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

Fri Dec 18 22:30:23 UTC 2015

On Friday, December 18, 2015 10:46:46 PM Anders Wallin wrote:
> 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.p
> ng
> 
> 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.

The OPA827 was merely used to illustrate the preferred gain setting and 
biasing architecture not the opamp to be used.

> 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?

T configuration   allows bypassing of residual power supply noise.
Divider only allows minimal filtering via the coupling capacitor.
You could also bias the bottom end of the transformer secondary and directly 
connect the noninverting input of the input amp to the other end of the 
transformer secondary.

> 
> 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 +10dBm input the corresponding SSB PN floor should be  around -163dBc/Hz.

> With an ADA4899-1 and lower value resistors I get about -4.5 dB improvement
> to 3.4 nV/sqrt(Hz) input-referred

Corresponds to an SSB PN noise floor of around -168dBc/Hz.

> 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?
> 

Try using an input stepup transformer eg 1:2 (voltage ratio) perhaps with a 
centre tapped secondary to allow use with higher input signal levels via tap 
selection. The use of a stepup input transformer reduces the required amplifier 
gain as well as reducing its effective input noise. As long as the input 
currentt noise isnt excessive the SSB PN floor should be reduced by about 
6dBc/Hz.

> thanks,
> Anders
> 

Bruce

> 
> 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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