Re: Digital AC/DC Voltmeter design
From: John Larkin (jjSNIPlarkin_at_highTHISlandPLEASEtechnology.XXX)
Date: 03/22/05
- Next message: Genome: "Re: Notice How Larry"
- Previous message: Genome: "Re: Notice How Larry"
- In reply to: Fred Bartoli: "Re: Digital AC/DC Voltmeter design"
- Next in thread: normanstrong_at_comcast.net: "Re: Digital AC/DC Voltmeter design"
- Messages sorted by: [ date ] [ thread ]
Date: Mon, 21 Mar 2005 19:09:47 -0800
On Sun, 20 Mar 2005 21:24:26 +0100, "Fred Bartoli"
<fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
>
>"John Larkin" <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> a écrit dans
>le message de news:kffr31tp6hvlpa2tbirivoccnal9lv49g7@4ax.com...
>> On Sun, 20 Mar 2005 10:18:35 +0100, "Fred Bartoli"
>> <fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:
>>
>> >
>> >"Fred Bloggs" <nospam@nospam.com> a écrit dans le message de
>> >news:423C9BA5.4030506@nospam.com...
>> >>
>> >>
>> >> John Larkin wrote:
>> >>
>> >> > Just take random samples using a fast ADC; square, average, square
>> >> > root. You can either do it the direct way and get DC-coupled true
>RMS,
>> >> > or subtract out the mean value from each sample and effectively AC
>> >> > couple.
>> >>
>> >>
>> >> That's not how it's done in practice- not even close. If you don't know
>> >> what you're talking about then why don't you just shut the hell up.
>> >>
>> >
>> >Sorry Fred but see the HP3406A.
>> >
>> >BTW, you don't need fast ADC. Just fast enough sampling.
>>
>>
>> Actually, you don't even need to sample fast; there's certainly no
>> Nyquist issue here, as we're just gathering statistics on a waveform,
>> not trying to reproduce it. The reason to sample randomly (or at least
>> at a not-exactly-periodically rate) is to avoid aliasing the signal or
>> its harmonics. If the signal is of a known frequency (say, 60 Hz) you
>> can sample at some fixed rate that dances betweeen the harmonics
>> safely; the math gets interesting. My old survey meter sampled at some
>> magic rate close to 27 Hz, as I recall.
>>
>> The adc s/h does have to have bandwidth compatible with all the signal
>> components. So you can wind up using a wide-bandwidth ADC fired
>> slowly, or mux'd between a lot of channels.
>>
>> The 3406 used a very fast s/h, essentially the full-bridge sampler
>> like in the 1 GHz 1810 sampling scope plugin, fired at a relatively
>> low rate. Anybody got details? Was the 3406 true RMS? A schematic
>> would be fun.
>>
>> John
>
>Sorry, bad wording again. I meant that the sampler had to have enough BW so
>as to see all the signal components.
>
>For the full operating & service manual see:
>ftp://bama.edebris.com/hp/3406a/
>
>Lots of other manuals there.
Very cool link... thanks.
I checked an old HP catalog, and the 3406 is indeed a sample averaging
instrument, not true RMS.
John
- Next message: Genome: "Re: Notice How Larry"
- Previous message: Genome: "Re: Notice How Larry"
- In reply to: Fred Bartoli: "Re: Digital AC/DC Voltmeter design"
- Next in thread: normanstrong_at_comcast.net: "Re: Digital AC/DC Voltmeter design"
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
