Offset Correction

The internal offset-correction amplifiers remove the DC

offsets present in the baseband amplifiers. The offset-

correction loop effectively AC-couples the baseband

signal path, yielding a -3dB highpass corner frequency

according to the following:

farads, across QDC, QDC and IDC, IDC.

For applications where the DC information must be

maintained through the signal path, the offset correc-

tion can be disabled by connecting QDC, QDC, IDC,

and IDC directly to ground. Disabling the offset correc-

tion will effectively limit the input dynamic range of the

MAX2102/MAX2105. Typical input dynamic range will

be approximately -45dBm to -19dBm for single-ended

LO drive, and -55dBm to -19dBm for differential LO

drive.

Layout Considerations

Observe standard RF layout rules. A ground plane is

essential; when connecting areas of ground plane

between layers, use vias liberally. Remove the ground

plane under the external VCO area to reduce parasitic

capacitance. If a ground plane is used under the low-

pass filters, note that the filter shape may be slightly off-

set due to parasitic capacitance.

In a direct-conversion receiver, LO leakage to the RF

input connector is a major issue, since filtering of the

LO is impossible (the LO operates at the same frequen-

cy as the RF input). The external VCO section should

be housed in a separate shielded compartment, if pos-

sible. Use of a differential (balanced) LO will dramati-

cally reduce LO leakage. Also, the use of coplanar,

waveguide transmission-line structures reduces LO

leakage (used on the MAX2102 EV kit).

Observe the power-supply bypass capacitor connec-

tions in the

Pin Description table, notably pins 1, 3, 4, 6,

9, 11, 12, 14, 19, 20, 23, and 24. Traces from these IC

pins to the bypass capacitors must be kept to an

absolute minimum. Where possible, make these con-

nections on the top side of the board.

The MAX2102 EV kit includes ferrite beads in series

with power-supply leads. The beads may not be

required for all applications.

Direct-Conversion Tuner ICs for

Digital DBS Applications

______________________________________________________________________________________

13

Table 1. Suggested Component Values for Discrete Lowpass Filters (0.1dB Ripple

Chebyshev Type)

Note: Suggested types: Inductors: Coilcraft 1008CS, tolerance = ±5%; Capacitors: use tolerance = ±2%. Refer to Figure 2 for circuit

diagram.

0.1dB Chebyshev,

40

0.1dB Chebyshev,

60

0.1dB Chebyshev,

90

ADC

SAMPLING

RATE (Msps)

FILTER

TYPE

100

100

100

(

Ω)

39

22

18

C1

(pF)

910

620

390

L1

(nH)

120

82

56

C2

(pF)

1500

910

620

L2

(nH)

150

100

68

C3

(pF)

1500

1000

680

L3

(nH)

120

82

56

C4

(pF)

10

10

10

(k

Ω)