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

[Magnus Danielson]

Transformer isolation isn't helping much at RF, as you will capacitively 
couple through the transformer. I've been bitten by that in real life, 
as I was called in to solve issues in someone elses design. It was only 
when I introduced an RF choke that we got conducted noise battled. It's 
also not enough, as the RF choke needs an RF path to ground in order to 
start rejecting effectively, which was the issue another time, so you 
want an RF choke with caps to ground on the inside.

The galvanic isolation can be done using transformer or capacitors after 
that.

There is an over believe in isolation, as it only takes one mistake to 
break the system. Another approach is to ground everything, cross-ground 
etc. and bring the DC/power-spurs down through conduction. It have 
proven itself easier to ensure RF properties when shield and chassi is 
tied hard to each other, as it provides good RF conduction and the cable 
does not act like an antenna against the shield for the RF power being 
unbalanced. The RF choke then acts to separate the chassi RF from that 
of the board, assisting in the balance.

Transformers can provide RF shielding, if they have double shields 
between the coils, and where the shield of each side is connected to 
it's ground. That way each coil will capacitively terminate in it's own 
shield, and the remaining capacitive coupling will mainly be between the 
shields and hence grounds. I rarely see people doing this.

I've been bitten multiple times by the capacitive coupling in 
transformers, and only when I found a way to handle it things have 
started to work. It's not all magnetics.

Cheers,
Magnus

On 12/19/2015 12:33 AM, Tim Shoppa wrote:
> All the inputs and outputs were deliberately transformer isolated. Why
> break the isolation by using capacitor from coax shield to chassis ground?
>
> I do realize that some isolation transformers have "extra floating turns"
> to give transformer action that cancels stray capacitive coupling. I don't
> think the capacitors tying coax shield to chassis ground can serve that
> purpose.
>
> Tim N3QE
>
> On Mon, Nov 30, 2015 at 3:02 PM, Anders Wallin <anders.e.e.wallin at gmail.com>
> wrote:
>
>> HI all,
>> I need to build a few distribution amplifiers (>90% for 10MHz, sometimes
>> maybe 5MHz) and instead of reinventing the wheel I decided to try to
>> modernize the TADD-1 into an all (almost) SMD design. Here are some draft
>> sketches:
>>
>> http://www.anderswallin.net/2015/11/frequency-distribution-amplifier-plans-a-k-a-smd-tadd-1/
>>
>> Does this sound/look reasonable or crazy?
>> Any suggestions for op-amps to try and/or compare to the AD8055?
>> What causes the extra phase-noise below 1 Hz offset in John A's result:
>> https://www.febo.com/pages/amplifier_phase_noise/amplifier_phase_noise.png
>>
>> Suggestions for a low noise DC-regulator circuit? The 12-24VDC supplied to
>> this board will most likely come from a switched-mode PSU, so filtering of
>> common-mode noise is mandatory.
>> I found the TI LP38798 shown in the schematic by googling - if someone has
>> a proven a measured design that would be a safer choice. In any case more
>> filtering (e.g. ferriites) is probably a good idea.
>>
>> This design will be available on my blog or on github when it is done - if
>> anyone is interested.
>>
>> Thanks,
>> Anders
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