[Elecraft] Keying Waveform Measurement

[Stephen W. Kercel]

Don:

Thanks very much. Yes, the ARRL setup is serious overkill. However, I have 
a Tek 465, and I thought it would be kind of nifty to try to set it up to 
look at my keyed waveform.

As for the matter of running a 50 Ohm source into a high Z scope input, Tek 
has a slick solution. They use these 50-Ohm 2-Watt terminators that you 
apply right at the BNC connector at the scope input. Essentially, the 
source, the 50-Ohm load and the high-Z scope impedance are tied in 
parallel. Of course the gotcha is that you have to use some fairly 
expensive high-power attenuators to bring the transmitter output down to 2 
Watts into the terminator. (The reason for running high power through 
attenuators instead of simply cranking down the rig power is that the test 
is intended to observe the keying waveform at full power.)

It had occurred to me that a cheaper strategy would be to run the rig into 
my Heath Cantenna (remember those?) and connect a regular high-Z 
compensated scope probe (the probe is good up to 100 MHz) across the dummy 
load resistance. Is there some gotcha to doing that? Maybe that is not such 
a good solution; 100 watts RF into a 50 Ohm load will have a voltage of 
something like 200 Volts peak to peak, and I expect that that is way more 
than the scope could handle. I also expect that to observe full power, 
you'd need to construct a high-Z voltage divider to tie across the dummy 
load, being very careful to keep its reactance low.

I also dimly recall that there was a piece in QST a few months (years?) 
back describing a little sampling device (something like a directional 
coupler) that you could insert in the coax line. The device was supposed to 
have trivially small insertion loss, but let you look at your on-air output 
on the scope direct and in real time. Any chance you remember when that 
came out?

The reason the ARRL test is so fancy is that it is intended to measure 
timing, the time delay between key down and the beginning of occurrence of 
RF output, and the shortening of the first dot in semi-QSK schemes.

Thanks for your help with this.

73,

Steve
AA4AK


At 02:24 PM 2/9/2005 -0500, you wrote:
>Steve,
>
>The setup pictured is IMHO overkill, but it covers all bases for any kind of
>transmitter.  No doubt the ARRL Lab has a semi-permanent setup for this
>test, but all that equipment may not be required depending on what you wish
>to conclude from your test
>
>The Keying Test Generator is nothing more than a keyer - but that one has a
>special output for triggering the 'scope.  In most cases, the 'scope can be
>triggered on the channel that the keyer output is connected to.
>
>The setup shown requires a 'scope with a 50 ohm input to properly load the
>attenuator.  Commonly available 'scopes have a high impedance input rather
>than a 50 ohm input.  If the 'scope and probe input will accept the voltage
>level presented by the transmitted signal, the attenuator may be replaced
>with a dummy load (keeping the power output under the speced limit for the
>'scope).  In fact if all you want to look at is the shape of the output
>waveform, you only need one channel connected directly to the RF output (and
>a dummy load)- just trigger on the input and display 2 or 3 dot times.
>
>If you need to measure the relative timing of the RF envelope with respect
>to the keying, a dual trace 'scope is needed.  Trigger the 'scope on the
>channel connected to the keyer output (trigger on the negative going slope)
>and you can read the delay from the onset of keying to the beginning of the
>RF wavefront.
>
>That is about all I can tell you other than those test setups shown will
>work and can tell you all you need to know about the keying characteristics
>of any transmitter.
>
>73,
>Don W3FPR
>
>
>
> > -----Original Message-----
> >
> > I was wondering if anyone on the reflector has tried to observe a
> > transmitted keying waveform using the technique described on page
> > 25.50 of
> > the 2005 ARRL Handbook, and depicted in Figures 25.86 and 25.87.
> >
> > The Handbook makes no mention of what the "Keying Test Generator"
> > is or how
> > to correctly set it up. I'd be most grateful if someone could
> > explain what
> > a "Keying Test Generator" really is.
> >
> > 73,
> >
> > Steve Kercel
> > AA4AK
> >
> >