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

Bob Camp kb8tq at n1k.org
Sat Dec 19 09:56:25 EST 2015


Hi

Another way to look at coax ….

You can (easily) have signals flowing on the *outside* of the shield. In an ideal world with perfect coax outside would be
outside and inside inside. Ideally the two signal sets would never interact. Once you put an isolated transformer on the end
of the cable, the “outside” signal shows up on the capacitive coupling. Since you are open circuit terminating the outer current,
you likely have a voltage peak at this point. Maximizing voltage on the capacitor pretty much guarantees maximum signal 
transfer. Since the transformer now has both the “inside” and “outside” signals on it, you have a problem. 

One solution to this is a common mode choke (the other term for the “outside” signal). Ferrite lumps are one way to do this, there
are other ways (coil up the cable). The idea here is to provide a high(er) impedance to the signal you do not want. In the case of 
ferrites, you may be able to provide a resistive component and convert them into heat. Just as you can buy cheap little RF transformers
these days, you can also buy cheap little common mode chokes as well. The down side is that they are designed for VHF and up 
type frequencies (cell phones …) rather than 10 MHz. For lower frequencies you generally are stuck with winding a number of turns
on a cheap core. The only way I know to get cores cheap is to buy them by the pound. That makes the design more of a “use what you 
have” deal than anything else.

Bob


> On 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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