DSO spec confusion

[Brett Pantalone]

Hello,

I am a software engineer with some theoretical (but little practical)
knowledge of electronics. I'm shopping for an inexpensive (< $1000)
digital storage scope to use in my software consulting business and for
my personal hobby projects. I'm a little confused by the way these
scopes are specified, and I'm hoping someone can clear it up for me.

When a scope is advertised as, for example, 60MHz bandwidth, but has a
100 Msa/s sampling frequency, what does this mean? Doesn't Nyquist
dictate that the sampling frequency must be at least 120 MSa/s to
properly characterize a 60MHz signal?

Also, what about a scope advertised as 60MHz bandwidth, but 1 Gsa/s
sampling frequency? What do I get with the additional oversampling?

Most of my work will be with audio signals and digital data at
frequencies less than 1MHz, but I will occasionally want to verify clock
signals of 50MHz or more. What features and specifications should I
look for when shopping for a DSO?

Thanks for any advice you can offer.

-- Brett

 

[none]

Brett Pantalone wrote:

Hello,

I am a software engineer with some theoretical (but little practical)
knowledge of electronics. I'm shopping for an inexpensive (< $1000)
digital storage scope to use in my software consulting business and for
my personal hobby projects. I'm a little confused by the way these
scopes are specified, and I'm hoping someone can clear it up for me.

When a scope is advertised as, for example, 60MHz bandwidth, but has a
100 Msa/s sampling frequency, what does this mean? Doesn't Nyquist
dictate that the sampling frequency must be at least 120 MSa/s to
properly characterize a 60MHz signal?

The net rate must be half the frequency but there is a technique

called equivalent time sampling where each a/d sample is delayed
in time on the next sweep. At 100msa/s each sample is at 10nsec
spacing. If you take ten sweeps with the sample point displaced
in time by an additional 1nsec, then you have a 1Gsps equivalent
rate. The tradeoff is a cheaper a/d for a slower data acquisition
rate. Many years ago we had boxcar integrators which effectively
took one data point per sample. By moving the delay, you assemble
a waveform. Clearly the digital scopes are better than this.

Also, what about a scope advertised as 60MHz bandwidth, but 1 Gsa/s
sampling frequency? What do I get with the additional oversampling?

At 1Gsps you get the waveform sampled with a 1nsec resolution so you

can actually see some details. If you only sample at just over 2
samples per cycle, the amplitude will be small and the waveform
will look odd.

Most of my work will be with audio signals and digital data at
frequencies less than 1MHz, but I will occasionally want to verify clock
signals of 50MHz or more. What features and specifications should I
look for when shopping for a DSO?

For what you want, most of the digital scopes from the Chinese imports

to the (probably chinese import) Tektronix will do fine.
The biggest adjustment you will have is that there are aliasing
effects when you adjust the sweep rate.

For audio and lower frequencies you can also do will with a good
used Tek 2465 family oscilloscope. Analog scopes are a pain when
you are looking at transients but they are good for continuous
signals. However, unless you like screen picturesm, the digital
are good for documenting results.

I have an assortment of scopes but I find I use my Tek TDS3032 almost
exclusively. They are expensive but are great scopes.

Thanks for any advice you can offer.

-- Brett

 

[Pierre-François]

My recomandation would be for a Tektronix 2465B (B is the must with auto
setup), it's the RR of the analog scopes.
Old, built in the 80's but much metter than a low cost digital scope in wich
all the person I know who purchased were desapointed.

pf



"Brett Pantalone" <pantalone2001@yahoo.com> a écrit dans le message de news:
13to50dclpt5m6e@corp.supernews.com...

Hello,

I am a software engineer with some theoretical (but little practical)
knowledge of electronics. I'm shopping for an inexpensive (< $1000)
digital storage scope to use in my software consulting business and for my
personal hobby projects. I'm a little confused by the way these scopes are
specified, and I'm hoping someone can clear it up for me.

When a scope is advertised as, for example, 60MHz bandwidth, but has a 100
Msa/s sampling frequency, what does this mean? Doesn't Nyquist dictate
that the sampling frequency must be at least 120 MSa/s to properly
characterize a 60MHz signal?

Also, what about a scope advertised as 60MHz bandwidth, but 1 Gsa/s
sampling frequency? What do I get with the additional oversampling?

Most of my work will be with audio signals and digital data at frequencies
less than 1MHz, but I will occasionally want to verify clock signals of
50MHz or more. What features and specifications should I look for when
shopping for a DSO?

Thanks for any advice you can offer.

