Datasheet

LTC6957-1/LTC6957-2/
LTC6957-3/LTC6957-4
23
6957f
For more information www.linear.com/LTC6957-1
Table 2 has the slew rate ranges most suitable for the four
different filter settings.
Table 2
FILTA FILTB INPUT SLEW RATE (V/µs)
Low Low >400
High Low 125 to 400
Low High 40 to 125
High High <40
Another way to look at this is to consider the case of sine
waves, for which the frequency ranges will depend on
input amplitudes, as illustrated in Table 3.
Table 3
FREQUENCY RANGE
INPUT
AMPLITUDE
(dBm)
FILTA = L,
FILTB = L
(MHz)
FILTA = H,
FILTB = L
(MHz)
FILTA = L,
FILTB = H
(MHz)
FILTA = H,
FILTB = H
(MHz)
10 >63 20 to 63 6.3 to 20 <6.3
5 >112 35 to 112 11 to 35 <11
0 >200 63 to 200 20 to 63 <20
–5 >112 35 to 112 <35
–10 >200 63 to 200 <63
Figure 7 has LTC6957-1 100MHz additive phase noise
measurements that illustrate the trade-offs between filter
settings at various input slew rates. Each of the three
charts has all four filter settings, and one input amplitude;
Figure7a has a +10dBm input, Figure 7b has a 0dBm input,
and Figure 7c has a –10dBm input. The four filter settings
are shown in the same colors throughout.
With +10dBm at 100MHz, the input slew rate is 628V/µs
and Table 2 indicates the best filter setting to use is FILTA
= FILTB = L, which is seen to be the case in Figure 7a.
The noise at the next filter setting is only slightly higher,
but for the maximum filtering case there is a full 10dB of
additional noise.
With 0dBm at 100MHz, the input slew rate is 198V/µs and
Table 2 indicates the best filter setting to use is FILTA = H,
FILTB = L. Again
this is seen to be the case in Figure 7b.
As
the input was decreased 10dB from Figure 7a to Figure 7b,
the blue trace rose 5dB while the green trace only rose 3dB.
With –10dBm at 100MHz, the input slew rate is 63V/µs and
Table 2 indicates the best filter setting to use is FILTA = L,
FILTB = H. Again this is seen to be the case in Figure 7c. As
the input was decreased 10dB from Figure 7a to Figure 7b,
and again to Figure 7c, the red trace rose just 3dB then
another 4dB, while the green and blue traces rose much
faster.
applicaTions inForMaTion
OFFSET FREQUENCY (Hz)
100
–165
PHASE NOISE (dBc/Hz)
–140
1k 10k 100k
69571234 F07a
–160
–150
–145
–155
1M
FILTA = H, FILTB = L
FILTA = FILTB = L
FILTA = FILTB = H
FILTA = L, FILTB = H
SINGLE-ENDED SINE WAVE INPUT,
100MHz AT 10dBm (2V
P-P
)
LTC6957-1
OFFSET FREQUENCY (Hz)
100
–165
PHASE NOISE (dBc/Hz)
–140
1k 10k 100k
69571234 F07b
–160
–150
–145
–155
1M
FILTA = H, FILTB = L
FILTA = FILTB = L
FILTA = FILTB = H
FILTA = L, FILTB = H
SINGLE-ENDED SINE WAVE INPUT,
100MHz AT 0dBm (632.5mV
P-P
)
LTC6957-1
OFFSET FREQUENCY (Hz)
100
–165
PHASE NOISE (dBc/Hz)
–140
1k 10k 100k
69571234 F07c
–160
–150
–145
–155
1M
FILTA = L, FILTB = H
FILTA = H, FILTB = L
FILTA = FILTB = L
FILTA = FILTB = H
SINGLE-ENDED SINE WAVE INPUT,
100MHz AT –10dBm (200mV
P-P
)
LTC6957-1
Figure 7. 100MHz Additive Phase Noise with Varying Input Amplitudes
(a) (b) (c)