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AD8348

Rev. A | Page 22 of 28

The level of the mixer output (or the output of the baseband

filter) can then be set by varying the setpoint voltage fed to

Pin 11 (VSET) of the AD8362.

Care should be taken to ensure that blockers—unwanted signals

in the band of interest that are demodulated along with the desired

signal—do not dominate the rms power of the AD8362 input.

This can cause an undesired reduction in the level of the mixer

output. To overcome this, baseband filtering can be implemented

to filter out undesired signals before the signal is presented to

the AD8362.

Figure 56 shows the effectiveness of the AGC loop in

maintaining a baseband amplifier output amplitude with less

than 0.5 dB of amplitude error over an IF input range of 40 dB

while demodulating a QPSK-modulated signal at 380 MHz.

The AD8362 is insensitive to crest factor variations and

therefore provides similar performance regardless of the

modulation of the incoming signal.

IFIP POWER INPUT (dBm, ZO = 200Ω)

–55

–45

120

110

100

140

130

–4

–2

–3

–1

–35

–25

–15

–5

QPSK

ERROR

–5.1dBm re 10kΩ

Figure 56. AD8348 Baseband Amplifier Output vs.

IF Input Power with AD8362 AGC Loop

BASEBAND FILTERS

Baseband low-pass or band-pass filtering can be conveniently

performed between the mixer outputs (IMXO and QMXO) and

the input to the baseband amplifiers. Consideration should be

given to the output impedance of the mixers (40 Ω).

1.2µH

AD8348

IMXO

VCMO

IAIN

60Ω

0.68µH

150pF

82pF

100Ω

4.7pF

8.2pF

TO AD8362

INPUT IF AGC

LOOP IS USED

Figure 57. Baseband Filter Schematic

Figure 57 shows the schematic for a 100 Ω, fourth-order elliptic

low-pass filter with a 3 dB cutoff frequency of 20 MHz. Source

and load impedances of approximately 100 Ω ensure that the

filter sees a matched source and load. This also ensures that the

mixer output is driving an overall load of 200 Ω. Note that the

shunt termination resistor is tied to the source driving VCMO

and not to ground. This ensures that the input to the baseband

amplifier is biased to the proper reference level. VCMO is not

an output pin and must be biased by a low impedance source.

The frequency response and group delay of this filter are shown

in Figure 58 and Figure 59.

FREQUENCY (MHz)

–80

–50

–40

–60

–70

–30

–20

–10

100

Figure 58. Baseband Filter Response

FREQUENCY (MHz)

100

Figure 59. Baseband Filter Group Delay

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