LT5506

7

5506fa

BLOCK DIAGRA

2

2

IF DET

EN

STBY

+

–

+

–

90

°

0

°

5506 BD

÷2

2

7

14

13

15

16

3

6

11

10

9

12

VGA

I-MIXER

Q-MIXER

CLIPPER

CLIPPER LPF

LPF

The LT5506 consists of variable gain amplifier (VGA),

I/Q demodulator, quadrature LO generator, hard clipping

amplifiers (clippers), lowpass filters (LPFs) and bias

circuitry.

The IF signal is fed to the inputs of the VGA. The VGA gain

is typically set by an external signal in such a way that the

amplified IF signal delivered to the I/Q mixers is constant.

The IF signal is then converted into I/Q baseband signals

using the I/Q down-converting mixers. The quadrature LO

signals that drive the mixers are internally generated from

the on-chip divide-by-two circuit. The I/Q signals are

passed through a pair of hard-clipping amplifiers (clip-

pers), which protect the subsequent lowpass filters from

overloading. After externally setting the required gain,

these amplifiers should not clip. However, in the event of

overload, they reduce the settling time of the (optional)

external AC coupling capacitors by preventing asymmetri-

cal charging and discharging effects. The second order

baseband lowpass filters remove the out-of-band noise

and harmonic content generated by the mixers and the

clippers. The I/Q baseband outputs are buffered by output

drivers.

VGA and Input Matching

The VGA has a nominal 60dB gain control range with a

frequency range of 40MHz to 500MHz. The inputs of the

VGA must have a DC return to ground. This can be done

using a transformer with a central tap (on secondary) or an

LC matching circuit with a matched impedance at the

frequency of interest and near zero impedance at DC. The

APPLICATIO S I FOR ATIO

differential AC input impedance of the LT5506 is about

200

Ω, thus a 1:4 (impedance ratio) RF transformer with

central tap can be used. In Figure 6, the evaluation board

schematic is shown using a 1:4 transformer. The mea-

sured input sensitivity of this board is about –82.6dBm for

a 10dB signal-to-noise ratio. In the case of an LC matching

circuit, the circuit of Figure 1 can be used. In Table 1 the

values are given for a range of IF frequencies. The match-

ing circuit of Figure 1 approaches 180

° phase shift be-

frequency. However, some amplitude mismatch occurs if

the circuit is not tuned to the center frequency. This leads

to reduced circuit linearity performance, because one of

the inputs carries a higher signal compared to the perfectly

balanced case. A 10% frequency shift from the center

frequency results in about a 2dB gain difference between

contribution from the input stage which leads to a 0.75dB

drop in IIP3. Moreover, the IIP2 of the circuit is also

reduced which can lead to a higher second order harmonic

contribution. The circuit can be driven single ended, but

this is not recommended because it leads to a 3dB drop in

gain and a considerable increase in IM5 and IM7 compo-

nents. The single-ended noise figure increases by 4dB if

one IF input is directly grounded and increases by 1.5dB

if one IF input is grounded via a 1

µH inductor. An IF input

cannot be left open or connected via a resistor to ground

because this will disturb the internal biasing, reducing the

gain, noise and linearity performance. For optimal perfor-

mance, it is important to keep the DC impedance to ground