June 19, 2012

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Oscilloscope Applications

Building on the o'scope basics covered earlier in our Apr 10th episode, we now delve into the many productive uses of the 'scope with our projects, with guest host and expert lecturer Alan Wolke W2AEW.


Back in our April 10th episode of CWTD, W2AEW gave us a great presentation on oscilloscope basics and fundamentals: a walk through the font panel controls of a common 'scope, how to optimally adjust the vertical, horizontal and triggering settings, and some simple typical uses of the instrument with our homebrew circuits on the bench.

This time, Alan delves into some pretty interesting 'scope "use cases" for particularly-relevant projects, including signal tracing through a circuit, monitoring critical points in the signal chain of a receiver and transmitter, component curve tracing,  and special cases requiring the use of delayed timebase settings.

So if you're just now starting to expand your use of the oscilloscope, or even if you think you've already got a handle on this powerful instrument, this session of CWTD will likely be wildly interesting.  Be ready for some great interactive Q&A!

73, George N2APB  & Joe N2CX

PS:  As some "homework" for this week's session, be sure to look back to the April 10th notes and even give a listen to the podcast in order to be refreshed and ready to dive in this week.

Audio Recording ... (Listen to the MP3 podcast)

Discussion Notes:

<20:01:11> "George - N2APB": Whiteboard for tonight's session .... http://dl.dropbox.com/u/43021514/CWTD/TeamspeakChat.html

<20:06:04> "Pete - WB2QLL": Bruce Vaughan NR5Q wrote lots of great articles for Electric Radio, he did!

<20:18:21> "Armand WA1UQO": Built this circuit a few months ago. Works great Alan - Thanks!!

<20:22:34> "Sam NV1P": i've used a 2MHz scope to see a 20m signal. worked great

<20:23:31> "Sam NV1P": good ol' tek 310A

<20:23:58> "George - N2APB": Taking a DIGITAL scope up to, or past, the BW limit is a different story?

<20:26:24> "George - N2APB": Square wave is composed of a never-ending series of odd hamonics of the fundamental.  And the faster the rising edges, the farther out they go.  A low-BW scope will act as a LPF to the signal being viewed, so the edges of the waveform may appear to be softer.  So benerally, the more BW scope you have, the better to see the fast edges.

<20:27:04> "George - N2APB": What causes the blurred signals in item #11?

<20:40:11> "George - N2APB": AC coupling is a great way to see noise/ripple on a power bus too.  Flip on AC coupling and then crank up your vertical gain while probing the V+ bus.

<20:42:25> "George - N2APB": The two sinusoidal waveforms shown on the whiteboard are actually the same signal, but with the trigger control set to + on the lower, and - on the upper.

<20:48:12> "Terry WB4JFI": A quick freq measurement "cheat" is to set the scope display period for some common value, such as 0.1us per division, then count the number of cycles on the total display.

<20:48:43> "Sophi": yes i can hear you

<20:51:32> "George - N2APB": Sophi:  Sorry for pointing it out that you were activating your PTT and "transmitting".  Either you were mistakenly pressing your PPT on the keyboard, or you have VOX on (which you should not have set.)  Thanks!

<20:56:19> "George - N2APB": "Outer Limits": "For the next 60 minutes, we have control of the vertical.  We have control of the horizontal. ..."  ==> Lissajous from the 60's!

<20:58:12> "George - N2APB": Our whiteboard material can be found at ... http://dl.dropbox.com/u/43021514/CWTD/TeamspeakChat.html

<21:07:15> "George - N2APB": Perhaps TDR can be the last one?

<21:15:38> "WA0ITP Terry": Awesome presentation Alan,  thanks very much.

<21:16:01> "Brooks - WA5OZE": web site URL where the videos are?

<21:16:50> "George - N2APB": It's YouTube.com.  Just click on any of the links for the individual videos we highlited tonight , for example ... https://www.youtube.com/watch?v=Il_eju4D_TM

<21:17:18> "George - N2APB": Or, while on YouTube.com, search for "Alan Wolke"

<21:18:06> "Alan W2AEW": http://www.youtube.com/w2aew

<21:20:11> "Armand WA1UQO": Thanks Alan, nice presentation. Any more planned for the future?

<21:20:46> "Alan W2AEW": Yes, I hope so!  Thank you!


NEWS  ... Surviving Technology, by Bruce Vaughan, NR5Q

This week's news item concerns a book that I recently ordered from Electric Radio and immediately fell in love with.  Bruce Vaughan, NR5Q is one of my personal heroes, as he is a homebrewer extraordinaire of tube-based radios ... and particularly in regen receivers.  A lot of his work is on the Internet and many people have replicated his designs, just as I am doing right now with his Ultimate Regen Receiver.  In his book "Surviving Technology", NR5Q chronicles his days growing up in the late 1940's, becoming enthralled with ham radio and doing everything he could to collect parts, build radios and be a ham in those early days.  Between collecting and nearly memorizing the QST magazines of those days and visiting other hams of that era who ultimately became household names to us hams of today, Bruce's storytelling is fascinating and educational to me even today as an avid homebrewer.  The second half of his book focuses on detailed plans and notes for his most popular radios ... you'll love it! 

