# Digital scope for $60 Thread Starter #### iONic Joined Nov 16, 2007 1,654 #### MrChips Joined Oct 2, 2009 26,791 zero_coke found a TDS210 for$100 on ebay.
If you can find another one or TDS220 for that price, grab it. You wouldn't regret it.

#### SgtWookie

Joined Jul 17, 2007
22,227
If you wouldn't ever want to look at more than 1 trace at a time that was up to a 100kHz square wave, you'd be OK with that.

The thing is, you most often need to look at two (or more) wave forms at once, so a single trace just doesn't cut the mustard.

I really like the older Tek analog scopes; you can actually fix them if they break, and calibrate them yourself.

#### thatoneguy

Joined Feb 19, 2009
6,359
For under $50 and portable, the DSO Nano is cool, but it's only 8 bit resolution, color display and even capture though. Good for showing kids how stuff works and into the audio band up to about 50kHz. They came out with the 4 channel DSO Nano V2 Quad which is much more capable 2 analog capable to 30 Mhz (72Ms/s per channel), + 2 digital inputs + trigger input, for$200 at SeeedStudio.

For benchtop, look for a Tektronix. DSO is nice, especially when looking for what is being sent digitally, though for the majority, an older Tek will cover a majority of uses. Some of the older (CRT display) are more crisp than the 1st gen greyscale LCD displays, and they had capture ability as well, just not as much as new scopes.

#### bountyhunter

Joined Sep 7, 2009
2,512
If you wouldn't ever want to look at more than 1 trace at a time that was up to a 100kHz square wave, you'd be OK with that.
Not so sure. It claims 1MHz bandwidth. A square wave has a lot of significant Fourier content at high frequencies that make up it's shape. I used to have a 5 Mhz scope and it mangled the leading edges. Low bandwidth will "round" everything off.

IMHO, a 1 MHz bandwidth scope is just about useless.

#### someonesdad

Joined Jul 7, 2009
1,583
As Dirty Harry said, "A scope's just gotta know it's limitations". I use the following rule of thumb: divide the manufacturer's stated bandwidth by (at least) 10 and that's the number you should stick into your brain as the generally-useful upper frequency limit of a scope. In fact, 20 is a better number because it's more conservative.

Now, why would one do that? It's because the purchase-lust-engorged brain of the hapless buyer sees only that 1 MHz bandwidth number and immediately translates that into "Oh, I can look at any signal up to that frequency" (the marketroids know this well). Even an experienced EE can make this mistake, even though they know what a reputable manufacturer means when he says a scope has a bandwidth of 1 MHz is that a unity amplitude sine wave of 1 MHz put into that scope will measure an amplitude of one-half of what a sine unity amplitude wave of 100 kHz will measure. So, if you could be happy measuring signals up to 50-100 kHz, go for it.

As Wookie said, the other major limitation of such a device is that lack of two channels. When you use a scope for troubleshooting, you will want multiple channels.

Next to a DMM, a scope is the most useful troubleshooting tool for electrical stuff. I agree with the folks who recommend getting a good lab-quality older scope, especially one that you can repair if it breaks. Then start sticking your spare change in a piggy bank and in a few years you'll have the wampum to get a really nice scope. You won't be sorry.

Thread Starter

#### iONic

Joined Nov 16, 2007
1,654
Alright, I get the message. I'll pass on this little guy as it is not as versatile as would be needed for any real diagnostics.

Thanks for the input.

The TDS210 series scope looks nice and does not take up the entire workbench. I told my friend his would make a very good boat anchor if it broke on him!

#### thatoneguy

Joined Feb 19, 2009
6,359
The lower bandwidth (< 100 kHz effective) scopes such as the one you linked and the DSO Nano will at least let you know if a low to mid range DMM's voltage reading is from PWM or not. Newer DMMs have frequency and duty cycle built in, which negates that need.

Also useful for audio if the vertical resolution isn't entirely worthless, although a mini amplifier and alligator clips works often, sometimes it is too much of a draw when probing around pre-amps.

So they do have utility, so if getting one for free or something, don't ignore it. If spending 3 to 5 digits, though, get one that does what you will need, mixed signal preferably so serial communications can be read, and at least 2 channels + external trigger. 1Gs/sec is a decent enough capture rate for most uses, and 350 to 500Mhz will cover most of what you'll be working with (Signals from 35-50Mhz, though higher can be used if the sampling rate is high enough). Any change from that, and you go from a 3 digit pricetag to a 5 digit pricetag in a very big hurry.

#### bountyhunter

Joined Sep 7, 2009
2,512
As Dirty Harry said, "A scope's just gotta know it's limitations".
More accurately, the scope USER has to know it's limitations.

I remember when we had a military inspector coming in to do a design review..... required that we display the output "noise" waveform on one of our high power switchers at full load, and it looked BAD. Very high spike noise and ringing. So my old boss John "Sleazeball" Brown walks over to the Tektronix 200 MHz scope and presses the tiny button labeled:

"10 MHz bandwidth Limit"

Instantly the high frequency spikes and ringing MAGICALLY disappeared.

