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Justin Richards | 1 Dec 2011 02:46
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Re: [EE] Measuring characteristic impedance of coax cable

Is there an implied frequency when discussing characteristic impedance.
When we need to analyze cables with respect to their RF
characteristics we end up with a large table S parameters that have
been recorded over a range of frequencies.

So when a cable is quoted at 75 ohms, that must be for a given
frequency.  If so what is it.

Justin
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Tamas Rudnai | 1 Dec 2011 02:59
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Re: [OT] Question about the Netherlands

Hi Sean,

If a two hours journey is acceptable for you then you can also thinking
about getting your tools in Germany (Bremen, Dusseldorf, Hannover etc are
all in a reachable distance, especially if you have a very fast car as you
can drive as fast as you want on a German motorway :-) ) Within Europe you
do not need to pay customs and you use the single currency so it does not
really matter where you find the shop.

Tamas

On 30 November 2011 20:42, Sean Breheny <shb7@...> wrote:

> Hi all,
>
> I have a question for all the Dutch piclisters :)
>
> Some of my colleagues are going to the Netherlands to install some
> equipment at a customer site. They are going to have to purchase some
> tools locally in order to do this work. They will need at least some
> regular hand drills as well as an air compressor with a fairly large
> tank (maybe 100 Liters), and a hammer drill (for inserting concrete
> anchors). Renting is a possibility but they would prefer to buy as
> they will be back again a few times over the next year and they would
> need the same tools again and they can store them on-site.
>
> This site is in Almelo. We can either buy these items at a physical
> local shop or we can buy them online and have them shipped. Can anyone
> advise me on either some shops in the Almelo area which would see
> these tools or on an online vendor who sells these tools (compatible
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Harold Hallikainen | 1 Dec 2011 03:20

Re: [EE] Measuring characteristic impedance of coax cable

Ideally characteristic impedance is independent of frequency. In reality,
it may vary some with frequency, but I don't think the variation is
substantial. The attenuation can vary substantially with frequency. I keep
thinking back to Smith charts. Also, I did some stuff with Hyperlynx
recently. It's pretty neat. It looks at traces on a board layout and
models them as transmission lines. It then shows a series string of
transmission lines representing the trace(s) and lets you look at pulse
waveforms at various points on the trace.

Harold

> Is there an implied frequency when discussing characteristic impedance.
> When we need to analyze cables with respect to their RF
> characteristics we end up with a large table S parameters that have
> been recorded over a range of frequencies.
>
> So when a cable is quoted at 75 ohms, that must be for a given
> frequency.  If so what is it.
>
> Justin
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> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

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peter green | 1 Dec 2011 03:30

Re: [EE] Measuring characteristic impedance of coax cable

Justin Richards wrote:
> Is there an implied frequency when discussing characteristic impedance.
> When we need to analyze cables with respect to their RF
> characteristics we end up with a large table S parameters that have
> been recorded over a range of frequencies.
>
> So when a cable is quoted at 75 ohms, that must be for a given
> frequency.  If so what is it.
>   
An ideal coaxial cable (made up of perfect conductors and perfect 
dielectrics in a perfect geometery) will have a constant characteristic 
impedance across all frequencies. I'm not sure on the details but my 
understanding is that if the characterstic impedance is changing 
significantly with frequency you are probablly above the cable's usable 
frequency range.

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Oli Glaser | 1 Dec 2011 06:18

[EE] Obscure screw size

Hi all,

I have a Hammond 1551R box into which fits a small PCB.
The recommended shape of the PCB is given in the datasheet:
(http://www.hammondmfg.com/pdf/1551R.pdf)
I have received the PCB and it fits perfectly, but the Hammond box only 
came with the 2 screw for the lid and none for the PCB mount holes. The 
sizes specified are 2.5mm for the PCB hole diameter, but the screws are 
given as 32 x 3/16" self tapping.
So the question is - what does this translate to in metric sizes? I get 
the 3/16" is probably the length, but what does 32 stand for? Is it for 
a 1/32" thread?
I have trawled through pages and pages of all the many different 
"standards" - Whitworth, BAS, UTS, etc but I can't fit 32 into anything 
that seems to fit properly.
The UTS (Hammond are US so I assumed this would be the most likely) is 
specified as X-Y where X is the diameter and Y is the thread turns per 
inch. 1/32 would give a diameter of 0.8mm and 3/16 means nothing in 
terms of turns per inch. Even if you swap them round it doesn't make sense.
Whilst linking to the above datasheet I noticed it's actually different 
from the one I have, and gives #2 x 3/16" instead of the aforementioned 
32 x 3/16". This makes more sense as #2 equates to 2.18mm diameter 
shank, but I'm still unsure as to the thread (not that it matters too 
much as it's self tapping, but I'd like to know anyway)
I'm going to order some number 2 screws, but can anyone help me to 
confirm exactly what the numbers mean for future reference?

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William Wilson | 1 Dec 2011 06:43
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RE: [EE] Obscure screw size

In imperial units the 2.5mm screw would be a 3/32 inch diameter screw
with 32 threads per inch and 3/16" long.

