lt5546 Linear Technology Corporation, lt5546 Datasheet - Page 8

• Manufacturer Part Number: lt5546
• Description: 40mhz To 500mhz Vga And I/q Demodulator With 17mhz Baseband Bandwidth
• Manufacturer: Linear Technology Corporation

LT5546
APPLICATIO S I FOR ATIO
To keep the DC resistance of L3 below 2Ω, 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
To compensate for this, the value of coupling capacitor C3
is lowered and will contribute some correcting reactance.
For low frequencies, it might not be possible to find any
practical inductor value for L3 with DC resistance smaller
than 2Ω. In that case it is recommended to use a trans-
former with a center 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.
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
8
Table 1. The Component Values of Matching Network L1, L2, L3,
C1, C2 and C3.
Figure 1. Example L-C IF Input Matching Network at 280MHz
+
f
IF
and IF
(MHz)
100
150
200
250
300
350
400
450
500
50
–
INPUT
inputs for a narrow frequency region, but the
IF
L1, L2(nH)
120nH
56pF
340
159
106
C3
80
64
53
45
40
35
32
L3
U
5.6pF
C1
C1, C2(pF)
56nH
U
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
5546 F01
1800
and IF
470
470
470
120
120
120
120
120
120
+
–
–
are equal.
U
C3(pF)
820
220
220
220
56
56
56
56
56
56
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 9.4dB, and the
maximum gain is about 55dB. 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 –78.3dBm 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
voltage V
range is from V
nominal gain range from 1.6dB to 56.8dB. The linear-in-
dB gain relation with the V
control voltages as low as –0.35V. This results in an
INPUT
Figure 2a. Simplified IF Input Matching Network at 280MHz
and Figure 2b. Simplified Circuit Schematic of the IF Inputs
IF
CTRL
L1
15nH
(2a)
can be set lower than 0.2V. The normal
CTRL
L2
15nH
C1
10pF
TO IF
TO IF
= 0.2V to 1.7V, which results in a
+
–
IF
CTRL
75Ω
+
voltage still holds for
1mA
V
BIAS
(2b)
1mA
CTRL
CTRL
5546 F02
75Ω
5546fa
IF
pin.
pin
–