[time-nuts] Unified VCXO Carrier Board
csteinmetz at yandex.com
Thu Oct 22 16:04:14 EDT 2015
>5.Board has 2 reference frequency inputs with LTC6957 receivers. One
>of them can interface the onboard VCXO to the CPLD.
As I have said before, there is very little if any advantage to using
an LTC6957 at 10MHz (as opposed to using a run-of-the-mill
comparator), and the LTC6957 is not as good, even with filtering on,
as a circuit with lower inherent jitter such as an optimized
LT published an app note that shows the improvement with filtering
enabled, using a 10MHz input. The relevant graph is attached below.
The graph compares an optimized 6957 implementation without filtering
and with optimum filtering. At an input level of -10dBm, the phase
noise floor is 7dB lower with filtering, and at an input of +10dBm,
the improvenent is <2.5dB. Extrapolating beyond the graph to the
right, at an input level of +13dBm (= 1Vrms, the customary level for
frequency references), the improvement with filtering will be very
near 0dB. It is not hard to add 10dB or more of gain at 10MHz with a
residual phase noise penalty in the -180dB range. So, with a 10MHz
input, even if the output level of the source is lower than +13dBm,
you can easily do just as well with a 6957 with the filtering off as
with it on. And with a squaring circuit that has inherently lower
residual PN than the 6957 (e.g., an optimized Wenzel-type squarer),
you can do better than with any permutation of the 6957.
These data are consistent with testing I have done of the LTC6957 and
other squaring circuits.
I realize that you did not say you expect to use the board only at
10MHz, and the LTC6957 with filtering may provide some improvement at
lower frequencies (compared to the 6957 without filtering). But even
with filtering on, the 6957 will not outperform an optimized
Wenzel-type squarer until you get well down into the kHz range. And
for the usual time-nuts case -- 10MHz at ~13dBm -- the 6957 is not
the best solution, even with filtering enabled.
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