lt5557 Linear Technology Corporation, lt5557 Datasheet - Page 13

• Manufacturer Part Number: lt5557
• Description: 400mhz To 3.8ghz 3.3v High Linearity Downconverting Rf Mixer
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
These equations give a good starting point, but it is usually
necessary to adjust the component values after building
and testing the circuit. The final solution can be achieved
with less iteration by considering the parasitics of L3 in the
above calculations. Specifically, the effective parallel re-
sistance of L3 (calculated from the manufacturer’s Q data)
will reduce the value of R
calculated values of L1 (=L2) and C6 (=C7). Also, the
effective parallel capacitance of L3 (taken from the manu-
facturers SRF data) must be considered, since it is in
parallel with X
value for L1 does not fall on a standard value for the
desired IF. In this case, a simple solution is to load the IF
output with a high-value external chip resistor in parallel
with L3, which reduces the value of R
standard value.
Discrete IF balun element values for four common IF
frequencies (190MHz, 240MHz, 360MHz and 450MHz)
are listed in Table 4. The 190MHz application circuit uses
a 3.3kΩ resistor in parallel with L3 as described above.
The corresponding measured IF output return losses are
shown in Figure 10. Typical conversion gain, IIP3 and LO-
IF leakage, versus RF input frequency, for all four ex-
amples is shown in Figure 11. Typical conversion gain,
IIP3 and noise figure versus IF output frequency is shown
in Figure 12.
Compared to the transformer-based IF matching tech-
nique, this network delivers approximately 1dB higher
conversion gain (since the IF transformer loss is elimi-
nated), though noise figure and IIP3 are degraded slightly.
The most significant performance difference, as shown in
Figure 12, is the limited IF bandwidth available from the
discrete approach. For low IF frequencies, the absolute
bandwidth is small, whereas higher IF frequencies offer
wider bandwidth.
Table 5. Discrete IF Balun Element Values (R
IF FREQUENCY
(MHz)
190
240
360
450
IF
(from table 3). Frequently, the calculated
U
L1, L2
120nH
100nH
56nH
47nH
U
IF
, which in turn influences the
W
C6, C7
OUT
6.0pF
4.7pF
3.0pF
2.2pF
= 50Ω)
IF
, until L1 is a
270nH || 3.3kΩ
U
150nH
82nH
47nH
L3
Figure 10. IF Output Return Losses with Discrete Balun Matching
Figure 12. Conversion Gain, IIP3 and SSB NF vs IF Output
Frequency Using Discrete IF Balun Matching
Figure 11. Conversion Gain, IIP3 and LO-IF Leakage
vs RF Input Frequency and IF Output Frequency
(in MHz) Using Discrete IF Balun Matching
–10
–20
–30
26
24
22
20
18
16
14
12
10
14
12
26
24
22
20
18
16
10
0
8
6
4
2
8
6
4
2
1700
150
50
IIP3
G
SSB NF
C
200
1800
150
IIP3
G
IF OUTPUT FREQUENCY (MHz)
190 MHz
RF INPUT FREQUENCY (MHz)
190IF
240IF
360IF
450IF
C
250
IF FREQUENCY (MHz)
240 MHz
1900
250
300
LOW-SIDE LO (–3dBm)
LOW-SIDE LO (–3dBm)
360 MHz
350
2000
350
450 MHz
400
RF = 1950MHz
LO-IF
2100
450
T
T
A
A
450
= 25°C
= 25°C
190IF
240IF
360IF
450IF
5557 F10
5557 F11
5557 F12
2200
550
500
–30
–40
–50
–10
–20
–60
–70
LT5557
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5557fa