Rainbow Electronics MAX7057 - Manual (Page 9)

nates the problems associated with crystal-pulling FSK
signal generation. The multiplying factor of the PLL is
set by a 16-bit number, with 4 bits for integer and 12
bits for fraction. The 12-bit fraction in the synthesizer
results in a tuning resolution that is equal to the refer-
ence frequency divided by 4096.
The MAX7057 has an internal variable shunt capacitor
connected at the PA output. This capacitor is controlled
using the SPI to maintain highly efficient transmission at
any frequency within a 1.47 to 1 (28/19) tuning range.
This means that it is possible to change the frequency
and retune the antenna to the new frequency in a very
short time. The combination of rapid-antenna tuning
ability with rapid-synthesizer tuning makes the
MAX7057 a true frequency-agile transmitter. The tuning
capacitor has a resolution of 0.25pF. The MAX7057
also features adjustable output power through an exter-
nal resistor to nearly +10dBm into a 50
Ω load at +2.7V.
The MAX7057 supports data rates up to 100kbps NRZ
in both ASK and FSK modes. In FSK mode, the fre-
quency deviation can be programmed as low as
±2kHz
and as high as ±100kHz.
Power Amplifier (PA)
The PA of the MAX7057 is a high-efficiency, open-drain
switching-mode amplifier. In a switching-mode amplifi-
er, the gate of the final-stage FET is driven with a very
sharp 25% duty-cycle square wave at the transmit fre-
quency. This square wave is derived from the synthe-
sizer circuit. When the matching network is tuned
correctly, the output FET resonates the attached match-
ing circuit with a minimum amount of power dissipated
in the FET. With a proper output-matching network, the
PA can drive a wide range of antenna impedances,
which include a small-loop PCB trace and a 50
Ω anten-
na. The output-matching network suppresses the carri-
er harmonics and transforms the antenna impedance to
an optimal impedance at PAOUT, which is from 125
Ω
to 250 Ω.
When the output-matching network is properly tuned,
the PA transmits power with a high overall efficiency of
up to 25%. The efficiency of the PA itself is more than
39%. The output power can be adjusted by changing
the impedance seen by the PA or by adjusting the
value of an external resistor at PAOUT.
Envelope Shaping
The MAX7057 features an internal envelope-shaping
resistor for ASK modulation, which connects between
PAVDD and ROUT. When connected to the PA pullup
inductor, the envelope-shaping resistor slows the turn-
on/-off time of the PA and results in a smaller spectral
width of the modulated PA output signal.
Variable Capacitor
The MAX7057 has a set of internal variable shunt
capacitors that can be switched in and out to present
different capacitor values at the PA output. The capaci-
tors are connected from the PA output to ground. This
allows changing the tuning network along with the syn-
thesizer divide ratio each time the transmitted frequency
changes, making it possible to maintain maximum trans-
mitter power while moving rapidly from one frequency to
another.
When the particular capacitance control bit is high, the
corresponding amount of shunt capacitance is added at
PAOUT. The 32 capacitor values are selected using the
SPI; the capacitance resolution is 0.25pF. The total
capacitance can vary from 0 to 7.75pF. For example, if
cap[1] and cap[3] are high, and cap[4], cap[2], and
cap[0] are low, this circuit will add 2.5pF at PAOUT. See
Table 1 for variable capacitor values and control bits.
Fractional-N Phase-Locked Loop (PLL)
The MAX7057 utilizes a fully integrated fractional-N PLL
for its transmit frequency synthesizer. All PLL compo-
nents, including the loop filter, are included on-chip.
The loop bandwidth is programmable to either 300kHz
or 600kHz. See Tables 2, 3, and 4 for “pllbw” bit
description. The 16-bit fractional-N topology allows the
transmit frequency to be adjusted in increments of
f
XTAL /4096. The allowable range of the f RF /f XTAL ratio is
approximately 19 to 28.
The fractional-N topology also allows exact FSK fre-
quency deviations to be programmed, completely elimi-
nating the problems associated with generating
frequency deviations by crystal oscillator pulling.
The integer and fractional portions of the PLL divider
ratio set the transmit frequency. The following example
shows how to determine the correct values to be
loaded to registers HIFREQ1, HIFREQ0, LOFREQ1, and
LOFREQ0. See Tables 2, 3, and 7–10 for a detailed
description of these registers.
MAX7057
300MHz to 450MHz Frequency-Programmable
ASK/FSK Transmitter
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Table 1. Variable Capacitor Values and
Control Bits
SPI REGISTER BITS
INCREMENTAL SHUNT
CAPACITANCE (pF)
cap[0] 0.25
cap[1] 0.5
cap[2] 1.0
cap[3] 2.0
cap[4] 4.0