The book can be purchased for about $25 from Ray Osterwald of Electric Radio Magazine [http://www.ermag.com/]



Bandwidth considerations:

  - what does it mean?

  - how much is enough?

  - how much is too much?


Probes and Terminations:

  - probe types

  - passive vs. active

  - affects of probe loading

  - 50ohm terminations and Z-zero probes

  - etc.


Practical applications

  - troubleshooting

  - monitoring

  - signal tracing

  - servicing

  - TDR

  - XY applications, such as curve tracing, etc.


Oscilloscope Types

  - Triggered sweep vs. recurrent sweep types

  - Analog vs. Digital

    - types of digital (real-time vs. equivalent sampling)

    - digital scope issues such as aliasing, memory considerations, etc.

    - advantages of digital over analog, and vice-versa


Advanced analog scope topics

  - delayed timebase operation (analog scope's version of zoom)

  - auxiliary inputs and outputs

  - etc.




LOTS of videos on scope applications, and ham radio uses for scopes ... Click on the links to view the actual videos. 


But for now, we'll look at some screen shots of key points in the videos as we talk through them here in CWTD ...


1) Monitoring your transmitter:  https://www.youtube.com/watch?v=y4Zt_LJX1Tc

This video shows a few ways that you can monitor your ham radio transmitter with an oscilloscope. Look at the RF carrier, the carrier envelope, the amplitude demodulated signal, as well as linearity "trapazoid" patterns (please see below). It shows a simple circuit I put together for a local ham radio club meeting and presentation.

I put this circuit together to show the radio club members examples of a sampler and a amplitude demodulator. Of course, since these were built onto the same "demonstrator" board with a common RF input, the linearity can not be measured with this since any non-linearity in the output would appear in both sensors. This is indicated by the notes in the video. In order to truly measure the non-linearity of an amplifier or a modulator/exciter, the X-input would normally be driven by the baseband signal, or the demodulated input to an amplifier, etc., and the RF output would be sensed with the sampler and applied to the Y input of the scope.




2) Basics of 1X/10X Probes: https://www.youtube.com/watch?v=SX4HGNWBe5M

This video discusses what 1X and 10X probes are, and how using 1X and 10X passive probes to connect your oscilloscope to your circuit will affect the circuit and the frequency response. It discusses why 1X or 10X probes are used, and how to properly compensate 10X probes



3) AC/DC Coupling: https://www.youtube.com/watch?v=Hkq-fvb5-NI

This video discusses the AC and DC vertical coupling modes on an oscilloscope. It shows that these settings are not really the same thing as the AC/DC settings on a multimeter, but rather they dictate how the signal is coupled to the vertical channel of the scope. In many cases, the DC coupling mode is very useful for looking at AC signals. Similarly, the AC coupling mode can be very useful when looking at DC signals. The choice of which vertical coupling mode to use depends upon the signals and circuits that you are looking at.



4) Triggering Controls: https://www.youtube.com/watch?v=OFGm-Pel4Hg

This video describes the Triggering controls on a typical Tektronix analog oscilloscope. Other analog scopes will typically be similar. The video presents the Trigger Level, Slope, Source, Coupling, Mode and Holdoff controls, and how each of them are used and their affect on the triggering capability of the scope.



5) Basics of Frequency measurement: https://www.youtube.com/watch?v=OQMnjaMSKn8

This is a "back to basics" video that I put together by request of some of my subscribers and ham radio friends. It discusses how to measure the frequency of a signal on an analog oscilloscope, and talks a little about the accuracy considerations. It shows how to properly adjust the vertical and horizontal controls to enhance the accuracy of the measurement.




6) Distortion measurements (or not!): https://www.youtube.com/watch?v=XHA4c82msqo

This video (by request) takes a look at making signal amplitude and distortion measurements on an oscilloscope and a spectrum analyzer. It runs through a quick example of measuring the signal amplitude on the scope in volts peak-to-peak, then converting this measurement to dBm and verifying that this is what is measured on the spectrum analyzer. The importance of maintaining a common load impedance for these measurements is discussed. The second half of the video discusses distortion measurements, and what you can and cannot see on a scope compared to a spectrum analyzer. It shows that while a scope is really good at making amplitude measurements (and can be used to measure frequency as shown in a previous video), it isn't necessarily the best tool to measure distortion at levels that are required to meet FCC Part 97 Emission requirements for amateur radio use.



7) Basics of Lissajous patterns: https://www.youtube.com/watch?v=t6nGiBzGLD8

Another "Back to Basics" video: This video takes a fairly detailed look at the basics of Lissajous patterns on an oscilloscope. There are a LOT of videos that show Lissajous patterns on YouTube, but not too many that describe the mechanism behind these patterns. Several of my YouTube viewers have asked for this topic, so here we are. Some low-speed waveforms are used so that the trace movement that draws the Lissajous figures can be seen and understood.



