[time-nuts] Don't use cheap cables -- a cautionary tale (BobCamp)

Tue Mar 5 13:06:17 EST 2013

Hi

Not quite sure how my name got into the thread title...

Coverage numbers aren't all they might be. A simple aluminum foil wrap lets
a vendor claim 100% coverage. The skin depth of the foil is near zero, so
it's not a big help shield wise at low frequencies. Same thing with labels,
just about any fly by night outfit can write nearly anything on a cable.  

Skin depth at 1 MHz is about 0.1 mm (4 mils). At that point, shielding is
compromised. Depending on cable length / required isolation / optical
coverage you might want 5 or even 10 skin depths. It's a bit tough to find a
flexible lab cable with 1 mm (40 mil) shielding on it.

As you go down in frequency, at some point your only real solution is a
twisted pair. Before that point, multiple isolated shields can help (good
luck with connectors). At 1 Hz (1 pps ..) you would need 0.1 m of shield to
get to one skin depth. That's not a cable I have in stock... 

Bob  

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Gregory Muir
Sent: Tuesday, March 05, 2013 12:08 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Don't use cheap cables -- a cautionary tale
(BobCamp)

I'm weighing in a little late on this one but wanted to make a few comments 
regarding cables and connectors in hopes that a prior poster may not have 
covered these thoughts.

With regards to single-shield coaxial cables, I think we are all aware that 
shielding effectiveness can vary widely between cables of the same type.  I 
have seen cable shielding as bad as 50% and as good approaching 90%.  Of 
course, anything below 100% means that they are leaky - for either inward or

outward bound signals.  But one little cautionary indication as to the 
quality of the cable and its shielding is stamped on the cable itself.  If 
you see a description saying RG-xx "Type," you are basically dealing with a 
cable of which the manufacturer is saying that it is like that specific type

of cable but does not necessarily meet the specifications one would expect. 
So, if you want a cable to meet a certain quality, use a cable without the 
"Type" designation.

In your posting, you mentioned the words "radio room."  That in itself is a 
red flag meaning possible sources of strong RF.  In situations like that you

must make sure that any circuits requiring good isolation from other 
interfering sources must contain adequate shielding be it circuit- or 
cable-wise.  I think we all are aware of that consideration.  I have seen 
instances of strong enough RF sources actually setting up circulating 
currents in adjacent coaxial cable shields that couples the stray power into

the innocent cable and also affects the impedance characteristics of same. 
This has also been seen in the larger solid copper transmission lines as 
well as microwave waveguides.

With regards to connector selection, during my life in Colorado working with

the US government, I once attended a RF connector course at the (then) NBS. 
It was quite an eye opener with regards to the connect-disconnect cycle life

of the typical coaxial cable connector.  If you want a reliable connection, 
don't start with a worn connector.  And the choice of manufacturer of the 
connector can be as important as well with regards to the construction and 
plating materials used.  When I compare the typical easily-obtained "Jameco"

or "Marlin P Jones" type of connector to a better made product such as a 
true MIL item on a network analyzer, one can see a noticeable difference 
between the impedance and VSWR characteristics. It has been long known that 
the simple nickel-plate connector can cause problems over time in an 
installation from changing contact resistance and such.  The manufacturer 
"RF Industries" still uses silver & gold plating on their types of RF 
connectors as compared to the cheaper nickel-plated types and I have found 
these to be very reliable with a cost slightly higher than the cheaper 
nickel plated types.

Greg

>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 4 Mar 2013 07:28:19 -0500
> From: Bob Camp <lists at rtty.us>
> To: Discussion of precise time and frequency measurement
> <time-nuts at febo.com>
> Subject: Re: [time-nuts] Don't use cheap cables -- a cautionary tale
> Message-ID: <7AD45AD9-6965-4F7A-B683-D75902992FAF at rtty.us>
> Content-Type: text/plain; charset=us-ascii
>
> Hi
>
> Termination is important. An open cable typically reflects energy down 
> both the inside and outside of the shield. I'd bet the same thing is true 
> to a much lesser extent of an open T connector.
>
> The list of nasties is quite long, so there is no one magic thing that 
> fixes all problems.
>
> Bob
>
> On Mar 3, 2013, at 10:36 PM, Mark Spencer <mspencer12345 at yahoo.ca> wrote:
>
>> This is a useful thread IMHO.
>>
>> Re the continuous beat note interference issue, I believe I've 
>> encountered this when evaluating a Datum1000B.   At first I saw a 
>> periodic change in frequency of several E-10, the typical period was 
>> several hundred seconds.   Turning off all the un needed gear in my lab 
>> except  for a few ocxo's that i don't want to turn off and using double 
>> shielded RG400 cables without adapters for all the interconnections 
>> seemed to make the issue go away.   All the outputs from the un used 
>> ocxo's were also terminated with bnc or sma terminators.    Even the BNC 
>> T connectors I typically leave connected to the inputs of my HP5370B's 
>> (along with 50 ohm terminators) seemed to cause issues in this 
>> application.
>>
>> This issue has also prompted me to give up on my plans to move my GPSDO's

>> from my radio room to my lab, as it's nice to be able to leave the 
>> GPSDO's running into a terminator vs having to shut them off.
>>
>> I've never really put much effort into tracking down the root cause of 
>> this issue but I suspect it is similar to what John mentioned.
>>
>> My FTS1050 (which IIRC is based on a datum 1000) doesn't seem to have 
>> this issue, building enclosures for my Datum1000's is on my post 
>> retirement to do list as I suspect running them without an enclosure may 
>> be contributing to this problem.
>>
>> As far as I know my BVA Ocxo is immune from this issue as well.
>>
>> Regards Mark Spencer

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