Author Topic: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?  (Read 945 times)

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Offline Dumble

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Hi all,

I am new to the forums but I've watched plenty of Dave's stuff on his YouTube channel.  :-+

I've recently got quite into RF design and also LC, RC experimentation and I've been seeing all these great videos with people using Spectrum Analyzers to measure and see the results. Also in a few of these videos they say you can use the FFT settings in the Oscilloscope to give some basic Analyzer readings but being a Scope noob, I'm not sure how this is done.

My scope is a Hantek DSO5102B rated to 100Mhz but I'm experimenting around in the VHF / UHF "Ham Band" region so I believe I need some sort of attenuation so I don't blow the smoke out of my scope.  :scared:

I simply cannot afford even a budget model analyzer at this point in time and doubt I will for a good long while at the prices even the older analogue analyzers go for so I've just got to make do with what I can for now to get by. Quite often I've used my cheapo RTL SDR Dongle and SDR Sharp as a poor mans analyzer but it's just not the same or detailed enough.  :--

What I would like to know is how I go about safely setting up / connecting up my scope correctly to work as an analyzer with an RF transmitter operating at 144Mhz giving out around 1W of power for example so I can experiment around with different RF Filters and measure and see what's happening?

Obviously I understand that it's not going to be as good as having a dedicated analyzer but I figure should get more information than I get from an SDR Dongle?  :-//

If anyone could point me in the right direction for where to get started, I would be very grateful.

Oh and I forgot to say I am a Radio License Holder in the UK. I currently hold a Foundation Level License.


Thanks in advance!!!   :)
 

Offline capt bullshot

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #1 on: November 16, 2017, 10:36:04 PM »
Oscilloscopes' FFT function is usually more or less useless ...
Yes, you could use it to see the typical spectra of a square or a ramp waveform, but that's the most useful thing I ever found to do with an DSO's FFT.
IMO, for your needs you're better fitted with the SDR than a FFT on DSO.
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Offline Ian.M

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #2 on: November 16, 2017, 10:47:27 PM »
There's the old-skool poor-man's spectum analyser method of slowly sweeping a VCO, and mixing it with the signal of interest in a carefully balanced mixer then putting it through a very narow band crystal bandpass filter then into an envelope detector.   Set up the scope for an XY plot of the detector output against the VCO control voltage.  Back in the day, one would use the ramp output from the CRO timebase to sweep the VCO as slow as you could tolerate the flicker, but DSOs typically dont offer you a timebase ramp output.

Calibrate it with a crystal marker pip generator, and if the crystal filter has a narrow enough bandwidth and the sweep is slow enough, its possible to get sub 100Hz resolution.
« Last Edit: November 16, 2017, 10:54:49 PM by Ian.M »
 

Offline tggzzz

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #3 on: November 16, 2017, 11:03:47 PM »
You need to define:
  • what you need to know
  • why you need to know it
  • to what "accuracy" you need to know it; N.B. "accuracy" in its widest sense!
and then you will be able to choose and use an instrument.

Usually a scope is the wrong tool for "looking at RF"; it has limited resolution, sensitivity, and is too non-linear. A scope is, however, fine for looking at baseband signals before modulation and after demodulation.

Consider using a £15 SDR dongle as a poor-man's spectrum analyser and modulation analyser.

As for your question about too much transmitter power, you need either an attenuator or a directional coupler.

I suggest you contact your local radio amateur society (RSGB website); they will be a mine of information. The ARRL handbook is also very useful; get a secondhand one since radio doesn't change much from decade to decade :)
There are lies, damned lies, statistics - and ADC/DAC specs.
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Online rhb

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #4 on: November 17, 2017, 03:00:17 AM »
The SDRplay RSP2 running SDRuno makes a pretty decent SA with up to 10 MHz span.  You can see some screen shots here:

http://www.eevblog.com/forum/testgear/'nose-source-bg7tbl-2016-03-06'/msg1320375/#msg1320375

I've not yet gotten around to trying the noise source with a filter other than the one I made to cleanup the output of the noise source.  I'll try to do that in the next few days.  I needed some perf board to mount things but couldn't find any.  Some finally crawled out of hiding a day or two ago.

The RSP 2 does have some strong spurs,  but anything you get will.  For VHF and higher an ADF43xx from eBay and an Arduino is an alternative to the noise source.

