Rainbow Electronics MAX3580 - Manual (Page 19)

MAX3580
Direct-Conversion TV Tuner
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RF Tracking Filter
The MAX3580 utilizes two narrowband RF tracking filters,
one for VHF and one for UHF. Each filter is comprised of
a fixed inductor and three digitally controlled variable
capacitors named series, shunt, and parallel capacitors.
The integrated RF tracking filters uses an external induc-
tor between IND1 and IND2 pins to set the filter’s center
frequency. The inductor value must be 68nH ±2% in
order to achieve the corner frequency response. The vari-
able capacitors are factory calibrated to this particular
inductor value. The value of each capacitor is also set to
compensate for process variation of each individual part
and to receive the desired RF channel.
The process variation is factory calibrated by determin-
ing the best capacitor values for three discrete frequen-
cies, which are stored in the on-chip ROM table. Upon
power-up these values (6 bytes total) have to be read
out of the MAX3580 ROM table and stored in the micro-
processor local memory.
When tuning the MAX3580 to a given Rx frequency, the
correct capacitor value has to be calculated using the
following linear formulas and written to the appropriate
registers. This is in addition to programming the PLL
with the desired frequency.
The formulas differ for VHF and UHF bands but are the
same for all three capacitor values. Since the factory
calibration coefficients stored on the MAX3580 can dif-
fer for each capacitor, the calculations have to be exe-
cuted for all three capacitor values separately.
VHF: Capacitor = ROM_value_VHF -
(RX_frequency_in_MHz - 200MHz ) / 10MHz
In other words, the capacitor values to be written to the
MAX3550 decrease 1 count per 10MHz above 200MHz
and increase accordingly below 200MHz.
UHF: Capacitor = ROM_value_UHF_lo -
(ROM_value_UHF_lo - ROM_value_UHF_hi)
x (RX_frequency_in_MHz - 470MHz ) / 390MHz
This means the capacitor values stored in the UHF_lo
entries of the MAX3580 ROM table are the correct values
for 470MHz reception and the UHF_hi values for 860MHz
reception. For any frequency in between, the capacitor
values are obtained by a simple linear interpolation.
Note: When tuning to frequencies above 860MHz
channel center frequency, do not use the formula
above, but rather keep programming the tracking filter
with the coefficients obtained for 860MHz.
Examples: Assuming the MAX3580 ROM table entries
are C
SERIES VHF = 8, C SERIES UHF_lo = 15, C SERIES
UHF_hi = 3
208MHz: C SERIES = 8 - round ( ( 208-200 ) / 10 ) = 7
(floating point division, round to nearest
integer after division)
8 - floor ( ( 208 - 200 + 5) / 10 ) = 7
(all calculations using signed integer values,
truncate result of division)
677MHz: C SERIES = 15 - round ( (15-3) x (677 - 470) /
390 ) = 9 (floating point division, round to
nearest integer after division) 15 - floor
( ( ( 15-3) x (677-470) + 195 ) / 390 ) = 9
(all calculations using signed integer values,
truncate result of division)
Power-Supply Layout
To minimize coupling between different sections of the
IC, the ideal power-supply layout is a star configuration,
which has a large decoupling capacitor at the central
V
CC node. The V CC traces branch out from this node,
with each trace going to separate V CC pins of the
MAX3580. Next to each V CC pin is a bypass capacitor
with a low impedance to ground at the frequency of
interest. Use at least one via per bypass capacitor for a
low-inductance ground connection.
The three ground pins (GND_PLL, GND_CP,
GND_TUNE) must be connected to the ground plane
by separate via holes and must not be directly connect-
ed to the exposed paddle.
Chip Information
PROCESS: BiCMOS