A few technical questions about oscilloscope use

[Planobilly]

Hi Guys,

First, I am new to using a scope. I don't fully understand how to use one and what they can and cannot do. I have a 100Mhz Owan DS7102V. It only cost around $500 so it is certainly not a high end scope. I does have many functions of more expensive scopes such as FFT. I only repair and build vacuum  tube guitar amps so my needs only relate to that sort of equipment.

I had an issue with a amp producing a low frequency noise of less than 100hz. On another occasion I was hearing 120hz noise coming from the power supply. In both cases I could not find a way to see the noise on the scope.

If I can hear a sound coming from a speaker, is there a way to display that sound on a scope in all cases?

Secondly, is there a way to set up a scope to only display signals of a certain frequency range, say 50hz to 150hz for example? Also can all frequencies above some range, say every thing above 20KHz be filtered out? Could I build some sort of RC filter to do that sort of thing?

Thanks,

Billy

[w2aew]

Hi Guys,

First, I am new to using a scope. I don't fully understand how to use one and what they can and cannot do. I have a 100Mhz Owan DS7102V. It only cost around $500 so it is certainly not a high end scope. I does have many functions of more expensive scopes such as FFT. I only repair and build vacuum  tube guitar amps so my needs only relate to that sort of equipment.

I had an issue with a amp producing a low frequency noise of less than 100hz. On another occasion I was hearing 120hz noise coming from the power supply. In both cases I could not find a way to see the noise on the scope.

If I can hear a sound coming from a speaker, is there a way to display that sound on a scope in all cases?

Secondly, is there a way to set up a scope to only display signals of a certain frequency range, say 50hz to 150hz for example? Also can all frequencies above some range, say every thing above 20KHz be filtered out? Could I build some sort of RC filter to do that sort of thing?

Thanks,

Billy

If you're used to building and repairing tube amps, then I'll assume you are already aware of the precautions to take around the high plate voltages, etc. that are present in tube gear.

To observe low-level AC signals (audio) that are riding on top of large DC bias voltage (plate voltage, etc.), use the AC Coupling function in your vertical settings.  This will remove the DC component from your signal, and allow you to adjust the vertical scale to a more sensitive setting to see the low level signals.

Especially at these voltages, ALWAYS use 10x probes.

Low frequency signals like these are almost always power supply related - especially since you mentioned <100Hz (the signal is likely 60Hz), and 120Hz.  60Hz (in the US) is the live voltage frequency, and 120Hz is typically of the "hum" you'll get after a full wave rectifier.  The "hum" is usually the result of bad power supply filter capacitors which are supposed to smooth these out.  However, sometimes you can get bad tubes where the heater supply gets into the signal path.  Use AC coupling as stated above and probe your supply voltages (provided the probes are rated for the DC voltage that is present), as well as probing signals in the audio path.

For power supply "hum" related issues, it is often very helpful to use the LINE triggering function on the scope.  That will synchronize the waveforms to the line frequency, making it easy to view the waveforms without any fiddling with trigger controls.

Scopes usually have a Low Pass Filter function in their vertical settings, but it is most commonly just a 20MHz filter - not really what you were looking for.  However, setting the horizontal timebase appropriately for the signals you want to see will be the most helpful to view the signals of interest.  For example, if you set the horizontal scale to 5ms/div, then the 60Hz and 120Hz signals will display a few cycles, but higher frequencies will just look like "fuzz" on the signal.  If you're triggering on the low frequency signal, then waveform averaging can often reduce the "fuzz".

Here are two videos that might be helpful.  The first discusses AC and DC coupling in the scope, and why/when each is useful.  The second is a much longer video that discusses scope operation in general - mainly in the context of probing signals inside an old tube AM radio - the probing around the radio occurs towards the end of the video, but you might find the rest of it useful too:

[TheoB]

That's a very good explanation. What I can add is that some scopes have a filter function. I have a Rigol DS1054Z that has a math function called filter. You can set high low or bandpass filter. You might want to check if your scope supports it.

[rstofer]

That's a very good explanation. What I can add is that some scopes have a filter function. I have a Rigol DS1054Z that has a math function called filter. You can set high low or bandpass filter. You might want to check if your scope supports it.

Another feature I didn't know I had!  I really must get around to reading the manual!
It is simply amazing how many features the DS1054Z provides.  I don't know that I'll even need the Filter functionality but it seems like a good thing to have.

[Planobilly]

Hi,

Thanks for the education and the links. As I have never had any "one on one" tutoring on how to effectively use a scope I am having to learn from you guys and the internet videos.

I also have a secondary possibility of using a recording software ( Sonar Professional) which I own. It is chocked full of spectrum analyzers and filters which could prove to be useful. I have several high quality microphones and think I can set them up in such a way as to not acquire too much additional added ambient noise.

Another friend of mine recommended this piece of equipment which could prove to be useful in conjunction with a scope.

http://www.ebay.com/itm/Rockland-85...654268?hash=item3ad3cee2fc:g:P9sAAOSw-0xYP1SG

Another possibility was to build a couple of "Biquad Filter" using LM 359s and cascade them for sharper cut off.

As I am pretty new to all this electronics technology, I have not yet learned enough to know what is possible or what standard methods of solving these type of issues are.

Also because there is not a lot of new tube equipment being produced now days many things that use to be common are no longer readily available such as affordable  regulated multi tap variable high voltage power supplies which are very useful to a tube amp builder. I guess I will just have to build one. Another project...lol

Thanks again for the education.

Cheers,

Billy   

[TheoB]

Secondly, is there a way to set up a scope to only display signals of a certain frequency range, say 50hz to 150hz for example? Also can all frequencies above some range, say every thing above 20KHz be filtered out? Could I build some sort of RC filter to do that sort of thing?

Use your FFT function. You can display only the frequencyband of interest. Advantage is that you can measure much smaller signal levels than you see on your screen. And if you only want to see 120Hz, that's also much easier.