LTC6948IUFD-2-TRPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC6948IUFD-2-TRPBF
Linear Technology
'LTC6948IUFD-2-TRPBF' PDF : 36 Pages View PDF
LTC6948
Applications Information
–40
–50
–60
–70
–80
fVCO ≈ 3GHz
–90 fPFD = 50MHz
LOOP BW = 130kHz
–100 CPLE = 1
N = 60
SWEPT NUM
–110
1k
10k
100k
1M
10M
FREQUENCY OFFSET FROM 3GHz (Hz)
6948 F15
Figure 15. Integer Boundary Spur Power vs Frequency
Offset from Boundary, LTC6948-1
will result in approximately 3dB more output power than
single-ended. Impedance matching to an external load in
both cases requires external chokes tied to VRF+. Measured
RF± S-parameters are shown below in Table 17 to aid in
the design of impedance matching networks.
Table 17. Single-Ended RF Output Impedance
FREQUENCY (MHz)
IMPEDANCE (Ω)
100
133.0 – j16.8
500
110.8 – j46.1
1000
74.9 – j57.0
1500
49.0 – j51.3
2000
34.4 – j41.4
2500
27.0 – j32.1
3000
23.2 – j24.1
3500
21.6 – j15.9
4000
20.9 – j7.7
4500
20.1 – j0.2
5000
18.1 + j7.4
5500
16.7 + j12.5
6000
17.1 + j16.1
6500
20.2 + j20.1
7000
26.9 + j24.6
7500
38.8 + j32.3
8000
52.9 + j43.1
S11 (dB)
–6.7
–6.8
–6.9
–6.7
–6.5
–6.5
–6.6
–7.1
–7.5
–7.4
–6.4
–5.6
–5.5
–6.2
–7.6
–8.8
–8.2
Single-ended impedance matching is accomplished using
the circuit of Figure 16, with component values found in
Table 18. 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
6948 F16
Figure 16. Single-Ended Output Matching Schematic
Table 18. Suggested Single-Ended Matching Component Values
fRF (MHz)
350 to 1500
LC (nH)
180
CS (pF)
270
1000 to 5800
68
100
Return loss measured on the DC1959 using the above
component values is shown in Figure 17. 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 18. LC should be a wirewound
inductor selected for maximum Q factor and SRF, such
as the Coilcraft HP series of chip inductors.
The LTC6948’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 more information www.linear.com/LTC6948
6948f
31
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