What a crook, but a good lesson. A switcher designer needs to remember the spikes that will be punching through his switch transistors can only be viewed on wide band analog scopes.

At national Semiconductor, digital was God and all the benches (except mine) had the Tek lunchbox digital scopes (I think they were 2 GHz sample rate?) Best available at the time. Got into a "discussion" once with my boss regarding the LIMITATIONS of digital: set up a switcher on the bench and showed the waveforms using the TEK digital side-by-side with a true analog scope (TEK 7904A, 350 MHz bandwidth). The analog scope showed a beautiful waveform of the voltage spike and ringing... the digital just showed a few dots. It just waved as the fast transient went by.

#### T.Jackson

Joined Nov 22, 2011
328
The analog scope showed a beautiful waveform of the voltage spike and ringing... the digital just showed a few dots. It just waved as the fast transient went by.
So you're basically saying that the DSO wasn't a 'real time' machine?

#### T.Jackson

Joined Nov 22, 2011
328
I think you've gotta spend at least 2K to get a DSO that isn't laughable. All these DSO with more colours than the rainbow on them with bull**** specifications. Kinda like these audio amplifiers that claim an output of 200WRMS, yet their DC supply is only good for 20W.
Load a bull.

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#### bountyhunter

Joined Sep 7, 2009
2,512
So you're basically saying that the DSO wasn't a 'real time' machine?
I'm saying the bandwidth of any scope "clips off" any waveform whose frequency content exceeds the BW. Any part of a waveform that rises or falls quickly has the highest frequency content. Put a 10 KHz square wave (with sharp edges) into a 5 MHz scope and the edges will be rounded off. Fast rising voltage spikes are not shown on a DSO because it only samples twice during the entire event. That voltage spike gets reduced to a couple of dots on the screen.

#### T.Jackson

Joined Nov 22, 2011
328
So you're talking about the propogation delay then.

#### thatoneguy

Joined Feb 19, 2009
6,359
So you're talking about the propogation delay then.
No, it's the limitation of the slew rate of input amps combined with slow sampling rates. Usually both are problems in early DSOs. It wasn't until the Tek "RealTime" displays and multi Ghz sampling rates were around that these signal impurities were captured, as noise is extremely wide spectrum.

#### bountyhunter

Joined Sep 7, 2009
2,512
So you're talking about the propogation delay then.
No sir, it has nothing to do with propogation delay. I am talking about how any waveform can be expressed as a series of sine waves of varying frequencies and amplitudes in a Fourier series. If the amplifier stages of your scope only pass signals to 5MHz, all content above that is attenuated or eliminated alltogether. The waveform displayed is the true waveform minus the higher components. For a square wave, it will have rounded off leading and falling edges.

This effect afflicts both analog and DSO, it is simply a case of the amplifier stages acting as a low pass figure attenuating significant high frequency signal components.

Few people realize a 1kHz square wave has a ton of significant content in the higher ranges. Filter it out and the waveform gets distorted. In general, any part of a waveform which rises or falls quickly has the highest frequency content.

Attached is the equation Fourier series of a square wave.

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#### T.Jackson

Joined Nov 22, 2011
328
Ah I know what you mean. I very mildly touched base on this one years back.

The fundamental AC waveform is the sine, and a square contains a multitude of many of them.

This is very complex theory that you're talking now. Way out of my immediate leauge.

#### T.Jackson

Joined Nov 22, 2011
328
Has someone got a simple way of explaining this to me?

I am a bit simple.

#### MrChips

Joined Oct 2, 2009
26,791
Here is a pictorial on what makes up a square wave.

On the left hand side is pure sine wave. On the graph below it is that of a pure sine wave of higher frequency and lower amplitude. This is then added to the previous collection (summation) of waves.

On the right hand side is what the sum of the previous waves look like.

The final graph is the waveform of just adding 5 sine waves.
If the scope were not band-limited this would go on forever to infinity and you would observe a perfect square wave. Since all scopes have a limited bandwidth, what you would observe would be a determined by the limitations of the scope, both the analog bandwidth and the sampling rate of a DSO.

(BTW, the Tektronix DS220 has a bandwidth of 100MHz and a sampling rate of 1000Msps.)

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#### bountyhunter

Joined Sep 7, 2009
2,512
Has someone got a simple way of explaining this to me?

I am a bit simple.
Fourier determined that any waveform that exists can be expressed as the sum of a series of sine waves which vary in amplitude and frequency. The first term is the "fundamental" which is the base frequency of the waveform. All additional terms are multiples of that frqeuency at reduced amplitudes.

Deriving the series of terms for the Fourier series is quite complex,.

#### T.Jackson

Joined Nov 22, 2011
328
Here is a pictorial on what makes up a square wave.
Thanks.

This is where a spectrum analyzer would come in useful correct?

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