[time-nuts] SMD TADD-1 distribution amplifier - seeking comments and suggestions?

Anders Wallin anders.e.e.wallin at gmail.com
Fri Dec 18 20:46:46 UTC 2015


Thanks for all the useful comments!
Things improved quite a bit just by wrapping the (insulated) board in
aluminium foil:
http://www.anderswallin.net/wp-content/uploads/2015/12/alufoil_and_battery.png

Op-Amps:
Maybe it wasn't clear enough on schematic, but I used the AD8055 (as in
TADD-1). The TL071 is there just because it comes standard with KiCad -
sorry for the confusion.
OPA827 doesn't have the slew-rate for +10 or +20 dBm at 10 MHz.
I was looking at ADA4899-1, but with the exposed pad it's not a direct
replacement and might require a new board.

Biasing:
I didn't understand why a "T" of resistors is better than a simple voltage
divider? The extra resistor is R3 in Bruce's second attachment.
What is the noise contribution of the biasing resistors?

Battery:
The lead acid battery was only used as a convenient DC source in the lab. I
would need to find a quiet SMPS or linear supply for permanent
installation.
Additional filtering on the 12VDC input might be still be required. Results
with the battery are good, so does that 'prove' the schematic is OK after
the linear regulator?

Isolation measurements:
Not done yet. I want to get one good channel first ;)
Would this be done by feeding a known amplitude RF (at 10.1 MHz or
something?) to an output-channel, and looking at the feedthrough to an
another output-channel or to the amplifier input with e.g. a spectrum
analyzer?

Far-out PN/AM is still 7dB short of the 6502!
Looks like an SNR issue to me, rather than some issue with the linear
regulator noise feeding through?!
AD8055 in non-inverting circuit with 1+2k7/2k7 gain has 9.6 nV/sqrt(Hz)
input-referred voltage noise PSD (if I calculated correctly..)
With an ADA4899-1 and lower value resistors I get about -4.5 dB improvement
to 3.4 nV/sqrt(Hz) input-referred
Could they run two or more amplifiers in parallel on the 6502 to get
1/sqrt(N) type improvements - or are there still cheap gains to be made
with my circuit?

thanks,
Anders


On Fri, Dec 18, 2015 at 6:32 PM, Pete Lancashire <pete at petelancashire.com>
wrote:

> When I used a Lead acid battery as a low noise and isolated power source I
> raided the kitchen. Put the batter inside
> a polyethylene container they type with a 'snap tight' lid. Then found in
> my junk a nylon barb to threaded fitting and
> some Tygon tubing to create an external vent. In another junk box a sealed
> MS socket and its mating plug.
>
> It solved three things venting to where it didn't matter, sealed becoming
> not sealed and puking and solving
> shorting from touching things.
>
>
>
>
>
> On Fri, Dec 18, 2015 at 7:22 AM, Chuck Harris <cfharris at erols.com> wrote:
>
> > One of my other hats involves advising electronics scrap and
> > recycling companies, and the repair of all manner of electronics
> > equipment.
> >
> > In all of the equipment I have rummaged through I can state the
> > following without reservation:
> >
> > I have never seen any sign of damage caused by properly float charged
> > sealed lead acid batteries.
> >
> > I have seen lots of serious damage caused by trickle charged nicads.
> >
> > I have seen some very serious damage caused by lithium iodide pacemaker
> > cells at end-of-life.
> >
> > I have seen lots of damage caused by carbon-zinc, and alkaline cells.
> >
> > Lithium ion cells behave like electrolytic capacitors.  They want to
> > have their initial inrush current limited to about 1C, and they must
> > have their final charge voltage limited to 4.2V.  They will tolerate
> > being floated at 4.2V for quite a while, but that will ultimately lead
> > to their capacity being compromised.
> >
> > A simple backup charger for a LiIon cell would be a constant voltage,
> > current roll back supply that is controlled by a timer that charges
> > the cell every time power is restored, and several times a year if
> > power doesn't fail.
> >
> > Discharge must be abruptly stopped when the cell voltage drops below
> > around 2V... the exact value varies by the manufacturer.
> >
> > -Chuck Harris
> >
> >
> > Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
> >
> >> On 17 Dec 2015 21:00, "Anders Wallin" <anders.e.e.wallin at gmail.com>
> >> wrote:
> >>
> >>>
> >>> First prototype assembled today, tested with 12 VDC SMPS wall-wart
> supply
> >>> and with 12 V lead-acid battery.
> >>>
> >>
> >> Anders
> >>>
> >>
> >> Is the lead acid battery supposed to be there so the unit continues to
> >> function if power is removed?
> >>
> >> If do, I believe that the choice of a lead acid battery is a poor one.
> I
> >> believe that even the sealed ones release very small amounts of
> sulphuric
> >> acid and when contained in equipment the acid results in damage in the
> >> long-term. I believe that people have reported damage to oscillators
> like
> >> the HP 10811A even on this list.
> >>
> >> I believe NiCd would be a better choice.  That said I somewhere read
> they
> >> were banned in Europe but that might have been for general consumer use,
> >> as
> >> I note that they are still shipped in some products - e.g. sone
> emergency
> >> lights I bought in the UK from Farnell,  although the lights were made
> in
> >> China. One can certainly still buy NiCd cells in Europe.
> >>
> >> I don't know if there is any simple way of slowly charging Lithium Metal
> >> Hydride batteries. Commercial chargers from reputable manufacturers have
> >> temperature sensors, voltage sensors and I assume a microprocessor to
> >> determine how to charge them and when to stop charging.  You don't need
> to
> >> fast-charge a distribution amplifier,  but I don't know if there's any
> >> relatively simple way of charging them.
> >>
> >> Dave.
> >>
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