LT5506EUF Linear Technology, LT5506EUF Datasheet - Page 7

• Manufacturer Part Number: LT5506EUF
• Description: IC DEMOD QUADRATURE VGA 16QFN
• Manufacturer: Linear Technology

Specifications of LT5506EUF

• Function: Demodulator
• Lo Frequency: 80MHz ~ 1GHz
• Rf Frequency: 40MHz ~ 500MHz
• Gain: 59dB
• Noise Figure: 6.8dB
• Current - Supply: 36mA
• Voltage - Supply: 1.8 V ~ 5.25 V
• Package / Case: 16-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• P1db: -

BLOCK DIAGRA
APPLICATIO S I FOR ATIO
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
U
U
W
2 LO
2 LO
V
CTRL
IF
IF
–
+
+
–
W
11
10
2
3
6
EN
9
VGA
U
2
I-MIXER
Q-MIXER
90
0
differential AC input impedance of the LT5506 is about
200 , 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-
tween IF
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
the IF
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 H 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
CLIPPER LPF
CLIPPER
+
and IF
+
LPF
and IF
STBY
12
–
inputs. This results in a 1.5dB higher IM3
16
15
14
13
–
7
5506 BD
in a broad range around its center
I
I
IF DET
Q
Q
OUT
OUT
OUT
OUT
+
–
+
–
LT5506
5506fa
7