-- Brett

 

[Rich Webb]

On Sat, 15 Mar 2008 14:23:08 -0400, Brett Pantalone
<pantalone2001@yahoo.com> wrote:

Hello,

I am a software engineer with some theoretical (but little practical)
knowledge of electronics. I'm shopping for an inexpensive (< $1000)
digital storage scope to use in my software consulting business and for
my personal hobby projects. I'm a little confused by the way these
scopes are specified, and I'm hoping someone can clear it up for me.

When a scope is advertised as, for example, 60MHz bandwidth, but has a
100 Msa/s sampling frequency, what does this mean? Doesn't Nyquist
dictate that the sampling frequency must be at least 120 MSa/s to
properly characterize a 60MHz signal?

Not really. If you have only one sample per half cycle, you'll be only
able to say that there's a signal there.

Also, what about a scope advertised as 60MHz bandwidth, but 1 Gsa/s
sampling frequency? What do I get with the additional oversampling?

Say your scope's A/Ds are 8-bits and you want to avoid aliasing
anything into that least significant bit. That means you'll want a
stop band that's down at least 48 dB at the Nyquist limit where
frequencies start "folding back" on you. With a 4th order filter,
you'll get that much attenuation in 0.6 decades or at about 240 MHz.
That's where your sampling Nyquist limit will need to be, so your
sample rate should be at least 480 MHz per channel or 960 MHz for two
channels.

Most of my work will be with audio signals and digital data at
frequencies less than 1MHz, but I will occasionally want to verify clock
signals of 50MHz or more. What features and specifications should I
look for when shopping for a DSO?

For audio freqs, an inexpensive analog scope is fine. You'll probably
mostly be looking at repeating waveforms (or at least would rarely
want/need to use single-shot).

For digital *data* I'd strongly recommend a logic analyzer rather than
a scope. There are some decent, inexpensive PC-based ones. I've been
using the LogicPort from <http://www.pctestinstruments.com/index.htm>
for some time now and like it quite a lot.

All that said, there seems to be a consensus around here that Instek
scopes are pretty close to today's sweet spot. Similar to the
Tektronix models but with a larger data buffer, RS232, & USB.
<http://www.tequipment.net/InstekGDS-2102.html> for one example. I'm
still using my "ancient" TDS220 but if it ever goes up in smoke ...

--
Rich Webb Norfolk, VA

 

[Joerg]

Pierre-François wrote:

My recomandation would be for a Tektronix 2465B (B is the must with auto
setup), it's the RR of the analog scopes.

Absolutamente! That's about the first thing I suggest to clients who try
to diagnose something with a fancy DSO, to go on EBay and snatch a 2465.
After it arrives they often feel like somebody turned on the lights.
"Hey, we can see!"

But since Brett is a SW-engineer he may have to look at digitals stuff
such as SPI sequences a lot and then a DSO wins.

Old, built in the 80's but much metter than a low cost digital scope in wich
all the person I know who purchased were desapointed.

pf



"Brett Pantalone" <pantalone2001@yahoo.com> a écrit dans le message de news:
13to50dclpt5m6e@corp.supernews.com...
Hello,

I am a software engineer with some theoretical (but little practical)
knowledge of electronics. I'm shopping for an inexpensive (< $1000)
digital storage scope to use in my software consulting business and for my
personal hobby projects. I'm a little confused by the way these scopes are
specified, and I'm hoping someone can clear it up for me.

When a scope is advertised as, for example, 60MHz bandwidth, but has a 100
Msa/s sampling frequency, what does this mean? Doesn't Nyquist dictate
that the sampling frequency must be at least 120 MSa/s to properly
characterize a 60MHz signal?

Also, what about a scope advertised as 60MHz bandwidth, but 1 Gsa/s
sampling frequency? What do I get with the additional oversampling?

Most of my work will be with audio signals and digital data at frequencies
less than 1MHz, but I will occasionally want to verify clock signals of
50MHz or more. What features and specifications should I look for when
shopping for a DSO?

Thanks for any advice you can offer.

-- Brett

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

Rich Webb wrote:

On Sat, 15 Mar 2008 14:23:08 -0400, Brett Pantalone
pantalone2001@yahoo.com> wrote:

Hello,

I am a software engineer with some theoretical (but little practical)
knowledge of electronics. I'm shopping for an inexpensive (< $1000)
digital storage scope to use in my software consulting business and for
my personal hobby projects. I'm a little confused by the way these
scopes are specified, and I'm hoping someone can clear it up for me.

When a scope is advertised as, for example, 60MHz bandwidth, but has a
100 Msa/s sampling frequency, what does this mean? Doesn't Nyquist
dictate that the sampling frequency must be at least 120 MSa/s to
properly characterize a 60MHz signal?

Not really. If you have only one sample per half cycle, you'll be only
able to say that there's a signal there.

Also, what about a scope advertised as 60MHz bandwidth, but 1 Gsa/s
sampling frequency? What do I get with the additional oversampling?

