer provides an additional way to
control the duration of the on time of the receiver.
The CPU must begin driving DIO low any time during
t
LOW
= t
CPU
+ t
RF
+ t
ON
. If the CPU fails to drive DIO
low at the end of t
ON
, DIO is pulled high through the
internal pullup resistor and the time sequence is restart-
ed, leaving the MAX7032 powered down. Any time the
DIO line is driven high while the DRX = 1, the DRX
sequence is initiated, as defined in Figure 10. In the
event that the CPU is processing data, after t
ON
expires, the CPU should keep the MAX7032 awake by
holding the DIO line low.
The data written to the t
ON
register (register 0x0B and
register 0x0C) are multiplied by the t
ON
time base
(Table 15) to give the total t
ON
time. See the example in
the
Off Timer
(
t
OFF
)
section. On power-up, the on-timer
register is reset to zero and must be written before
using DRX mode.
Transmitter Low-Frequency Register (TxLOW)
The TxLOW register sets the divider information of the
fractional-N synthesizer for the lower transmit frequency
in FSK mode. See the example given in the
Fractional-N
PLL
section. In ASK mode, TxLOW determines the carri-
er frequency.
Transmitter High-Frequency Register (TxHIGH)
The TxHIGH register sets the divider information of the
fractional-N synthesizer for the upper transmit frequency
in the FSK mode. In ASK mode, the content of TxHIGH
is not used. The 16-bit register contains the binary rep-
resentation of the TX PLL divider ratio, which is shown in
the example in the
Fractional-N PLL
section.
Applications Information
Output Matching to 50
ΩΩ
When matched to a 50Ω system, the MAX7032’s PA is
capable of delivering +10dBm of output power at V
DD
= +2.7V. The output of the PA is an open-drain transis-
tor that requires external impedance matching and
pullup inductance for proper biasing. The pullup induc-
tance from the PA to PAV
DD
serves three main purpos-
es: it resonates the capacitive PA output, provides
biasing for the PA, and becomes a high-frequency
choke to prevent RF energy from coupling into V
DD
.
The network also forms a bandpass filter that provides
attention for the higher order harmonics.
Output Matching to PCB Loop Antenna
In most applications, the MAX7032 must be impedance
matched to a small-loop antenna. The antenna is usual-
ly fabricated out of a copper trace on a PCB in a rec-
tangular, circular, or square pattern. The antenna has
an impedance that consists of a lossy component and
a radiative component. To achieve high radiating effi-
ciency, the radiative component should be as high as
possible, while minimizing the lossy component. In
addition, the loop antenna has an inherent loop induc-
tance associated with it (assuming the antenna is termi-
nated to ground). For example, in a typical application,
the radiative impedance is less than 0.5Ω, the lossy
impedance is less than 0.7Ω, and the inductance is
approximately 50nH to 100nH.
Layout Considerations
A properly designed PCB is an essential part of any
RF/microwave circuit. On high-frequency inputs and
outputs, use controlled-impedance lines and keep
them as short as possible to minimize losses and radia-
tion. At high frequencies, trace lengths that are on the
order of λ/10 or longer act as antennas, where λ is the
wavelength.
