GaAsFET can serve this function. This circuitry is usual-

ly required in order to meet system noise-figure require-

ments, may provide a buffered F-connector output, and

may also be required to meet stringent LO leakage

requirements. The PIN attenuator is typically controlled

by the same voltage as the MAX2102/MAX2105 AGC

control pin so that, overall, a single AGC line from the

baseband processor can control the entire tuner.

In some applications, a varactor-tuned preselection

bandpass filter is added between the discrete LNA and

the MAX2102/MAX2105. This is usually required only

for very high-linearity tuners, such as those designed

for single channel-per-carrier (low-data-rate) applica-

tions. The filter provides a means of broadly filtering

adjacent interferers, thus improving the tuner’s inter-

modulation performance. Additionally, the filter

removes the RF interference at twice the LO frequency,

which would otherwise add to cochannel interference

(the MAX2102/MAX2105 alone reject this carrier to typi-

cally -32.3dBc).

External Oscillator

Since the MAX2102/MAX2105 are direct-conversion

receivers, the external LO must tune to the same fre-

quencies as the desired RF input signals.

The MAX2102/MAX2105 oscillator input port (LO, LO)

accepts either a single-ended or differential (balanced)

LO signal. A differential LO offers reduced LO leakage

to the RFIN port, as well as lower spurious downconver-

sion levels of RF signals, which are at twice the LO fre-

quency. Refer to Figure 3 for differential LO

connections. For best performance, ensure that the LO

and LO traces are symmetrical.

The LO drive levels should be maintained to within the

specified limits. If the LO drive falls below the specified

range, quadrature performance may be affected.

Driving LO above the specified limits will cause a high-

er LO leakage level at RFIN; this may be acceptable in

some applications. The MAX2102/MAX2105 offset-cor-

rection loop can withstand LO leakage levels corre-

sponding to at least 0dBm of LO input power drive.

Prescaler

Typical stand-alone tuner applications will not use the

MAX2102/MAX2105 prescaler function, but instead use

a commercial synthesizer IC such as the Philips

TSA5055, which has an internal prescaler. To disable

the MAX2102/MAX2105 prescaler, disconnect the

PSGND pin (leave open). The prescaler will cause an

output spur in the baseband spectrum, to a level of

about -20dBc (referred to 500mVp-p baseband output

level) that may land within the desired signal bandwidth

in some applications.

To use the MAX2102/MAX2105 prescaler, connect the

PSGND pin to ground. In some applications, the

prescaler may be toggled on and off using a MOSFET

to switch PSGND to ground. PSGND should be forced

to within 100mV of ground, and the MOSFET must be

capable of sinking 15mA. PSOUT is capable of driving

a typical CMOS load of 10k

Ω in parallel with 5pF. A

2k

Ω pull-down resistor must be connected from PSOUT

to GND.

The prescaler requires a stable level at the MOD pin

12ns before the falling edge of PREOUT to assert the

desired modulus. The level at MOD must remain static

until 3ns after this falling edge.

Baseband Amplifiers

The MAX2102/MAX2105 baseband amplifiers provide

over 2Vp-p swing at IOUT and QOUT, and are capable

of driving 100

Ω. IOUT and QOUT must be AC-coupled

to any lowpass filters. In a typical application, IOUT and

QOUT drive a 5th or 7th-order lowpass filter for ADC

anti-aliasing purposes (see the

Systems Considerations:

Lowpass Filters in Direct-Conversion Tuners section).

After the filters, in some cases, additional gain may be

required. This can be accomplished with a pair of video-

speed op amps, such as the MAX4216 dual-video op

amp. Alternatively, the MAX1002/MAX1003 dual ADC

has built-in gain ahead of the ADCs, digitizing levels as

low as 125mVp-p. Contact Maxim for more information

about the MAX4216 or the MAX1002/MAX1003.

Direct-Conversion Tuner ICs for

Digital DBS Applications

12

______________________________________________________________________________________

MAX2102

MAX2105

22

Ω

56

Ω

22pF

LO

22

21

22

Ω

56

Ω

22pF

LO POWER:

-15dBm TO -5dBm,

POWER INTO LO, LO.

LO, LO DRIVEN DIFFERENTIALLY

LO

Figure 3. Differential LO Drive