lt5506 Linear Technology Corporation, lt5506 Datasheet - Page 8

• Manufacturer Part Number: lt5506
• Description: 40mhz To 500mhz Quadrature Demodulator With Vga
• Manufacturer: Linear Technology Corporation

LT5506
APPLICATIO S I FOR ATIO
of both IF inputs lower than 2 . In the matching network
of Figure 1, inductor L3 is used for supplying the DC bias
current to the IF
below 2 , 120nH is used. This disturbs the matching
network slightly by causing the frequency where the S11
is minimal to be lower than the frequency where the
amplitudes of IF
this, the value of coupling capacitor C3 is lowered and will
contribute some correcting reactance. For low frequen-
cies, it might not be possible to find any practical inductor
value for L3 with DC resistance smaller than 2 . In that
case it is recommended to use a transformer with central
tap. The tolerance for the components in Figure 1 can be
10% for a return loss higher than 16dB and a gain
reduction due to mismatch less than 0.3dB.
It is possible to simplify the input matching circuit and
compromise the performance. In Figure 2a, the simplified
matching network is given.
8
Table 1. The Component Values of Matching Network L1, L2, L3,
C1, C2 and C3.
f
IF
(MHz)
100
150
200
250
300
350
400
450
500
50
Figure 1. IF Input Matching Network at 280MHz
INPUT
IF
L1, L2(nH)
120nH
56pF
+
340
159
106
+
C3
80
64
53
45
40
35
32
L3
input. To keep the DC resistance of L3
and IF
U
5.6pF
C1
–
C1, C2(pF)
56nH
U
are equal. To compensate for
L1
15.9
10.6
8.0
6.4
5.3
4.5
4.0
3.5
3.2
34
L2
56nH
W
C2
5.6pF
L3(nH)
TO IF
TO IF
5506 F01
1800
470
470
470
120
120
120
120
120
120
+
–
U
C3(pF)
820
220
220
220
56
56
56
56
56
56
This matching network can deliver equal amplitudes to the
IF
phase difference between the inputs will not be exactly 180
degrees. In practice, the phase shift will be around 145
degrees, depending on the quality factor of the network.
This will result in a reduction in the gain. The higher the
chosen quality factor, the closer the phase difference will
approach 180 degrees. However, a higher quality factor
will reduce bandwidth and create more loss in the match-
ing network. For minimum board space, 0402 compo-
nents are used. The measured noise figure for maximum
gain with this matching network is about 8dB, and the
maximum gain about 57dB. Assuming 0402 inductors
with Q = 35, the insertion loss of this network is about
2.5dB. The tolerance for the components in Figure 2a can
be 10% for a return loss higher than 10dB and a gain
reduction due to mismatch less than 0.5dB. The measured
input sensitivity for this matching network (see also Fig-
ure 11) is about –82.7dBm for a 10dB signal-to-noise
ratio.
The gain of the VGA is set by the voltage at the V
For high gain settings, both the noise figure and the input
IP3 will be low. From a noise figure point of view, it is
advantageous to work as closely as possible to the maxi-
mum gain point. However, if the voltage at the V
is increased beyond the maximum gain point (where
additional increase in control voltage does not give an
increase in gain), the response time of the gain control
circuit is increased. If control speed is crucial, a few dB of
gain margin should be allowed from the highest gain point
to be sure that at all temperatures, the maximum gain
setting is not crossed. At low gain settings, the noise figure
and the input IP3 will be high. Optionally, the control
INPUT
+
Figure 2a. Simplified IF Input Matching Network at 280MHz
and Figure 2b. Simplified Circuit Schematic of the IF Inputs
and IF
IF
–
L1
15nH
inputs for a narrow frequency region, but the
(2a)
L2
15nH
C1
10pF
TO IF
TO IF
+
–
IF
75
+
1mA
V
BIAS
(2b)
1mA
CTRL
CTRL
5506 F02
75
5506fa
pin.
IF
pin
–