Am 22.10.2015 um 22:04 schrieb Charles Steinmetz:
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),
What do you consider a run-of-the-mill comparator? LM139, LMV7219,
AD8561, ADCMP580?
and the LTC6957 is not as good, even with filtering on, as a circuit
with lower inherent jitter such as an optimized Wenzel-style squarer.
What optimizations? I have seen the "Wenzel" circuit in cheapish
frequency counter inputs in
the late seventies, maybe with a diode bridge added as input protection..
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.
so it shows that one can replace filtering by signal power :-)
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.
But in the KHz range, the filter corner frequency should be much lower
than in the 6957...
The 100 MHz range would be more interesting. The Wenzel squarer would
then need
"real transistors" with higher 1/f corner. The PLL would have a sub-Hz
BW in this application,
so the race for the lowest floor seems not too important. OK, maybe for
the 1pps.
The 6957 takes one square centimeter including transformer and works for
all frequencies.
Any objections against the AD9901 phase comparator? I have a tube of them.
regards, Gerhard
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