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

Bob Camp kb8tq at n1k.org
Sat Jan 28 11:57:38 EST 2017


The real question is: Do you have an application where < 100 ps matching matters? If so do you 
need to match both at the amplifier *and* at the ends of the cables? 

Other than a phased array radar, I can’t think of to many situations where the answer is yes …
Put another way, for the normal stuff we do, it is not a significant issue. If you know the offset
you can take it out in any calculations where it might matter. 


> On Jan 28, 2017, at 6:58 AM, Anders Wallin <anders.e.e.wallin at gmail.com> wrote:
> 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
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