MAX2120
Complete, Direct-Conversion Tuner for DVB-S
and Free-to-Air Applications
______________________________________________________________________________________ 17
Applications Information
The MAX2120 downconverts RF signals in the 925MHz to
2175MHz range directly to the baseband I/Q signals. The
devices are targeted for digital DBS tuner applications.
RF Input
The RF input of the MAX2120 is internally matched to
75Ω. Only a DC-blocking capacitor is needed. See the
Typical Operating Circuit
.
RF Gain Control
The MAX2120 features a variable-gain low-noise amplifi-
er providing 73dB of RF gain range. The voltage-control
(VGC) range is 0.5V (minimum attenuation) to 2.7V
(maximum attenuation).
Baseband Variable-Gain Amplifier
The receiver baseband variable-gain amplifiers provide
15dB of gain-control range programmable in 1dB
steps. The VGA gain can be serially programmed
through the SPI™ interface by setting bits BBG[3:0] in
the Control register.
Baseband Lowpass Filter
The MAX2120 includes a programmable on-chip 7th-
order Butterworth filter. The -3dB corner frequency of
the baseband filter is programmable by setting the bits
LPF[7:0] in the Lowpass register. The value of the
LPF[7:0] is determined by the following equation:
where f
-3dB
is in units of MHz.
The filter can be adjusted from approximately 4MHz to
40MHz. Total device supply current depends on the fil-
ter BW setting, with increasing current commensurate
with increasing -3dB BW.
DC Offset Cancellation
The DC offset cancellation is required to maintain the I/Q
output dynamic range. Connecting an external capacitor
between IDC+ and IDC- forms a highpass filter for the I
channel, and an external capacitor between QDC+ and
QDC- forms a highpass filter for the Q channel. Keep the
value of the external capacitor less than 47nF to form a
typical highpass corner of 400Hz.
XTAL Oscillator
The MAX2120 contains an internal reference oscillator,
reference output divider, and output buffer. All that is
required is to connect a crystal through a series, 1nF
capacitor. To minimize parasitics, place the crystal and
series capacitor as close as possible to pin 14 (XTAL
pin). See Table 14 for crystal (XTAL) ESR (equivalent
series resistance) requirements. The typical input
capacitance is 40pF.
VCO Autoselect (VAS)
The MAX2120 includes 24 VCOs. The local oscillator fre-
quency can be manually selected by programming the
VCO[4:0] bits in the VCO register. The selected VCO is
reported in the Status Byte-2 register (see Table 13).
Alternatively, the MAX2120 can be set to autonomously
choose a VCO by setting the VAS bit in the VCO regis-
ter to logic-high. The VAS routine is initiated once the
N-divider LSB register word (REG 2) is loaded.
In the event that only the R-divider register or N-
divider MSB register word is changed, the N-divider
LSB word must also be loaded last to initiate the
VCO autoselect function. The VCO value pro-
grammed in the VCO[4:0] register serves as the start-
ing point for the automatic VCO selection process.
During the selection process, the VASE bit in the Status
Byte-1 register is cleared to indicate the autoselection
function is active. Upon successful completion, bits
VASE and VASA are set and the VCO selected is
reported in the Status Byte-2 register (see Table 13). If
the search is unsuccessful, VASA is cleared and VASE
is set. This indicates that searching has ended but no
good VCO has been found, and occurs when trying to
tune to a frequency outside the VCO’s specified
frequency range.
Refer to the MAX2112/MAX2120 VAS application note
for more information.
LPF[7:0]dec =
()
.
,
fMHz
MHz
dB−
−
+
3
4
029
12
SPI is a trademark of Motorola, Inc.
