LTC6945 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC6945' PDF : 28 Pages View PDF
LTC6945
APPLICATIONS INFORMATION
Measured RF± s-parameters are shown below in Table
11 to aid in the design of impedance matching networks.
Table 11. Single-Ended RF Output Impedance
FREQUENCY (MHz) IMPEDANCE (Ω)
500
102.8 – j49.7
1000
70.2 – j60.1
1500
52.4 – j56.2
2000
43.6 – j49.2
2500
37.9 – j39.6
3000
32.7 – j28.2
3500
27.9 – j17.8
4000
24.3 – j9.4
4500
22.2 – j3.3
5000
21.6 + j1.9
5500
21.8 + j6.6
6000
23.1 + j11.4
6500
25.7 + j16.9
7000
29.3 + j23.0
7500
33.5 + j28.4
8000
37.9 + j32.6
S11 (dB)
–6.90
–6.53
–6.35
–6.58
–7.34
–8.44
–8.99
–8.72
–8.26
–8.02
–7.91
–8.09
–8.38
–8.53
–8.56
–8.64
Single-ended impedance matching is accomplished using
the circuit of Figure 18, with component values found in
Table 12. Using smaller inductances than recommended
can cause phase noise degradation, especially at lower
center frequencies.
VRF+
RF+(–)
LC
CS
50Ω
VRF+
RF–(+)
LC
CS
TO 50Ω
LOAD
6945 F18
Figure 18. Single-Ended Output Matching Schematic
Table 12. Suggested Single-Ended Matching Component Values
fRF (MHz)
350 to 1500
LC (nH)
180nH
CS (pF)
270pF
1000 to 5800
68nH
100pF
Return loss measured on the DC1649 using the above
component values is shown in Figure 19. A broadband
match is achieved using an (LC, CS) of either (68nH, 100pF)
or (180nH, 270pF). However, for maximum output power
and best phase noise performance, use the recommended
component values of Table 12. LC should be a wirewound
inductor selected for maximum Q factor and SRF, such as
the Coilcraft HP series of chip inductors.
0
68nH, 100pF
–2
180nH, 270pF
–4
–6
–8
–10
–12
–14
–16
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
FREQUENCY (GHz)
6945 F19
Figure 19. Single-Ended Return Loss
The LTC6945’s differential RF± outputs may be combined
using an external balun to drive a single-ended load. The
advantages are approximately 3dB more output power than
each output individually and better 2nd-order harmonic
performance.
For lower frequencies, transmission line (TL) baluns such as
the M/A-COM MABACT0065 and the TOKO #617DB-1673
provide good results. At higher frequencies, surface mount
(SMT) baluns such as those produced by TDK, Anaren,
6945f
23
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