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

Bob Camp kb8tq at n1k.org
Sat Dec 19 12:27:53 EST 2015


Hi

There is a very significant difference between coax and twisted pair when it comes
to magnetic induction. The twist “cancels out” the signal on the pair. The shield has
the signal induced on it’s outer surface. Transformers work better on twisted pair than
on coax. If you look at a “normal” ethernet twisted pair interface (magnetics) you will 
likely see both a transformer and a common mode choke inside. That of course assumes
that the schematic is showing you what really is in there. Even with twisted pair the 
choke helps.

Bob


> On Dec 19, 2015, at 9:23 AM, Tim Shoppa <tshoppa at gmail.com> wrote:
> 
> I think there is a valid heritage in transformer isolation in time and
> frequency distribution, and it goes back to when telephone wiring was used
> to distribute audio-type IRIG signals around a campus or other facility.
> Even if a bunch of 60Hz or a local AM station was leaking through the IRIG
> signaling was quite impervious to it. (Heh, the aircraft VHF radio getting
> into Spinal Tap's lead guitar was hardly noticeable at that air force base,
> for that matter!!!)
> 
> But something feels "off" with lifting grounds on coax if the environment
> is just a test lab.
> 
> CAT 5/6 and Ethernet transformers work great at 10MHz but most all test
> equipment is expecting coax and a BNC.
> 
> Tim N3QE
> 
> On Sat, Dec 19, 2015 at 8:29 AM, Magnus Danielson <
> magnus at rubidium.dyndns.org> wrote:
> 
>> 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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