Question About the CP-1 Coupler

[Paul T. Rubin]

Hi All:

 

I am not sure I understand the procedure for terminating the sampling ports.
If a port is unused it says to terminate it into the internal 50 ohm load to
maintain balance. This makes sense. But if I connect a port (or ports)
through a short piece of 50 ohm coax to a scope which has a high impedance
input, is this port properly terminated?

 

Thanks,

Paul N8NOV

Houston

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[Ron D'Eau Claire-2]

With a high impedance input to the scope, yes. Some scopes offer a 50-ohm
termination at the scope as well. If you use that, you must not use the 50
ohm termination at the CP1. Be careful though. Many built-in terminations in
equipment like an oscilloscope has a very limited power handling capability.
My scope limits the input voltage to 5V RMS when using its internal 50 ohm
termination.

I use my 10:1 isolation probes with the scope so the CP1's 50 ohm
termination is perfect. Actually, I'd have preferred they provide a test
point for such probes, but hooking them to the "hot" end of the 50 ohm
termination resistor on the CP1 board words too...

Ron AC7AC

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Paul T. Rubin
Sent: Monday, October 02, 2006 4:45 PM
To: [hidden email]
Subject: [Elecraft] Question About the CP-1 Coupler


Hi All:

 

I am not sure I understand the procedure for terminating the sampling ports.
If a port is unused it says to terminate it into the internal 50 ohm load to
maintain balance. This makes sense. But if I connect a port (or ports)
through a short piece of 50 ohm coax to a scope which has a high impedance
input, is this port properly terminated?

 

Thanks,

Paul N8NOV

Houston

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[N8LP]

If the scope is Hi-Z, DO NOT terminate the coupler internally. Instead,
use a BNC Tee and terminate at the scope input with a 50 ohm
termination. Otherwise, the feedline to the scope will provide a
reactive load to the coupler port.

Larry N8LP


Paul T. Rubin wrote:

> Hi All:
>
>  
>
> I am not sure I understand the procedure for terminating the sampling ports.
> If a port is unused it says to terminate it into the internal 50 ohm load to
> maintain balance. This makes sense. But if I connect a port (or ports)
> through a short piece of 50 ohm coax to a scope which has a high impedance
> input, is this port properly terminated?
>
>  
>
> Thanks,
>
> Paul N8NOV
>
> Houston
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft   
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>
>
>
>  

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[Don Wilhelm-3]

Paul,

If you use a 50 ohm coax between the CP1 and the 'scope, you should use a 50
ohm termination right at the 'scope to achieve proper patterns and voltage
readings on the 'scope - the in-line type terminators work best in this
situation, and you would not use the CP1 built-in termination for this
situation since the termination is at the 'scope input.

A good method IMHO is to use a 10X probe (properly calibrated) on the 'scope
and place the termination right at the place where the scope probe is
placed - remember that this only works correctly with a 10x probe on the
scope.  If you place the 10x probe across the CP1 50 ohm terminator (with
the built-in terminator switched in), then the 'scope should read correctly.

The 'rule of thumb' (if I can generalize) in all cases of RF power (or
voltage) measurement is to attach the termination at the far end of any coax
used for the connection(s) - if the terminations is applied at any other
point in the system then erroneous readings are likely to result.  This is
just like antenna feedline considerations - if you connect an unterminated
chunk of feedline to an antenna, it will act as an inductor or capacitor
(depending on the length) and modify the voltage at the attachment point as
well as at the far end of the stub.  When doing RF measurements be certain
you have not created random stubs in the measurement connections.

73,
Don W3FPR

> -----Original Message-----
>
> I am not sure I understand the procedure for terminating the
> sampling ports.
> If a port is unused it says to terminate it into the internal 50
> ohm load to
> maintain balance. This makes sense. But if I connect a port (or ports)
> through a short piece of 50 ohm coax to a scope which has a high impedance
> input, is this port properly terminated?
>

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