LT5546

9

5546f

APPLICATIO S I FOR ATIO

pin is a very sensitive input because of its high input

impedance and therefore should be well shielded. Signal

noise floor in the I/Q baseband outputs. It can degrade the

linearity performance and it can cause asymmetry in the

two-tone test. If control speed is not important, 1

µF

ground.

A fast responding peak detector is connected to the VGA

input, sensitive to signal levels above the signal levels

where the VGA is operating in the linear range. It is active

from –22dBm up to 5dBm IF input signal levels. The DC

output voltage of this detector (IF DET) can be used by the

baseband controller to quickly determine the presence of

a strong input level at the desired channel, and adjust gain

accordingly. Figure 3a shows the simplified circuit sche-

matic of the IF DET output.

I/Q Demodulators

The quadrature demodulators are double balanced mix-

ers, down-converting the amplified IF signal from the VGA

into I/Q baseband signals. The quadrature LO signals are

generated internally from a double frequency external CW

signal. The nominal output voltage of the differential I/Q

depending on the linearity requirements. The magnitudes

of I and Q are well matched and their phases are 90

° apart.

Quadrature LO Generator

The quadrature LO generator consists of a divide-by-two

circuit and LO buffers. An input signal (2xLO) with twice

the desired IF signal frequency is used as the clock for the

divide-by-two circuit, producing the quadrature LO signals

for the demodulators. The outputs are buffered and then

drive the down-converting mixers. With a fully differential

approach, the quadrature LO signals are well matched.

Second harmonic content (or higher order even harmon-

ics) in the external 2xLO signal can degrade the 90

° phase

shift between I and Q. Therefore, such content should be

minimized. In disable or standby mode, the divide-by-two

stage is powered down. After enabling the circuit, the phase

relation between the IF signal and the baseband (I or Q)

signals can be either 0

° or 180°, since the circuit cannot

distinguish between the two subsequent identical sinusoi-

dal waveforms of the 2xLO input signal. The phase rela-

tion between I and Q is always 90

°, i.e. I always leads Q by

90

°. Figure 3b shows the simplified circuit schematic of

the 2xLO inputs. Depending on the application, different

2xLO input matching networks can be chosen. In Figure

4, three examples are given. The first network provides the

best 2xLO input sensitivity because it can boost the 2xLO

differential input signal using a narrow-band resonant ap-

proach. The second network gives a wide-band match, but

the 2xLO input sensitivity is about 2dB lower. The third

network gives a simple and less expensive wide-band

match, but 2xLO input sensitivity drops by about 9dB. The

IF input sensitivity doesn’t change significantly using any

of the three 2xLO matching networks.

Baseband Circuit

The baseband circuit consists of I/Q low-pass filters, I/Q

hard limiters (clippers) and I/Q output buffers. The hard

limiters operate as linear amplifiers normally. However, if

a high level input temporarily overloads a linear amplifier,

then the circuit will limit symmetrically, which will help to

prevent the output buffer from overloading. This speeds

Figure 3a. Simplified Circuit Schematic of the

IF DET Output and Figure 3b. The 2xLO Inputs

3.8k

1k

8k

8k

5546 F03

IF DET

+

–

400mV

(3a)

(3b)

Figure 4. 2xLO Input Matching Networks for 4a) Narrow Band

Tuned to 570MHz, 4b) Wide Band, 4c) Single-Ended Wide Band

39nH

5546 F04

2xLO

INPUT

2xLO

INPUT

2xLO

INPUT

240

Ω

1:4

3.3pF

100pF

100pF

3.3pF

56

Ω

(4a)

(4b)

(4c)