Rainbow Electronics MAX3540 - Manual (Page 12)

MAX3540
Complete Single-Conversion Television Tuner
12 ______________________________________________________________________________________
Applications Information
RF Inputs
The MAX3540 features separate UHF and VHF inputs
that are matched to 75Ω. Both inputs require a DC-block-
ing capacitor. The input registers select the active inputs.
In addition, the input registers enable or disable the low-
pass filter, which can be used when the VHF input is
selected. For 54MHz to 100MHz, select the VHF_IN with
the LPF filter enabled (INPT = 00). For 100MHz to
300MHz, select VHF_IN with LPF disabled (INPT = 01).
For 300MHz to 860MHz, select UHF_IN (INPT = 10).
RF Gain Control
The gain of the RF low-noise amplifier can be adjusted
over a typical 45dB range by the RFAGC pin. The
RFAGC input accepts a DC voltage from 0.5V to 3V,
with 3V providing maximum gain. This pin can be con-
trolled with the IF power-detector output to form a
closed RF gain-control loop. See the
Closed-Loop RF
Gain Control section for more information.
RF Tracking Filter
The MAX3540 includes a programmable tracking filter for
each band of operation to optimize rejection of out-of-
band interference while minimizing insertion loss for the
desired received signal. VHF low, VHF high, or UHF track-
ing filter is selected by the TF register. The center fre-
quency of each tracking filter is selected by a switched-
capacitor array, which is programmed by the TFS[7:0]
bits in the Tracking-Filter Series Cap register and the
TFP[5:0] bits in the Tracking-Filter Parallel Cap register.
To accommodate part-to-part variations each part is fac-
tory-calibrated by Maxim. During calibration the y-inter-
cept and slope for the series and parallel tracking
capacitor arrays is calculated and written into an internal
ROM table. The user must read the ROM table upon
power-up and store the data in local memory (8 bytes
total) to calculate the optimal TFS[7:0] and TFP[5:0] set-
tings for each channel. Table 16 shows the address and
bits for each ROM table entry. See the
Interpolating
Tracking Filter Coefficients section for more information
on how to calculate the required values.
Reading the ROM Table
Each ROM table entry must be read using a two-step
process. First, the address of the ROM bits to be read
must be programmed into the TFA[3:0] bits in the
Tracking Filter ROM Address register (Table 11).
Once the address has been programmed, the data
stored in that address is transferred to the TFR[7:0] bits
in the ROM Table Data Readback register (Table 13).
The ROM data at the specified address can then be
read from the TFR[7:0] bits and stored in the micro-
processor’s local memory.
MSB LSB
DATA BYTE DESCRIPTION ADDRESS
D7 D6 D5 D4 D3 D2 D1 D0
IFOVLD 0x0 OD2 OD1 OD0 X X X X X
VHF Low Series/
Parallel Y-Intercept
0x1 LS0[5] LS0[4] LS0[3] LS0[2] LS0[1] LS0[0] LS1[3] LS1[2]
VHF High Series/
Parallel Y-Intercept
0x2 LS1[1] LS1[0] LP0[5] LP0[4] LP0[3] LP0[2] LP0[1] LP0[0]
UHF Series/
Parallel Y-Intercept
0x3 LP1[3] LP1[2] LP1[1] LP1[0] HS0[3] HS0[2] HS0[1] HS0[0]
VHF Low Series Slope 0x4 HS1[3] HS1[2] HS1[1] HS1[0] HP0[3] HP0[2] HP0[1] HP0[0]
VHF High Parallel Slope 0x5 HP1[3] HP1[2] HP1[1] HP1[0] US0[7] US0[6] US0[5] US0[4]
VHF Low Parallel Slope 0x6 US0[3] US0[2] US0[1] US0[0] US1[5] US1[4] US1[3] US1[2]
VHF High Parallel Slope 0x7 US1[1] US1[0] UP0[7] UP0[6] UP0[5] UP0[4] UP0[3] UP0[2]
UHF Parallel Slope 0x8 UP0[1] UP0[0] UP1[5] UP1[4] UP1[3] UP1[2] UP1[1] UP1[0]
Table 16. ROM Table