Direct-Conversion TV Tuner

______________________________________________________________________________________

19

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

MAX3580 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

UHF_hi = 3

208MHz:

(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:

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

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