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MAX7034 View Datasheet(PDF) - Maxim Integrated

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MAX7034 Datasheet PDF : 14 Pages
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MAX7034
315MHz/434MHz ASK Superheterodyne
Receiver
LPARASITICS and CPARASITICS include inductance and
capacitance of the PCB traces, package pins, mixer input
impedance, etc. These parasitics at high frequencies can-
not be ignored, and can have a dramatic effect on the tank
filter center frequency. The total parasitic capacitance is
generally between 4pF and 6pF.
Intermediate Frequency and RSSI
The IF section presents a differential 330Ω load to provide
matching for the off-chip ceramic filter. The six internal
AC-coupled limiting amplifiers produce an overall gain of
approximately 65dB, with a bandpass-filter-type response
centered near the 10.7MHz IF frequency with a 3dB band-
Mixer
A unique feature of the MAX7034 is the integrated image
rejection of the mixer. This device eliminates the need
for a costly front-end SAW filter for most applications.
width of approximately 10MHz. The RSSI circuit demodu-
lates the IF by producing a DC output proportional to the
log of the IF signal level, with a slope of approximately
14.2mV/dB.
Advantages of not using a SAW filter are increased sen-
sitivity, simplified antenna matching, less board space,
and lower cost.
Applications Information
Crystal Oscillator
The mixer cell is a pair of double balanced mixers that
perform an IQ downconversion of the RF input to the
10.7MHz IF from a low-side injected LO (i.e., fLO = fRF
- fIF). The image-rejection circuit then combines these
signals to achieve 44dB of image rejection. Low-side
injection is required due to the on-chip image-rejection
architecture. The IF output is driven by a source follower
biased to create a driving-point impedance of 330Ω; this
provides a good match to the off-chip 330Ω ceramic
IF filter.
The IRSEL pin is a logic input that selects one of the
three possible image-rejection frequencies. When VIRSEL
= 0V, the image rejection is tuned to 315MHz. VIRSEL
= VDVDD/2 tunes the image rejection to 375MHz, and
VIRSEL = VDVDD tunes the image rejection to 434MHz.
The IRSEL pin is internally set to VDVDD/2 (image rejec-
tion at 375MHz) when it is left unconnected, thereby elimi-
nating the need for an external VDVDD/2 voltage.
Phase-Locked Loop
The crystal oscillator in the MAX7034 is designed to
present a capacitance of approximately 3pF between the
XTAL1 and XTAL2. 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. For example, a
4.7547MHz crystal designed to operate with a 10pF load
capacitance oscillates at 4.7563MHz with the MAX7034,
causing the receiver to be tuned to 315.1MHz rather than
315.0MHz, an error of about 100kHz, or 320ppm. It is
very important to use a crystal with a load capaci-
tance that is equal to the capacitance of the MAX7034
crystal oscillator plus PCB parasitics.
In actuality, the oscillator pulls every crystal. The crystal’s
natural 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 already accounted for in the specification of
The PLL block contains a phase detector, charge pump, the load capacitance. Additional pulling can be calculated
integrated loop filter, VCO, asynchronous 64x clock if the electrical parameters of the crystal are known. The
divider, and crystal oscillator driver. Besides the crystal, frequency pulling is given by:
this PLL does not require any external components. The
VthCeORFg,eInFe, raantedscaryslotawl-sfriedqeu= LeOnc. iTesheisregliavetionnbsyh:ip between f P
CM
2
C CASE
1
+
CLOAD
-
C CASE
1
+
C SPEC
×
10 6
f
XTAL
=
f
RF- f IF
32 × M
where:
M = 1 (VXTALSEL = VDVDD) or 2 (VXTALSEL = 0V)
To allow the smallest possible IF bandwidth (for best sen-
sitivity), minimize the tolerance of the reference crystal.
where:
fP is the amount the crystal frequency pulled in ppm.
CM is the motional capacitance of the crystal.
CCASE is the case capacitance.
CSPEC is the specified load capacitance.
CLOAD is the actual load capacitance.
When the crystal is loaded as specified (i.e., CLOAD =
CSPEC), the frequency pulling equals zero.
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