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@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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.