[Elecraft] Slightly off topic directional coupler question

[Don Brown]

Hi

Many scopes will indicate quite a bit above the bandwidth. They just will 
not be usable for accurate measurement. The usual spec on the vertical 
amplifier is, the scope will measure 3 db down at the specified bandwidth. 
The rise time relates to bandwidth as BW/.35 for example a 100 MHz scope 
will have about a 3.5 nanosecond rise time. If you want to make accurate 
measurements of Pk to Pk amplitudes then you should have about twice the 
bandwidth of the measured signal. So assuming the HF bands of up to a little 
below 30 MHz a 60 MHz scope would work for you. If you just want an 
indicator for looking a modulation envelops and the exact vertical is just a 
relative measurement then a scope of 30 MHz would work.

One other thing to consider the lower bandwidth scopes may not have a fast 
enough time base to display individual cycles or the trigger circuits may 
not work much above the bandwidth. Low cost scopes also have CRT that work 
with acceleration voltages of 4 to 8 Kv. This causes them to get very dim at 
high sweep speeds. Most of the Tek scopes have 20-24+ Kv CRT's

One more thing you can bandwidth limit the 475 at 100 and 50 MHz. So you 
could test your setup at 50 MHz to see if a low cost 50-60 MHz scope would 
work for you

Don Brown

KD5NDB






----- Original Message ----- 
From: "Rich McCabe" <r.mccabe at 1967z28.com>
To: <elecraft at mailman.qth.net>
Sent: Tuesday, January 31, 2006 7:49 PM
Subject: [Elecraft] Slightly off topic directional coupler question


I have a question :)

I only ask it here because its the best place for all my technical 
questions. OK, enough sucking up.

I just built a new Elecraft directional coupler with the intent of using it 
for a station monitor.

I have a Tektronix 475a that seems to work OK for what I play with.

I want to get a smaller (physical size) digital scope to use with my K2 and 
this new coupler and use it for misc troubleshooting with HF gear.

How far can I safely drop from my current 200mhz scope and get by?

Thanks and 73,

Rich
kd0zv

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