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308MHz/315MHz/418MHz/433.92MHz
Low-Power, FSK Superheterodyne Receiver
Crystal Oscillator
The XTAL oscillator in the MAX7042 is used to generate
the LO for mixing with the received signal. The XTAL oscil-
lator frequency sets the received signal frequency as:
f RECEIVE = (f XTAL x 32) + 10.7MHz
The received image frequency at:
f IMAGE = (f XTAL x 32) - 10.7MHz
is suppressed by the integrated quadrature image-
rejection circuitry.
The XTAL oscillator in the MAX7042 is designed to pre-
sent a capacitance of approximately 3pF between
XTAL1 and XTAL2. In most cases, this corresponds to a
4.5pF load capacitance applied to the external crystal
when typical PCB parasitics are added. It is very impor-
tant to use a crystal with a load capacitance that is equal
to the capacitance of the MAX7042 crystal oscillator plus
PCB parasitics. If a crystal designed to oscillate with a
different load capacitance is used, the crystal is pulled
away from its intended operating frequency, introducing
an error in the reference frequency. Crystals designed to
operate with higher differential load capacitance always
pull the reference frequency higher.
In reality, the oscillator pulls every crystal. A crystal’s nat-
ural frequency is really below its specified frequency, but
when loaded with the specified load capacitance, the
crystal is pulled and oscillates at its specified frequency.
This pulling is accounted for in the specification of the
load capacitance.
Additional pulling can be calculated if the electrical
parameters of the crystal are known. The frequency
pulling is given by:
Frequency Tolerance
The frequency tolerance of the crystal, the frequency
and bandwidth tolerance of the IF filter, and the desired
modulation bandwidth of the signal are all interrelated.
The combination of these characteristics should be such
to ensure that the modulated signal bandwidth stays
within the passband of the IF filter after downconversion.
As is shown below, a 50ppm tolerance crystal in combi-
nation with a 280kHz bandwidth IF filter is sufficient for
most FSK-modulated signals.
Smaller IF filter bandwidths can be used if high-tolerance
crystals are used for generating both transmitter and
MAX7042 receiver PLL references. The modulated spec-
trum of the transmitted signal must be downconverted by
the MAX7042 to fall within the passband of the IF filter.
The crystal tolerances must take into account the initial
+25°C tolerance, aging, load capacitance tolerances,
and temperature drift for both the transmitter and
MAX7042 receiver. To achieve acceptable signal recep-
tion, the following equation must hold:
2 x ( ? F TX + ? F RX + ? F IF + F DEV + 5 x F MOD ) < IFBW min
where:
? F TX = (transmitter crystal tolerance in ppm) x (carrier
frequency in MHz). This includes aging, load capaci-
tance, and temperature effects for the crystal tolerance.
? F RX = (MAX7042 crystal tolerance in ppm) x (carrier
frequency in MHz). This includes aging, load capaci-
tance, and temperature effects for the crystal tolerance.
? F IF = The center frequency tolerance of the selected
IF filter. This includes temperature drift of the IF filter
center frequency.
C m ? 1 1 ?
?
? x 10
? C
f p =
2 ? case + C load C case + C spec ?
6
F DEV = ±FSK frequency deviation from carrier frequency.
F MOD = One half of NRZ data rate, or the data rate if
Manchester coding is used.
where:
f p is the amount the crystal frequency is pulled in ppm.
C m is the motional capacitance of the crystal.
C case is the case capacitance.
C spec is the specified load capacitance.
C load is the actual load capacitance.
When the crystal is loaded as specified, i.e., C load =
C spec , the frequency pulling equals zero.
IFBW min = The minimum bandwidth of the selected IF
filter.
As an example, assume 315MHz carrier frequency,
±50ppm crystal tolerances for both transmitter and
MAX7042, ±30kHz IF filter center frequency tolerance,
±50kHz frequency deviation, and 4.8kHz Manchester
data rate:
2 x [(315 x 50) + (315 x 50) + 30000 +50000 + 5 x
4800] = 271kHz < IFBW min
This operating condition necessitates a 280kHz IF filter.
12
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