----------
> From: Jon Ogden <jono@enteract.com>
> To: John Fielding <johnf@futurenet.co.za>; amps@contesting.com
> Subject: Re: [AMPS] Parasitic suppressors/another question,
To: <amps@contesting.com>
> Date: 07 April 1999 09:41
>
> >As far as point #1 goes.
> >
> >Keying up an amp without a load connected and then finding that it
> >oscillates is not a
> >sure way of telling if the amp is potentionally unstable.
>
> And why not? If the amp oscillates with no load, then it IS potentially
> unstable. Is it unstable in a place where you don't need it to be
> stable? Possibly. But if it does NOT oscillate with no load, then it
> rarely will oscillate terminated into a 50 Ohm load either.
>
> This is somewhat of the principle behind a load pull test. You either
> short out or open circuit the output of an amp and then rotate it around
> the Smith chart (via Ctune). Also, the tank inductor generally provides
> a large enough reactive impedance to VHF energy that it likely doesn't
> matter wether you have a load on or not.
>
> Having designed commercial 800 MHz amps in the past, I prefer to have
> them stable (as much as possible) across all possible load conditions.
>
> > Few high high gain
> >amplifier devices are
> >unconditionally stable, and removing the output or input terminating
> >impedance is almost
> >certain to provoke oscillation.
>
> Few properly designed amps. I disagree with your statement.
>
> >
> >Point #2
> >
> >How does this prove the amplifier is potentionally unstable?
>
> If the amplifier starts oscillating after being "rung" by high speed
> keying it IS unstable. QED! It's called transient response.
> >
> >Point #3
> >
> >How can you tell the difference between VHF energy which is caused by
> >oscillation and VHF
> >energy caused by harmonic power by using the colour of a neon tube?
>
> Because if the tube is keyed but has NO input signal, it should have NO
> output signal. If it does, it AIN'T harmonic energy but HF or VHF
> oscillation.
Then how can the neon glow orange as you stated if no drive is applied? Surely
this must
be a test with drive applied. And if it is with no drive applied and the neon
glows
orange it means it is oscillating on or near the band selected - not so?
John ZS5JF
>
> 73,
>
> Jon
> KE9NA
>
> --------------------------------------------------------------------------
> Jon Ogden
>
> jono@enteract.com
> www.qsl.net/ke9na
>
> "A life lived in fear is a life half lived."
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