Metric equivalent is  M2.5x0.45 4.5mm long

_____________________________________________________________________________
Cris Wilson
Information Resource Consultant
College of Architecture, Arts, and Humanities
Clemson University
864-656-6081
________________________________________
From: piclist-bounces@... [piclist-bounces <at> mit.edu] On Behalf Of Oli
Glaser [oli.glaser <at> talktalk.net]
Sent: Thursday, December 01, 2011 12:18 AM
To: Microcontroller discussion list - Public.
Subject: [EE] Obscure screw size

Hi all,

I have a Hammond 1551R box into which fits a small PCB.
The recommended shape of the PCB is given in the datasheet:
(http://www.hammondmfg.com/pdf/1551R.pdf)
I have received the PCB and it fits perfectly, but the Hammond box only
came with the 2 screw for the lid and none for the PCB mount holes. The
sizes specified are 2.5mm for the PCB hole diameter, but the screws are
given as 32 x 3/16" self tapping.
So the question is - what does this translate to in metric sizes? I get
the 3/16" is probably the length, but what does 32 stand for? Is it for
a 1/32" thread?
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Oli Glaser | 1 Dec 2011 07:24

Re: [EE] Obscure screw size

On 01/12/2011 05:43, William Wilson wrote:
> In imperial units the 2.5mm screw would be a 3/32 inch diameter screw
> with 32 threads per inch and 3/16" long.
>
> Metric equivalent is  M2.5x0.45 4.5mm long

Thanks, that's very helpful. So the 32 is tpi and the 3/16" is length?
Trouble is my PCB holes are the recommended 2.5mm, so I'll need a bit 
smaller I guess. I'll get the number 2 imperial which by my calculations 
is 2.18mm.
Couple more questions:
How can we be sure of the diameter when only given 32 x 3/16" for the 
screw size?
What standard does this comply with?
Is there any decent reference out there that explains this?

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Oli Glaser | 1 Dec 2011 07:33

Re: [EE] Obscure screw size

I forgot to mention that on Mouser, I can only find one screw (out of 
1204) with a 32 thread size and it's not the right length.
On Digikey I get nothing either. there are 6 results for 3/16" but they 
are either 6/32" 8/32" or 4/40".
Is this screw size really that hard to find or am I missing something?

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Spehro Pefhany | 1 Dec 2011 08:18
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Re: [EE] Obscure screw size

At 12:18 AM 12/1/2011, you wrote:
>Hi all,
>
>I have a Hammond 1551R box into which fits a small PCB.
>The recommended shape of the PCB is given in the datasheet:
>(http://www.hammondmfg.com/pdf/1551R.pdf)
>I have received the PCB and it fits perfectly, but the Hammond box only
>came with the 2 screw for the lid and none for the PCB mount holes. The
>sizes specified are 2.5mm for the PCB hole diameter, but the screws are
>given as 32 x 3/16" self tapping.
>So the question is - what does this translate to in metric sizes? I get
>the 3/16" is probably the length, but what does 32 stand for? Is it for
>a 1/32" thread?

I don't see a "32" on that drawing, I see #2 x 3/16"  32 threads per
inch (North American screws use the reciprocal of the pitch) is certainly
plausible- typically this type of screw has a much coarser pitch than the
equivalent diameter machine screw.

In any case, this would be approximately a M1.8 Thread Forming fastener,
also called K18, but you'd have to verify the size to be sure. Pitch at
0.8mm is about right.

Here's one maker, along with some design rules for various plastics:-
http://www.semblex.com/files/PT-Semblex.pdf

Also note the Hammond part number 1551ATS100 for 100 pieces of screw,
about 6 cents each.. if you don't feel like screwing around with substitutes.

I would not be particularly gobsmacked if they were actually metric--
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Richard Prosser | 1 Dec 2011 08:26
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Re: [EE] Measuring characteristic impedance of coax cable

On 1 December 2011 15:30, peter green <plugwash@...> wrote:
> Justin Richards wrote:
>> Is there an implied frequency when discussing characteristic impedance.
>> When we need to analyze cables with respect to their RF
>> characteristics we end up with a large table S parameters that have
>> been recorded over a range of frequencies.
>>
>> So when a cable is quoted at 75 ohms, that must be for a given
>> frequency.  If so what is it.
>>
> An ideal coaxial cable (made up of perfect conductors and perfect
> dielectrics in a perfect geometery) will have a constant characteristic
> impedance across all frequencies. I'm not sure on the details but my
> understanding is that if the characterstic impedance is changing
> significantly with frequency you are probablly above the cable's usable
> frequency range.
>

Justin,

Cable characteristics are normally referred to at a standard
frequency, depending on cable likely application. 200MHz, 400MHz and
1GHz being typical but impedance should be pretty stable above a few
MHz.

Mathematically the impedance is give by Z=sqrt((R+jWL)/(G+jWC))
Since G is normally close to zero in most cases this reduces to
sqrt(L/C) at high frequencies - where WL is >> R.
(W = 2 * PI * f)
.
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