Say your scope's A/Ds are 8-bits and you want to avoid aliasing
anything into that least significant bit. That means you'll want a
stop band that's down at least 48 dB at the Nyquist limit where
frequencies start "folding back" on you. With a 4th order filter,
you'll get that much attenuation in 0.6 decades or at about 240 MHz.
That's where your sampling Nyquist limit will need to be, so your
sample rate should be at least 480 MHz per channel or 960 MHz for two
channels.

Most of my work will be with audio signals and digital data at
frequencies less than 1MHz, but I will occasionally want to verify clock
signals of 50MHz or more. What features and specifications should I
look for when shopping for a DSO?

For audio freqs, an inexpensive analog scope is fine. You'll probably
mostly be looking at repeating waveforms (or at least would rarely
want/need to use single-shot).

For digital *data* I'd strongly recommend a logic analyzer rather than
a scope. There are some decent, inexpensive PC-based ones. I've been
using the LogicPort from <http://www.pctestinstruments.com/index.htm
for some time now and like it quite a lot.

All that said, there seems to be a consensus around here that Instek
scopes are pretty close to today's sweet spot. Similar to the
Tektronix models but with a larger data buffer, RS232, & USB.
http://www.tequipment.net/InstekGDS-2102.html> for one example. I'm
still using my "ancient" TDS220 but if it ever goes up in smoke ...

Yep, I just wanted to suggest Instek. I've got the big one here
(GDS-2204) and it runs circles around Tektronix. Also very nice USB
connectivity. If you don't have a laptop at hand just pop in a USB stick
and it stores the images and raw data on there.

Tektronix has inferior sample memory depth and the TDS210/220 series is
prone to 40/80kHz noise, probably from its backlight inverter. IMHO a
disgrace to Tektronix. There was also some kind of safety recall because
of a sub-par ground connection. You might want to check that.

Brett: Get something with at least 500MSPS, better 1GSPS and 100MHz
bandwidth or more. If you do stuff with FPGA or fast micro controllers
you'll need it. An old rule is that the sample rate should be at least
10 times the highest spectral component you want to look at. Check out
Instek, Newark has them. But don't wait too long since the Dollar is
slipping a bit.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Rich Webb]

On Tue, 18 Mar 2008 00:10:45 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Tektronix has inferior sample memory depth and the TDS210/220 series is
prone to 40/80kHz noise, probably from its backlight inverter. IMHO a
disgrace to Tektronix. There was also some kind of safety recall because
of a sub-par ground connection. You might want to check that.

Mine's a fairly late model TDS220 and was produced well after the
range that was affected by the power supply recall -- thanks for the
tip, though! It's also quiet with respect to the inverter noise so I
don't really have any excuses to ditch it... ;-)

--
Rich Webb Norfolk, VA

 

[Joerg]

Rich Webb wrote:

On Tue, 18 Mar 2008 00:10:45 GMT, Joerg
notthisjoergsch@removethispacbell.net> wrote:



Tektronix has inferior sample memory depth and the TDS210/220 series is
prone to 40/80kHz noise, probably from its backlight inverter. IMHO a
disgrace to Tektronix. There was also some kind of safety recall because
of a sub-par ground connection. You might want to check that.

Mine's a fairly late model TDS220 and was produced well after the
range that was affected by the power supply recall -- thanks for the
tip, though! It's also quiet with respect to the inverter noise so I
don't really have any excuses to ditch it... ;-)

Then it should be ok. Except that the ones I used had no connectivity, I
had to snap digicam pictures. I did use them for lots of serial bus
debugging though and they performed those jobs nicely.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Brett Pantalone]

Thank you to everyone for your helpful advice!

-- Brett

 

On 3ÔÂ16ČŐ, ÉĎÎç2Ęą23ˇÖ, Brett Pantalone <pantalone2....@yahoo.com> wrote:

Hello,

I am a software engineer with some theoretical (but little practical)
knowledge of electronics. I'm shopping for an inexpensive (< $1000)
digital storage scope to use in my software consulting business and for
my personal hobby projects. I'm a little confused by the way these
scopes are specified, and I'm hoping someone can clear it up for me.

When a scope is advertised as, for example, 60MHz bandwidth, but has a
100 Msa/s sampling frequency, what does this mean? Doesn't Nyquist
dictate that the sampling frequency must be at least 120 MSa/s to
properly characterize a 60MHz signal?

Also, what about a scope advertised as 60MHz bandwidth, but 1 Gsa/s
sampling frequency? What do I get with the additional oversampling?

Most of my work will be with audio signals and digital data at
frequencies less than 1MHz, but I will occasionally want to verify clock
signals of 50MHz or more. What features and specifications should I
look for when shopping for a DSO?

Thanks for any advice you can offer.

-- Brett

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