Datasheet

LTC6946
24
6946fa
APPLICATIONS INFORMATION
The LTC6946 achieves an in-band normalized phase noise
floor of –226dBc/Hz (typical). To calculate its equiva-
lent input phase noise floor L
M(IN)
, use the following
Equation 11:
L
M(IN)
= –226 + 10 • log
10
(f
REF
) (11)
For example, using a 10MHz reference frequency gives
an input phase noise floor of –156dBc/Hz. The reference
frequency source’s phase noise must be at least 3dB better
than this to prevent limiting the overall system performance.
IN-BAND OUTPUT PHASE NOISE
The in-band phase noise produced at f
RF
may be calcu-
lated by using Equation 12.
L
M(OUT)
= –226 +10 s log
10
f
PFD
()
+ 20 s log
10
f
RF
f
PFD
⎛
⎝
⎜
⎞
⎠
⎟
or
L
M(OUT)
= –226 +10 s log
10
f
PFD
()
+ 20 s log
10
N
O
⎛
⎝
⎜
⎞
⎠
⎟
(12)
As can be seen, for a given PFD frequency f
PFD
, the output
in-band phase noise increases at a 20dB-per-decade rate
with the N divider count. So, for a given output frequency
f
RF
, f
PFD
should be as large as possible (or N should be as
small as possible) while still satisfying the application’s
frequency step size requirements.
OUTPUT PHASE NOISE DUE TO 1/f NOISE
In-band phase noise at very low offset frequencies may
be influenced by the LTC6946’s 1/f noise, depending upon
f
PFD
. Use the normalized in-band 1/f noise of –274dBc/
Hz with Equation 13 to approximate the output 1/f phase
noise at a given frequency offset f
OFFSET
.
L
M(OUT-1/f)
f
OFFSET
()
= –274+ 20 •log
10
f
RF
()
–10 • log
10
f
OFFSET
()
(13)
Unlike the in-band noise floor L
M(OUT)
, the 1/f noise
L
M(OUT 1/f)
does not change with f
PFD
, and is not constant
over offset frequency. See Figure 15 for an example of
in-band phase noise for f
PFD
equal to 3MHz and 100MHz.
The total phase noise will be the summation of
LM(OUT)
and L
M(OUT 1/f)
.
Figure 15. Theoretical In-Band Phase Noise, f
RF
= 2500MHz
OFFSET FREQUENCY (Hz)
10
PHASE NOISE (dBc/Hz)
–110
–100
100k
6945 F15
–120
–130
100
1k
10k
–90
TOTAL NOISE
f
PFD
= 3MHz
TOTAL NOISE
f
PFD
= 100MHz
1/f NOISE
CONTRIBUTION
RF OUTPUT MATCHING
The RF
±
outputs may be used in either single-ended or dif-
ferential configurations. Using both RF outputs differentially
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 V
RF
+
. Measured
RF
±
s-parameters are shown below in Table12 to aid in
the design of impedance matching networks.
Table 12. Single-Ended RF Output Impedance
FREQUENCY (MHZ) IMPEDANCE (Ohms) S11 (dB)
500 102.8 – j49.7 –6.90
1000 70.2 – j60.1 –6.53
1500 52.4 – j56.2 –6.35
2000 43.6 – j49.2 –6.58
2500 37.9 – j39.6 –7.34
3000 32.7 – j28.2 –8.44
3500 27.9 – j17.8 –8.99
4000 24.3 – j9.4 –8.72
4500 22.2 – j3.3 –8.26
5000 21.6 + j1.9 –8.02
5500 21.8 + j6.6 –7.91
6000 23.1 + j11.4 –8.09
6500 25.7 + j16.9 –8.38
7000 29.3 + j23.0 –8.53
7500 33.5 + j28.4 –8.56
8000 37.9 + j32.6 –8.64