Oscillioscope - spectrum analyzer

Can a cheap LCD digital oscilloscope be setup to analyze frequency? Or is that the whole purpose of spectrum analyzer? I just want to teach myself how to make high/low cut-off (or WHATEVER) audio filters, and measure them on the screen.

I just learned how to view square pulse & pwm signals, and can see the line jump from ground, +5v, +12v. When I bought the scope, I thought I was going to be able to see audio waves through it, and see how a store bought equalizer changes the signal on the screen, so I can learn how to copy those circuits for myself.

I suppose I could just run a pre-amplified audio signal straight into my line-in jack on the computer, and use software - but if these tests can be done on my scope, I would like to learn how.

Hi,
What do you have that can produce audio signals??

You need a “sweep generator” that can produce audio tone that goes low to high through the band you are interested in. Then look at the output with your oscilloscope. Sync the oscilloscope to the start of the sweep and you have a “sort of spectrum analyzer”…

There are Apps for a smartphone that can do a lot of this. Example HERE and HERE

And stuff like THIS

Can a cheap LCD digital oscilloscope be setup to analyze frequency?

Some can.

I recall someone using a sound card on a PC as an audio spectrum analyser.

Here is a start.

http://www.radio.imradioha.org/pc_based_test_gear.htm

Also some SDR (software defined radio) dongles for TV reception can be used as a 50 MHz to 2GHZ spectrum analyser which is rather impressive for about 20.

terryking228:
Hi,
What do you have that can produce audio signals??

I'm building MIDI synthesizers, and learning different chips. Tone() output Arduino, 555 chip, SN76489 sound generator, Speakjet.

The smartphone applications is a good idea - thanks. I had a "kit" I bought that hooked into my sound-card to use as oscilloscope. I got confused with the noisy signals on the screen, and did not feel confident that my hand soldering, parts in the kit, possibly un-shielded materials affected the data on the screen.

I think the 555 chip can handle every audible frequency so I guess I can use that as a sweep generator. Or tone() function in Arduino, but that gets confusing with timers - it's easier for me to use the Arduino as a controller to other circuits to generate audio.

The audio circuits have to filter out some raw oscillations from the output pins, clean up the signal - without dampening the volume gain so low it makes the amplifier ineffective.

I think I was looking for a high resolution live bar graph, to monitor changes when I swap components.

SDR TV dongles? -- I'm confused. Am I hooking rabbit ears up to my sound card? A box between satellight TV?

DocStein99:
SDR TV dongles? -- I'm confused. Am I hooking rabbit ears up to my sound card? A box between satellight TV?

No, a USB or HDMI stick that plugs directly into a modern telly to receive

They connect via broadband hubs.Only if you have a local hub though.
Sounds like you have a rural location, needs a high capacity link.

In the RF band they cost little for amateur use compared with similar equipment in the 80' s.
10 of thousands.

As a spectrum analyser no link is required , just powerful processing.

Boardburner2:
No, a USB or HDMI stick that plugs directly into a modern telly to receive

They connect via broadband hubs.Only if you have a local hub though.
Sounds like you have a rural location, needs a high capacity link.

The "telly"s here in America, are usually connected to a cable, from a digital box - through to a telephone pole via fiber optic or coaxial cable. We get about 10,000 channels of commercial censored garbage to watch.

If I understand right - I could just take RF input (usually used for a television), plug that into a USB adc device, and find software to analyze that signal.

For a good and cheap starting point have a look a spectrum lab.
http://www.qsl.net/dl4yhf/spectra1.html
It turns your sound card / hardware into a very powerful spectrum analyser, which should be fine for your purpose.
I've been using it for years for VLF/LF radio, which is around audio frequencies.
Just watch voltages as most cheap sound card / hardware don't like large voltages!

Did you get the Rigol Oscilloscope? Here is a link to EEVblog that might do what you are looking for. WFM file viewer

It might be time consuming, but it’s free.

I have gw-instek gds-2102. I wanted Rigol, but I do not have the experience to know the difference in specifications between the two, and the oWan. All 3 of them have 2 channels, output to a usb device somehow. I figured they were just competitive with the same features for just about the same price.

I have gw-instek gds-2102.

That scope has an FFT function. Try it.

DocStein99:
Can a cheap LCD digital oscilloscope be setup to analyze frequency? Or is that the whole purpose of spectrum analyzer?

I just bought one of those cheap digital oscilloscopes. It can do some analysis, report on max and average voltage and it can also generate limited waveforms and frequencies. But better than nothing for waveform - the waves can be produced while the two channel scope is running. And the damn thing fits in the palm of your hand. Of course, you can get screenshots - the biggest issue I have is that I don't understand all of the triggering modes. Great item, poor documenation .

DocStein99:
The "telly"s here in America, are usually connected to a cable, from a digital box - through to a telephone pole via fiber optic or coaxial cable. We get about 10,000 channels of commercial censored garbage to watch.

If I understand right - I could just take RF input (usually used for a television), plug that into a USB adc device, and find software to analyze that signal.

RTL-SDR refers to a particular type of USB dongle that designed and marketed to receive European standard over the air digital TV broadcasts (DVB-T). Some years ago a hacker discovered that such devices using a particular decoder chip (Realtek rtl2832, hence the "RTL"*) could be configured to bypass the DVB-T decoder and stream down-converted and digitized radio frequency signals to a PC. This digitized signal could then be processed by the PC software in essentially arbitrary ways, hence a the PC and USB dongle comprise a "software defined radio" receiver or "SDR". One useful way to process the data is to calculate and display the frequency spectrum of the received signal which is the essence of a spectrum analyzer.

