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From: John Larkin <jl@997PotHill.com>
Newsgroups: sci.electronics.design
Subject: Re: Favourite Test Equipment
Date: Fri, 05 Apr 2024 08:55:29 -0700
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On Fri, 5 Apr 2024 11:38:43 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 2024-04-05 11:31, John Larkin wrote:
>> On Fri, 5 Apr 2024 09:24:53 +0100, liz@poppyrecords.invalid.invalid
>> (Liz Tuddenham) wrote:
>> 
>>> Jan Panteltje <alien@comet.invalid> wrote:
>>>
>>>> Giggle Hertz oscillations are not happening in LC circuits of specific
>>>> kind where those can happen you can figure from the parts used, no giggle
>>>> Hz in a BC109.
>>>
>>> Back in the 1970s I found that a BC109 could be used as a self-
>>> oscillating transmitter output stage at 100 Mc/s on a 1.5v supply (it
>>> was for small animal cardiography).  Most "R.F." transistors wouldn't
>>> work under those circumstances.
>> 
>> I learned at an early age that emitter followers tend to oscillate. I
>> did a powerup reset circuit with an RC feeeding a 2N2219 emitter
>> follower feeding a TTL schmitt gate. The NPN oscillated at 100 MHz or
>> so and never pulled up the gate. A series gate resistor fixed it.
>> 
>> My recent 50 MHz SAV541-based Colpitts oscillator couldn't be tamed
>> with a gate resistor or with a bead, so I gave up on the phemt. I
>> think the wire bonds are a basic hazard.
>> 
>> https://www.dropbox.com/scl/fi/bak4p5bty2os2wtrs0091/9.jpg?rlkey=91e37ctc70189hvka9g1ufar3&raw=1
>> 
>> 
>> 
>
>Try putting BLF03VK600 beads in source and drain. Besides being rated at 
>3 GHz instead of 100 MHz, it has really nice low Q everywhere.
>
>My cascoded lab amp proto is using three SAV-331+'s in parallel like 
>that, running about 2.5 mA each, and shows no sign of trouble.
>
>Cheers
>
>Phil Hobbs

We tried various fixes, no joy. The circuit is complex, lots of diodes
and comparators and varicaps and stuff, so there are too many resonant
stubs.

We prototyped the new sorta-Colpitts circuit, using the BUF602 as the
gain element, and it's great. Having a closed-loop near-perfect
follower is nice.