Hi

Even if you are running 50 ohm loads, with 4 outputs, a power splitter is a very real solution. You will be down 6 db on each output. If you start with +13 you will have +7 dbm. Your target devices probably are happy as can be with anything over +3 dbm.

If you are worrying about WWVB problems, don’t just look at the cables. Check the connectors as well. BNC’s loose shielding capability much quicker than they get to a high insertion loss. I’ve had a lot of grief from them over the years.

Bob

Hi

You can also discover interesting things about the transformer it’s self in some circuits. They do what they do, but they also bring along a new set of issues. Sometimes simpler is better. If you don’t need what the transformer does, why add more complications ?

If you don’t lift the ground, the transformer isn’t doing much for you isolation wise.

If you do lift the ground, you need a lot more than just the transformer to make it all work and isolate properly. Once you add all that “stuff” you don’t have a simple circuit anymore.

——————

One example:

The most likely “threat” to your distribution system is a cell or portable phone. The beast fires up at random times and spews RF around everywhere. That’s the signal you want to isolate and shield against. It’s at some frequency between 40 MHz and 6 GHz depending on what it is. What ever you do needs to work over that range for isolation / shielding. It also needs to pass 10 MHz. Don’t have a phone in sight? How about WiFi, Bluetooth, and RF based remote controls ….

Why is it a threat? The RF comes back into your amp and changes bias levels / inter modulates with the 10 MHz. Either way, your 10 MHz moves around a bit. Is this purely theoretical? Nope, it happens far more often than you’d think. I’ve seen it a number of times.

A simple transformer at 10 MHz is not going to retain balance and isolation over 40MHz to 6 GHz. Even if it could the coax connector will imbalance it pretty badly. No balance = no isolation.

Good cables and a passive splitter have their issues, so do logic gates. Both have their advantages. More complicated is not always better.

23 dBm out of a KS box going down to +7 out gives you 16 db of room for attenuation. At 3 db for each 2:1 split, that’s 2^5 splits. It’s rare to find a need for 32 outputs. You can get 8 outputs at +13 dbm. That’s pretty hot for a distribution amp output.

Bob

Yep.  I found a lot of RF related issues appeared to go away when I switched to double shielded cables, stopped using T connectors for distributing signals, purchased and used HP distribution amps, terminated un used ports with 50 ohm loads etc.

I also found ferrite cores applied on the outside of longer cables to be helpful as well.  (I found it was useful to measure the actual currents flowing in the cable shields before and after adding the ferrite cores.)

I still have a couple of issues that occur when I occasionally transmit at the 300 watt level on VHF but the day to day issues where by WWV on 5 and 10 Mhz was hard to hear at times due to signal leakage and many low power transmissions could be counted on to influence a time lab frequency plot are gone now.

I expect issues are still there but they are below the detection threshold for me.  My time nuts gear and radio gear are separate but still in the same house.

Mark Spencer

On 2014-11-27, at 1:47 PM, Bob Camp kb8tq@n1k.org wrote:

Hi

If you are really going to get everything “tight” at RF, solid grounds (properly done) and good shielding (well thought out)  are going to do more for you than a lot of messing with circuits. Common mode chokes (outside cores) are way more likely to help than transformers.

That assumes you are after RF. If you have a 60Hz problem (due to the mega power AC welder) you may need to approach things a bit differently.

Bob

John wrote:

You certainly need the shield grounded at RF, but you don't
necessarily need it grounded all the way down to DC.  A fairly common
solution that works well for many applications is to use isolated
connectors (not connected directly to the chassis) with a 10nF
capacitor from each connector shield to the chassis immediately
adjacent to the connector.  The capacitor grounds the shield at RF
but allows very little current to flow at the mains frequency.  NOTE:
You want the bypass capacitor right at the connector, to make sure
the RFI current loop is tiny and cannot radiate.

That said, I often build distribution amplifiers with
chassis-grounded connectors and have never had ground loop
problems.  But then, I pretty much always design transformer-isolated
DC supplies with low-field transformers and linear regulators into
everything I build, and do not use wall-warts, desk-warts, or
switching supplies.

If one were building the dist amp I posted the other day and wanted
to use chassis-grounded connectors, the per-stage transformers could
be simplified from 1:1:1 to 1:1 -- omitting the third winding -- with
the transistor collectors capacitor-coupled to the output
connectors.  In this case, I would put ~100k resistors across each
output to hold the connector ends of the capacitors at 0v even with
an open load.

For isolation amplifiers used with mixer-type frequency comparators,
which can be very sensitive to ground loops because of the low mixer
output frequency, I do use RF transformers and bypass the shields to chassis.

Hear, hear.

Best regards.

Charles

Thanks for the kind words, Dick. :-)

However, a couple of notes on the TADD-1 bandpass filter:

1. I don't recommend installing it unless there is a good reason.  As
   others have noted, any tuned circuit is a thermometer and will degrade
   ADEV performance.  Other than the original test unit, I haven't built
   the filter into any of the several TADD-1s I've deployed in my lab.

2. The component values for the example filter in the TADD-1 manual are
   wrong.  I haven't gotten around to fixing that, mainly because of point 1.

And since the topic of isolated grounds has come up: the TADD-1 uses
transformer isolation on each input and output, and provides DC
isolation by putting a 10nf cap between ground and the coax shield (with
a very short path).  This seems to have worked pretty well for most users.

John

On 11/27/2014 11:41 AM, Richard Solomon wrote:

No problem, always glad to be of help to those in need.

73 es HH, Dick, W1KSZ

On 12/2/2014 2:27 PM, John Ackermann N8UR wrote: