Rainbow Electronics MAX7030 - Manual (Page 12)

MAX7030
Low-Cost, 315MHz, 345MHz, and 433.92MHz
ASK Transceiver with Fractional-N PLL
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Detailed Description
The MAX7030 315MHz, 345MHz, and 433.92MHz
CMOS transceiver and a few external components pro-
vide a complete transmit and receive chain from the
antenna to the digital data interface. This device is
designed for transmitting and receiving ASK data. All
transmit frequencies are generated by a fractional-N-
based synthesizer, allowing for very fine frequency
steps in increments of f
XTAL / 4096. The receive LO is
generated by a traditional integer-N-based synthesizer.
Depending on component selection, data rates as high
as 33kbps (Manchester encoded) or 66kbps (NRZ
encoded) can be achieved.
Receiver
Low-Noise Amplifier (LNA)
The LNA is a cascode amplifier with off-chip inductive
degeneration that achieves approximately 30dB of volt-
age gain that is dependent on both the antenna-match-
ing network at the LNA input and the LC tank network
between the LNA output and the mixer inputs.
The off-chip inductive degeneration is achieved by
connecting an inductor from LNASRC to AGND. This
inductor sets the real part of the input impedance at
LNAIN, allowing for a more flexible match for low-input
impedances such as a PC board trace antenna. A
nominal value for this inductor with a 50
Ω input imped-
ance is 12nH at 315MHz and 10nH at 434MHz, but the
inductance is affected by PC board trace length.
LNASRC can be shorted to ground to increase sensitiv-
ity by approximately 1dB, but the input match must
then be reoptimized.
The LC tank filter connected to LNAOUT consists of L5
and C9 (see the Typical Application Circuit). Select L5
and C9 to resonate at the desired RF input frequency.
The resonant frequency is given by:
where L TOTAL = L5 + L PARASITICS and C TOTAL = C9 +
C PARASITICS .
L PARASITICS and C PARASITICS include inductance and
capacitance of the PC board traces, package pins,
mixer-input impedance, LNA-output impedance, etc.
These parasitics at high frequencies cannot be
ignored, and can have a dramatic effect on the tank fil-
ter center frequency. Lab experimentation should be
done to optimize the center frequency of the tank. The
total parasitic capacitance is generally between 5pF
and 7pF.
Automatic Gain Control (AGC)
When the AGC is enabled, it monitors the RSSI output.
When the RSSI output reaches 1.28V, which corre-
sponds to an RF input level of approximately -55dBm,
the AGC switches on the LNA gain-reduction attenua-
tor. The attenuator reduces the LNA gain by 36dB,
thereby reducing the RSSI output by about 540mV to
740mV. The LNA resumes high-gain mode when the
RSSI output level drops back below 680mV (approxi-
mately -59dBm at the RF input) for a programmable
interval called the AGC dwell time (see Table 1). The
AGC has a hysteresis of approximately 4dB. With the
AGC function, the RSSI dynamic range is increased,
allowing the MAX7030 to reliably produce an ASK out-
put for RF input levels up to 0dBm with a modulation
depth of 18dB. AGC is not required and can be dis-
abled (see Table 1).
f
L C
TOTAL TOTAL
=
×
1
2 π
Pin Description (continued)
PIN NAME FUNCTION
28 AGC2 AGC Enable/Dwell Time Control 2 (MSB). See Table 1. Bypass to GND with a 10pF capacitor.
29 AGC1 AGC Enable/Dwell Time Control 1. See Table 1. Bypass to GND with a 10pF capacitor.
30 AGC0 AGC Enable/Dwell Time Control 0 (LSB). See Table 1. Bypass to GND with a 10pF capacitor.
31 XTAL1 Crystal Input 1. Bypass to GND if XTAL2 is driven by an AC-coupled external reference.
32 XTAL2 Crystal Input 2. XTAL2 can be driven from an external AC-coupled reference.
EP GND Exposed Paddle. Solder evenly to the board’s ground plane for proper operation.