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

[Garry Thorp]

Bruce wrote:
 
A little more detail is required such as:

1) What was the divided down output of the 74AC163 compared with?

The E5052B contains 2 uncorrelated test systems, and uses its 2 internal
synthesisers (with separate 10MHz ref OCXOs) as references. The signal
under test is split at the input, and cross-correlation is used to
reduce the effect of reference noise.

2) An image of the breadboard would also be useful.

It was 2 years ago, and it doesn't exist any more! The IC was an SO-16
package, stuck upside down to a piece of copper-clad board.

3) A circuit diagram showing component values and manufacturer's part
nos.

Circuit attached. I don't remember the supply filter resistor value, but
it was probably ~100R. I adjusted the supply to give 5V on the IC when
it was operating.

I've read the E5052B manual but there's insufficient detail to have
confidence in the calibration technique used when a square wave input is
used.

The E5052B doesn't tune the DUT, but digitally locks its synthesisers to
the signal, so there is no VCO constant to calibrate. It uses analogue
mixers as phase detectors. I assume it measures the slope at zero
crossing as it appears able to work with high harmonic levels, e.g.
passing undistorted signals through amplifiers driven into compression
and re-measuring doesn't change the measured close-in phase noise (in
the 1/f^3 region). Below 100Hz where the oscillator's phase noise
dominates, the 12.5MHz curve tracks it 18dB lower, which gives
confidence that the instrument has got the calibration right.

>From what I understand from the manual / various application notes, the
E5052B samples the baseband signal at 250MHz, and processes the whole
thing in a single band. (This is how it achieves such good
far-from-carrier performance. In the time it takes to do a single
measurement at 1Hz offset for example, it does 128000 correlations at
>6.25MHz, giving 25.5dB improvement.) From this, I think one can assume
that if the cal is valid close-in, it will be valid at all offsets.

I would be interested in any suggestions about possible measurement
errors. The instrument settings are shown on the plot I posted
yesterday. In theory, 74AC phase noise should tend to a very low value
as f tends to zero, assuming the power supply is quiet, as one is just
left with FETs with a low ON resistance.

Garry

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