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Author: John LarkinJohn Larkin Date: Aug 23, 2008 13:13
We recently designed an 8-channel complex waveform generator. Each
output stage is composed of a DAC, a lowpass filter, an output
amplifier, a test relay, and an output connector. It's this one:
http://www.highlandtechnology.com/DSS/V346DS.html
You can see the gold output connectors, and the relays are hiding just
behind the front panel.
The harmonic distortion seemed a bit high, in the -40 dBc range at 32
MHz and max level output. We were poking around with a spectrum
analyzer and happened to do a 0-3 GHz sweep and lo, a big line at
about 1 GHz. Something's oscillating!
Cut to the bottom line: the eight output amps, 1.5 GHz current-mode
opamps, are individually stable, but oscillate together. Futzing with
some amps may affect the outputs of others, several channels away. And
the ensemble oscillations have multiple stable modes, including the
occasional "off."
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Author: Michael A. TerrellMichael A. Terrell Date: Aug 23, 2008 14:00
John Larkin wrote:
>
> We recently designed an 8-channel complex waveform generator. Each
> output stage is composed of a DAC, a lowpass filter, an output
> amplifier, a test relay, and an output connector. It's this one:
>
> http://www.highlandtechnology.com/DSS/V346DS.html
>
> You can see the gold output connectors, and the relays are hiding just
> behind the front panel.
>
> The harmonic distortion seemed a bit high, in the -40 dBc range at 32
> MHz and max level output. We were poking around with a spectrum
> analyzer and happened to do a 0-3 GHz sweep and lo, a big line at
> about 1 GHz. Something's oscillating!
>
> Cut to the bottom line: the eight output amps, 1.5 GHz current-mode
> opamps, are individually stable, but oscillate together. Futzing with
> some amps may affect the outputs of others, several channels away. And
> the ensemble oscillations have multiple stable modes, including the ...
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Author: Joel KoltnerJoel Koltner Date: Aug 23, 2008 13:59
Your front panel is attached to grounded mounting holes on the PCB at roughly
the top and bottom then, eh?
Interesting. Thanks for writing this up!
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Author: John LarkinJohn Larkin Date: Aug 23, 2008 14:41
On Sat, 23 Aug 2008 13:59:36 -0700, "Joel Koltner"
yahoo.com> wrote:
>Your front panel is attached to grounded mounting holes on the PCB at roughly
>the top and bottom then, eh?
>
The front panel is not strapped to the PCB ground plane, per VME
convention, but is screwed into the card cage, which effectively RF
grounds the ends. There's also a pcb trace that connects to the panel
mounting screws and to the SCSI connector shell, for esd/emi reasons,
and that's apparently participating in the resonance too. It is all,
well, complex.
Modern current-mode opamps can easily oscillate at a GHz or more. If
you don't have a scope or a spectrum analyzer that spots this, the
low-frequency symptoms can be bizarre.
John
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Author: Bill WardBill Ward Date: Aug 23, 2008 15:05
On Sat, 23 Aug 2008 13:13:21 -0700, John Larkin wrote:
>
>
> We recently designed an 8-channel complex waveform generator. Each output
> stage is composed of a DAC, a lowpass filter, an output amplifier, a test
> relay, and an output connector. It's this...
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Author: Sir FrederickSir Frederick Date: Aug 23, 2008 16:00
The most improbable, yet significant, event
for me was when I was working at Jackass Flats,
Nevada, on the nuclear rocket engine program.
This obviously was before any atmosphere test ban
treaties. Anyhow, the liquid hydrogen ran out and the
reactor proceeded to really bake out!
Of course it ruined the reactor.
See for general background :
http://en.wikipedia.org/wiki/Nuclear_rocket
For specifics on the style I worked on right out of college :
http://en.wikipedia.org/wiki/Nuclear_thermal_rocket#Practical_testing
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Author: Paul Hovnanian P.E.Paul Hovnanian P.E. Date: Aug 23, 2008 17:26
This is why I stay away from anything over 60 Hz and less than 500 kCMil
conductor size.
--
Paul Hovnanian mailto:Paul@ Hovnanian.com
------------------------------------------------------------------
Do not mold, findle or sputilate.
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Author: ImmortalistImmortalist Date: Aug 23, 2008 22:14
On Aug 23, 1:13 pm, John Larkin
highNOTlandTHIStechnologyPART.com> wrote:> We recently designed an 8-channel complex waveform generator. Each
> output stage is composed of a DAC, a lowpass filter, an output
> amplifier, a test relay, and an output connector. It's this one:
>
> http://www.highlandtechnology.com/DSS/V346DS.html
>
> You can see the gold output connectors, and the relays are hiding just
> behind the front panel.
>
> The harmonic distortion seemed a bit high, in the -40 dBc range at 32
> MHz and max level output. We were poking around with a spectrum
> analyzer and happened to do a 0-3 GHz sweep and lo, a big line at
> about 1 GHz. Something's oscillating!
>
> Cut to the bottom line: the eight output amps, 1.5 GHz current-mode
> opamps, are individually stable, but oscillate together. Futzing with
> some amps may affect the outputs of others, several channels away. And
> the ensemble oscillations have multiple stable modes, including the ...
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Author: Tim WilliamsTim Williams Date: Aug 23, 2008 22:34
I've had one or two like that, so far. Not nearly so improbable, being much
simpler circuits, I suppose you could say primarily due to my inexperience.
Take, for instance,
http://webpages.charter.net/dawill/Images/20MHz_Amp.gif
The 2N5179s oscillated at some ungodly frequency, way beyond my 200MHz scope
(unsurprisingly, as they are >1.3GHz transistors). Probably a combination
of "bad" wiring and a not-too-great design (the next revision did away with
the 3904 emitter follower drivers, which probably coupled too much
end-to-end, exacerbating the problem).
Besides shitty performance that just didn't look right, I discovered this
one by touching the circuit. Oftentimes, parasites respond to the most
useful tool, the finger: it's dextrous, a thermometer (sometimes a
voltmeter!), has a reasonably consistent capacitance and resistance.
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Author: John LarkinJohn Larkin Date: Aug 23, 2008 22:56
On Sat, 23 Aug 2008 22:14:04 -0700 (PDT), Immortalist
yahoo.com> wrote:
>On Aug 23, 1:13 pm, John Larkin
> highNOTlandTHIStechnologyPART.com> wrote:>> We recently designed an 8-channel complex waveform generator. Each
>> output stage is composed of a DAC, a lowpass...
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