[time-nuts] Distribution amplifier (again!) - now mostly ok but has gain peaking

[Anders Wallin]

Hi all,
I've been tinkering with another distribution amplifier design and made
some measurements earlier this week.
The goal is roughly 1:8 fan-out, gain of 0 dB, for good quality (Cs, maser,
OCXO) 5 or 10 MHz signals in the range of maybe +0 dBm to +15 dBm - in a 1U
form-factor.

Earlier I made an SMD version of the TADD-1 design [1] which showed about
-156 dBc/Hz far-out phase-noise but was quite sensitive to external noise
and required 12VDC power from a lead-acid battery as well as shielding in
aluminium foil for a 'quiet' PN-spectrum.
I then did some SPICE simulations [2] (never trust them without testing ;)
which indicated ADA4899 would be a good op-amp. In practice the
slew-rate/distortion was limiting and the AD4899 version didn't show better
PN.

This new version is inspired by looking inside a 6502[3] - and in the
mean-time I also measured and Ettus Octoclock [4] - but its performance
isn't so exciting..

My current design is now here: https://goo.gl/photos/WB8fYd4jzba7nXH18
So far my observations are:
- phase noise around -162 dBc/Hz at 10 MHz
- nice quiet PN-spectrum when unshielded and powered from lab-supplies
- this probably means the supply-section with common-mode choke, BNX025
filter and LT1963/LT3015 is working OK. I should probably build a
10Hz-100kHz LNA (e.g. [5]) to verify. I've used 2k at 100MHz ferrites a lot
and an RC-filter on all supply pins - maybe overkill?
- an undesired feature is gain-peaking which increases from output ch1 to
ch8, shown here: https://goo.gl/photos/6QkoKakSPDdT7Acj7
I tried to improve it a bit by adding a 100pF cap at the start of the long
trace that feeds the output stages, but some gain-peaking still remains:
https://goo.gl/photos/qrkLzZ21ptcHxFsw6
- reverse isolation around 120 dB
- channel-to-channel isolation around 80 dB
- at 10MHz 1dB compression between +14 and +15 dBm
- IP3 perhaps +27dBm to +30 dBm.

Any ideas on how to deal with the long 'feeder-trace' that seems to be the
cause of the gain-peaking?
Anyway if not used at 100MHz perhaps my next version will have reduced BW
where the feeder-trace is not an issue..
Another issue is that the voltage regulators get quite hot when fed at
+/-12V and producing +/-6V. They should probably be positioned as far away
from the input/output amps and thermally disconnected if possible. I have a
+/-12V AC/DC brick on order - but a DIY linear PSU producing e.g. +/-8VDC
for the regulators might be better.

The picture gallery also shows a pulse distribution amp for 1PPS. It has an
LT1711 comparator feeding an 74AC14 buffer with length-matched traces to
74AC04's at the outputs. So far my length-matching didn't give zero
output-skew between the outputs - I see around 150-200ps skew which I tried
to tune a bit with wires and 0R resistors - without very much success.. any
ideas for improving this - or just leave it at 200ps skew?

cheers,
Anders

[1]
http://www.anderswallin.net/2015/12/frequency-distribution-amplifier-first-tests/
[2]
http://www.anderswallin.net/2015/12/frequency-distribution-amplifier-v2-simulations/
[3]
http://www.anderswallin.net/2016/02/symmetricom-6502-distribution-amplifier/
[4]
http://www.anderswallin.net/2016/09/ettus-octoclock-distribution-amplifier/
[5] http://cds.linear.com/docs/en/application-note/an83f.pdf