[time-nuts] Advice on 10 MHz isolation/distribution / Phase Noise of 74AC gates

[Garry Thorp]

Having followed the discussion for a while, I get the feeling that some
people dismissed my results on the basis that 'CMOS ICs can't be
anywhere near that good, therefore the measurement must be faulty,
therefore it's not even worth discussing it'.
 
I knew from other people's measurements that 74AC was capable of better
than -160dBc/Hz when used to make a phase detector at 10MHz, but I
wanted to do a quick feasibility check on a divider for an application a
couple of years ago.
 
The 74AC163 was powered from a linear bench supply via a long (many
seconds) RC time constant plus local decoupling. I adjusted the supply
to give 5V at the IC when it was operating.
 
The 100MHz OCXO, which gave 18dBm into 50 ohm, was AC-coupled into the
clock input, which was biased to half the supply voltage. The counter
was left dividing by 16, as its propagation delay and set-up times are
too long to programme it to divide by 10 with 100MHz clock rate. The
output was AC-coupled directly to the E5052B input, without any
filtering. I had to use the Qc output as the SSA doesn't work below
10MHz.
 
The attached plot shows the 12.5MHz phase noise plus that of the 100MHz
OCXO. The divider phase noise tracks 18dB below the OCXO at low offsets
as expected, before its flicker noise and eventual noise floor
predominate. I was primarily interested in seeing what the flicker noise
was like, but I was surprised when I saw how low the floor was!
 
The E5052B does the necessary calibration automatically before doing a
measurement - from my experience with the instrument I have no reason to
doubt the validity of the result. (The indicated 100MHz phase noise in
the ~1-50kHz region is actually limited by the E5052B, owing to my
setting only 100 correlations. However it shows it low enough to
indicate that the CMOS noise dominates over that range.)
 
I realise that a divider is very different from a simple inverter, but I
think this gives an useful indication of what AC logic is capable of.
 
Garry
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