8) Simple Component Curve Tracer: https://www.youtube.com/watch?v=Gwo3pEH7hUE

This video shows an example of performing a simple component test (curve tracer) on an oscilloscope. The simple circuit shown is often called an Octopus (I don't know why), and it's been around for decades, in many variants. You can Google search on Octopus component tester, oscilloscope curve tracer, etc. and find dozens and dozens of variants. In the video, I boil the circuit down to its simplest implementation, describe how it works, and demonstrate how you use it.




9) Delayed Timebase operation: https://www.youtube.com/watch?v=J-7MycqHPOs

This short video shows an example of how you would use the delayed timebase on an oscilloscope. This example uses a Tektronix 485 oscilloscope and bursted sinewave signal from a function generator



10) Three videos on viewing two signals clearly:

  1) https://www.youtube.com/watch?v=LonFeZh2WL4

  2) https://www.youtube.com/watch?v=hk0ZSRV8zIQ

  3) https://www.youtube.com/watch?v=j4IxbF0oFRE



11) Bandwidth Considerations: https://www.youtube.com/watch?v=oZ1Dv2dVGkU

I'm often asked by hams and hobbiests - I want to buy an oscilloscope, what bandwidth scope do I need? I usually answer - buy as much as you can afford, even if you are working on low frequency circuits. This video shows an example of why. Even simple audio circuits might have some hidden evils!



12) ALT, CHOP, etc. vertical controls: https://www.youtube.com/watch?v=zsy-pxpfB4o

This video (by request) describes the function and use of the ALT, CHOP, ADD and INVERT CH2 controls that are often found on analog oscilloscopes. The ALT and CHOP setting control how the scope shows multiple channels simultaneously. The ADD and INVERT controls help you to visualize the voltage difference between two points in a circuit.



13) Poor Man's TDR - measure length and Z of coax: https://www.youtube.com/watch?v=Il_eju4D_TM

This video shows one way to use a scope and function generator to measure the length of a piece of coax transmission line as well as estimate its impedance. It uses a "poor man's TDR" type of measurement by launching a pulse into the coax and measuring how long it takes to return after being reflected by the open circuit end. This same technique can be used to determine the distance to a fault (open or short). A simple method for determining the impedance of the line is also shown.

This video touches briefly on transmission line and reflection theory, but is definitely not intended to dive deep into these topics. There are literally books written about this topic - so that won't be covered here.



14) Test an RF Power Detector: https://www.youtube.com/watch?v=vvqVTD-VEow

This video shows how I used a scope and a signal generator to perform a quick check on the proper behavior of a broadband RF Power Detector that I recently picked up at a hamfest. The signal generator is setup to produce a pulsed RF signal, and the scope is used to look at both the RF signal as well as the RF detector output signal. The delayed timebase feature of the scope is used to zoom into the rising and falling edges of the RF pulse so that the response speed of the detector can be observed. It is also shown that for this type of detector, the response is a strong function of the load presented at the detector output.



15) Rear Panel I/O:

    - create a "marker": https://www.youtube.com/watch?v=QCFBBiIm1h0

    - Connect a counter, expand vertical scale: https://www.youtube.com/watch?v=tyA-4Qv8RDs

    - Display video! https://www.youtube.com/watch?v=5FYF5uhCzAM





LOTS of videos on scope applications, and ham radio uses for scopes ...

  1. Monitoring your transmitter: https://www.youtube.com/watch?v=y4Zt_LJX1Tc

  2. Basics of 1X/10X Probes: https://www.youtube.com/watch?v=SX4HGNWBe5M

  3. AC/DC Coupling: https://www.youtube.com/watch?v=Hkq-fvb5-NI

  4. Triggering Controls: https://www.youtube.com/watch?v=OFGm-Pel4Hg

  5. Basics of Frequency measurement: https://www.youtube.com/watch?v=OQMnjaMSKn8

  6. Distortion measurements (or not!): https://www.youtube.com/watch?v=XHA4c82msqo

  7. Basics of Lissajous patterns: https://www.youtube.com/watch?v=t6nGiBzGLD8

  8. Simple Component Curve Tracer: https://www.youtube.com/watch?v=Gwo3pEH7hUE

  9. Delayed Timebase operation: https://www.youtube.com/watch?v=J-7MycqHPOs

  10. Three videos on viewing two signals clearly:

  11.   1) https://www.youtube.com/watch?v=LonFeZh2WL4

  12.   2) https://www.youtube.com/watch?v=hk0ZSRV8zIQ

  13.   3) https://www.youtube.com/watch?v=j4IxbF0oFRE

  14. Bandwidth Considerations: https://www.youtube.com/watch?v=oZ1Dv2dVGkU

  15. ALT, CHOP, etc. vertical controls: https://www.youtube.com/watch?v=zsy-pxpfB4o

  16. Poor Man's TDR - measure length and Z of coax: https://www.youtube.com/watch?v=Il_eju4D_TM

  17. Test an RF Power Detector: https://www.youtube.com/watch?v=vvqVTD-VEow

  18. Rear Panel I/O:

  19.     - create a "marker": https://www.youtube.com/watch?v=QCFBBiIm1h0

  20.     - Connect a counter, expand vertical scale: https://www.youtube.com/watch?v=tyA-4Qv8RDs

  21.     - Display video! https://www.youtube.com/watch?v=5FYF5uhCzAM



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