>
>
>
>----------
>> 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.
? amen. In parasite-arced bandswitches, damage occurs primarily on the
10m and 15m taps. .
>>
>> Having designed commercial 800 MHz amps in the past, I prefer to have
>> them stable (as much as possible) across all possible load conditions.
>>
? However, in uhf amplifiers the anode-resonance is the tank. In hf
amplifiers there are usualy two resonant circuits connected to the anode.
.
>> > 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?
>
? In my experiences, g-g triodes can occasionally oscillate in steady
state without drive above the grid self-resonance. In a 3-500Z, this is
approx. 80MHz.
- later, John
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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