[Elecraft] Keying Waveform Measurement

Stephen W. Kercel kercel1 at suscom-maine.net
Wed Feb 9 16:18:22 EST 2005


Thanks to several commentors. Looks like I need to check the resistance of 
my 30-year-old cantenna.

73,

Steve
AA4AK


At 04:11 PM 2/9/2005 -0500, you wrote:
>Steve,
>
>I have no problem with my 10x probes at 100 watts and a 50 ohm load.  I use
>a Tek 465 here too.  But watch that Cantenna - check its actual resistance,
>they tend to climb in resistance as they age - just helped a guy out whose
>Cantenna was now a good 68 ohm dummy load.
>
>Yes, I know about those 50 ohm terminators - but as you know, they are power
>limited, and not commonly available except from Tektronics.  The UHF 'scopes
>(or rather vertical plug-ins) that I have encountered have only 50 ohm
>inputs, and not many hams have those available.
>
>I didn't really say so in my post to the reflector, but if you are going to
>catch the first pulse, you will need a storage 'scope of some variety.
>Which reminds me - if you are looking at the time delay between keying and
>RF, put your 'scopes' vertical amplifiers on chop rather than alternate -
>that will assure you the keying input you are looking at does indeed produce
>the RF that is shown on the other channel.  If the timing of each pulse is
>exactly the same it won't make a difference, but it is good to double check
>anyway.
>
>73,
>Don W3FPR
>
> > -----Original Message-----
> > From: elecraft-bounces at mailman.qth.net
> > [mailto:elecraft-bounces at mailman.qth.net]On Behalf Of Stephen W. Kercel
> > Sent: Wednesday, February 09, 2005 3:31 PM
> > To: elecraft at mailman.qth.net
> > Subject: RE: [Elecraft] Keying Waveform Measurement
> >
> >
> > 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
> > > >
> > > >
> >
> >
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