That said, the utility of RTL-SDR is radio frequency spectrum analysis, not audio frequency spectrum analysis as is your current need. A PC sound card plus software is, as noted above, the equivalent basis for audio frequency spectrum analysis on the cheap. I've found RTL-SDR useful in looking at Arduino RF transmitter issues such as "Is my cheap 433 MHz transmitter actually emitting RF energy when and where I think it should be?"

A good resource for RTL-SDR is http://www.rtl-sdr.com/. Note that the first few articles at that link reference higher performance (and an order of magnitude more expensive) receiver modules.

    • The rtl2832 contains the digitizer, DVB-T decoder (not used in the SDR context), and USB interface functions. It is paired with one of a few common radio frequency tuner chips. The Rafael Micro R820T2 tuner is generally the best of currently available options. RTL2832 / R820T2 DVB-T dongles can be found on eBay (for example) for less than $20 US.

I just learned how to view square pulse & pwm signals, and can see the line jump from ground, +5v, +12v. When I bought the scope, I thought I was going to be able to see audio waves through it, and see how a store bought equalizer changes the signal on the screen, so I can learn how to copy those circuits for myself.

You can see audio SIGNAL, in real time . Audio WAVES suggests you are referring to the Frequency response or spectrum of the audio device, as opposed to the real time signal. I have used cheal 20 Mhz scopes to troubleshoot and tune audio circuits by putting ch-1 on the test signal and ch-2 on the test points. I built a 10 band audio equalizer using LM307H (metal can) op amps and Walter jung's Active Filter Cookbook and an audio engineer tested it and declared it studio quality. I think the fact that I used preciion resistors and expensive caps was mostly responsible for the good performance. A friend once asked me to troubleshoot his voice disquiser circuit because it didn't work at all and I found all the copmponents were the correct value but were the absolute cheapest components available. I replaced them all with 1 % resistors and 5% mylar caps and it worked perfectly. I used a cheap 20Mhz scope .

MrMark:
RTL-SDR refers to a particular type of USB dongle that designed and marketed to receive European standard over the air digital TV broadcasts (DVB-T). Some years ago a hacker discovered that such devices using a particular decoder chip (Realtek rtl2832, hence the "RTL"*) could be configured to bypass the DVB-T decoder

Yes by broadband comment was a slip.

I was told that the designer of that dongle was a radio amateur who saw the possibilities, asked for and got permission to modify the silicon to make it possible.

Boardburner2:
I was told that the designer of that dongle was a radio amateur who saw the possibilities, asked for and got permission to modify the silicon to make it possible.

(Rtl-sdr - rtl-sdr - Open Source Mobile Communications), suggests the pass through was to enable software demod of FM radio by the closed-source Windows software and a guy doing a Linux port figured it out . . .

A 555 will not make a good VCO for a swept frequency analyzer. The output is a rectangular wave, that is heavy in harmonic frequencies. This will cause an analysis that means nothing.

You need something that makes a pretty good sine wave. How about some of the DDS chips? Direct Digital Synthesis. The AD9833 has 28 bit resolution on frequency and a 10 bit DAC. It can be used as a sweep frequency generator. It has outputs for sine, triangle, and square wave. You can get it on a breakout board with SMA connector for $12 to $15 USD.

Page 12 of the datasheet shows an example of a 60kHz output with the first harmonic about 62dB down from the fundamental.
AD9833 Datasheet

App Note AN-1044 about making a sweep generator out of a similar Analog Devices DDS:

AD9833 Waveform Generator

Conversation about AD9833 to a Nano Arduino on this forum

As per the conversation at the above link, you can decrease the clock crystal or supply an external clock signal at a lower frequency to get better frequency resolution for audio.

Amazon AD9833 from IC Station

IC Station AD9833 direct from IC Station

That sounds much more believable.
Could never figure out why it might have been 'allowed'.

Of course, you can get sound cards that capture at 192ksps @ 24 bit, and software that can use that for a pretty good audio spectrum analyzer. Some include the ability to use one channel as an output to provide a swept sine wave, and the other as an input at the same time. Both your sound card and the software must support this.

This one looks impressive.

Cobracom - Waveguide
Cobracom Oscilloscope PC based

Oscilloscope, Realtime spectrum analyzer, Impedance meter, RLC bridge and signal generator for Windows. Is a Windows application that converts your PC into a powerful dual-trace signal analyzer (oscilloscope, FFT etc...) . Uses your PC sound card as an Analog-to-Digital a Converter to digitize any input waveform and as Digital-to-analog Converter for the signal generator. True 24 bit adc/dac 48K/96k/192k sampes/sec.

MrMark:

    • The rtl2832 contains the digitizer, DVB-T decoder (not used in the SDR context), and USB interface functions. It is paired with one of a few common radio frequency tuner chips. The Rafael Micro R820T2 tuner is generally the best of currently available options. RTL2832 / R820T2 DVB-T dongles can be found on eBay (for example) for less than $20 US.

I think I found the kit on ebay, is much less than $20. It has an antenna, dongle software and a remote. It's an interesting gizmo I can afford. Thank you. I also was looking to experiment with r/c and radio waves too is a useful skill.