Discussion:
Source for DC-blocking 50-ohm terminators? (Spectracom 8140)
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paul swed
2018-06-04 22:56:39 UTC
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Julien
Thanks for the link I downloaded the manual and it all makes sense now.
Yes just lift the inductors to clear the DC. Its is actually for line
powered amplifiers as someone mentioned.
You would of thought there would be a power jumper per BNC.
Good luck.
Paul
wb8tsl

On Mon, Jun 4, 2018 at 1:50 PM, Taka Kamiya via time-nuts <
time-***@febo.com> wrote:

> I had the same issue looking for a DC blocking 50ohm terminator. They
> simply do not exist. So I just made one. Took a BNC male connector with a
> chassis mount flange. 50ohm and 0.015 microfarad capacitor in series.
> (because that's what I had on my desk at the time) The frequency is low
> enough, level is high enough, and some reflections are not going to cause
> any issues. It has been working for me for quite some time.
> ---------------------------------------
> (Mr.) Taka Kamiya
> I'm stuck in a wormhole.... Hello, worms!
>
> On ‎Monday‎, ‎June‎ ‎4‎, ‎2018‎ ‎12‎:‎10‎:‎15‎ ‎AM‎ ‎EDT, Julien
> Goodwin <time-***@studio442.com.au> wrote:
>
> I'm looking at using a Spectracom 8140 for 10Mhz distribution, and they
> specify using a DC-blocking 50-ohm terminator on each run.
>
> These seem to be odd enough that none of my usual sources have them
> (75-ohm DC-blocking terminators yes, 50-ohm, no).
>
> While it's easy enough to chain a DC block and a terminator, I'd prefer
> a single module I can more obviously label as the line termination.
> _______________________________________________
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paul swed
2018-06-04 14:56:34 UTC
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Yes I would say you have to build it.
The only reason make such a requirement is if the amplifier is not using a
split power supply. But instead is a single supply with the output at 50%
of the supply voltage. It could be very reasonable if what I suggest is
true to open the amp and install the caps within it.
Just a thought.
Regards
Paul
wb8tsl

On Mon, Jun 4, 2018 at 10:06 AM, Julien Goodwin <time-***@studio442.com.au>
wrote:

> Making my own that fits in either the mini Pomona boxes (which has
> BNCs)[1] or the nice really tiny Crystek SMA housings[2] with a BNC to
> SMA looks like it's probably the sensible option.
>
> 1: https://www.pomonaelectronics.com/products/boxes/boxes-with-connectors
>
> 2: http://www.crystek.com/microwave/spec-sheets/
> enclosure/SMA-KIT-1.3MF.pdf
>
> On 04/06/18 22:52, Bob kb8tq wrote:
> > Hi
> >
> > DC blocked 50 ohm terminations are indeed a bit odd. One answer is to
> simply use the
> > 75 ohm version and move on. The gotcha there is that they may / may not
> have a block
> > that works well at 10 MHz ( = the cap is to small ). The one thing *not*
> to do is to run the
> > device with no termination on the cable. The gizmo will be perfectly
> happy. Your lab will
> > be a mess. The coax will normally be long enough to make a great antenna
> at 10 MHz.
> > Leaving the end un-terminated puts it in “antenna mode”. You will have
> RF all over everything.
> > I have a lot of experience with this specific problem …..
> >
> > Bob
> >
> >> On Jun 4, 2018, at 12:09 AM, Julien Goodwin <time-***@studio442.com.au>
> wrote:
> >>
> >> I'm looking at using a Spectracom 8140 for 10Mhz distribution, and they
> >> specify using a DC-blocking 50-ohm terminator on each run.
> >>
> >> These seem to be odd enough that none of my usual sources have them
> >> (75-ohm DC-blocking terminators yes, 50-ohm, no).
> >>
> >> While it's easy enough to chain a DC block and a terminator, I'd prefer
> >> a single module I can more obviously label as the line termination.
> >> _______________________________________________
> >> time-nuts mailing list -- time-***@febo.com
> >> To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> >> and follow the instructions there.
> >
> > _______________________________________________
> > time-nuts mailing list -- time-***@febo.com
> > To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> > and follow the instructions there.
> >
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Bob kb8tq
2018-06-04 16:36:03 UTC
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Hi

Without the taps, the Spectracom system really does not do much for you at all. There is no
isolation / no buffering / no nice multiple output frequency stuff. Much cheaper and easier to
just use a simple 10 MHz amp if that is all you want to do.

Since you have significant voltage on the line, just dropping a 50 ohm load on there is a bad idea ….

Bob

> On Jun 4, 2018, at 11:24 AM, Arthur Dent <***@gmail.com> wrote:
>
> If you're not going to use the Spectracom taps that are the
> reason for the D.C. being on the coax, you can easily remove
> the inductors used to superimpose the D.C. on the coax lines.
> Without the 12V.D.C. on the outputs you can use standard 50
> ohm terminations. The open toroids are reachable if you take
> out a few screws to tip the back panel out and clip the leads
> to remove the parts.Using the 8140 as a DA without disabling
> the 12V.D.C. is a good way to fry some piece of test gear.
> Here is a photo of the schematic.
>
> -Arthur
> <8140 output.jpg>_______________________________________________
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Dave M
2018-06-04 15:43:37 UTC
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Julien,
I have a Spectracom 8140 also, and have the same problem. I found a couple
of the 10 MHz Line Taps on Ebay at reasonable price, however, I would like
to eliminate them. The +12C at the 10MHz outputs is the power for the line
taps, which are meant to drive long distribution lines in a plant.
If you don't have a copy of the manual for the 8140, you can download it
from Spectracom's web site
(https://spectracom.com/sites/default/files/document-files/8140_manual.pdf).
Look at the schematic, you can see that you can remove the inductors
associated with the outputs (L9, L10, L11 and L12), and the DC will be gone
from the outputs. Much easier and cheaper than DC blocks, and safer for
your equipment in case you forget to install a DC block..

Dave M


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[time-nuts] Source for DC-blocking 50-ohm terminators? (Spectracom 8140)
Julien Goodwin Sun, 03 Jun 2018 21:10:45 -0700

I'm looking at using a Spectracom 8140 for 10Mhz distribution, and they
specify using a DC-blocking 50-ohm terminator on each run.These seem to be
odd enough that none of my usual sources have them
(75-ohm DC-blocking terminators yes, 50-ohm, no).

While it's easy enough to chain a DC block and a terminator, I'd prefer
a single module I can more obviously label as the line termination.
_______________________________________________
time-nuts mailing list -- time-***@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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