FFTs on scopes depend very much on brand.  On the Rigol DS1102E it's too short to be usable.  On the Instek MSO-2204EA it works well up through 250 MHz though the UI is a PITA.

In both cases, the normalization function of an SA is missing.  If your scope will download a trace in CSV format, Octave will handle the math chores.
We all get what we deserve whether we want it or not, either as individuals or members of a group.  Sometimes this is as punishment and sometimes it's a blessing.  Which is always ambiguous and depends entirely upon what we do next.
 

Offline Bicurico

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #5 on: November 17, 2017, 04:14:10 AM »
Take a look at my blog: http://vma-satellite.blogspot.pt/

If you combine the SMA/NWT range of devices with my "VMA Simple Spectrum Analyser" software, you get a pretty usefull spectrum analyser for TV/CATV/SAT applications starting at 50$/€/£.

Let me know if you need help in this matter.

Regards,
Vitor
 

Offline Dumble

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #6 on: November 17, 2017, 07:36:45 PM »
Hello everyone, thanks for replying.

Obviously that's not the news I wanted to hear but I was more or less expecting that to be the case. If scopes made good Spectrum Analyzers, nobody would be buying the Analyzers, right?
Looks like I'll be sticking to my RTL SDR Dongle and SDR Sharp Software for the foreseeable future.

I guess I don't urgently need an Analyzer at this stage but one would certainly be beneficial for the future.

It's a bit like when I first purchased my Oscilloscope. I didn't think I needed one at that particular time but I purchased it anyway when it was at a good price because I knew I would progress and grow into needing it. And from the day it arrived it has been absolutely invaluable. As much as it is a piece of test equipment, it's also an educational tool to me too. As a Multi-Meter gives you one picture, a Scope gives you a more complete picture and I feel the same about an SDR Vs. a dedicated Analyzer.

When I got my scope, it opened up a whole new world of possibilities and opportunities to design, build and test things I'd never thought possible for me. As much as I've learned about electronics along the way I've learned equally as much about my scope as well. One example is a project I've been working on for a while on and off which uses an Atmega but is designed to take very little power at sleep and uses MOSFETS to turn on peripheral devices when required. Prior to getting my scope, my project was still drawing 16mA and uses relays and transistors as switches. Thanks to my scope I was able to switch over to using MOSFETS, something I had zero experience with before and avoided them like they were some voodoo relic. And just by analyzing the signals on the scope and comparing the Gate and Drain voltages I was able to visually see what I needed to see and it made the world of difference to my understanding. Not to mention what I'd learned about using my scope in the process. My project now draws less than 1mA and has shrunk by half it's original size by using Surface Mount Components - something else I'd not used before. You would be surprised how much just having my little scope has pushed me and helped me learn and I love to learn!

Okay I probably could have rigged up two multi-meters to see what was going on but having the scope to hand is what has made me want to progress more and more into more advanced areas just because I knew I had the means to test it all out with reasonably accurate budget equipment and so I am able to see what's going on and I think that's key for me. I'm a highly visual learner. Monkey see, monkey do. I get a better picture of what's going on if I have some visual information. It must just be me lol.

With the MOSFET situation, as I say, I could have just used two multi-meters but then I've got to mentally translate and visualize it where as the scope lays it all out for me to see and I can see the change over of the MOSFET pins as one pin goes low and the source voltage flows on the scope trace which is so much easier and it would be the same if I had a dedicated Spectrum Analyzer.

I guess the best way to put it is I feel hindered and handicapped not having one to hand and it kinda puts me off progressing with my experimenting with RF because I know I'll reach a point where I do need the correct test equipment and I can't get it.

Maybe this year Santa can accidentally leave behind £10,000 in a sack under my Christmas tree LOL.... And the year after he can do it again so I can get a Network Analyzer hahah.


Thanks all!



« Last Edit: November 17, 2017, 07:41:26 PM by Dumble »
 
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Offline tggzzz

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #7 on: November 17, 2017, 08:26:44 PM »
It's a bit like when I first purchased my Oscilloscope. I didn't think I needed one at that particular time but I purchased it anyway when it was at a good price because I knew I would progress and grow into needing it. And from the day it arrived it has been absolutely invaluable. As much as it is a piece of test equipment, it's also an educational tool to me too. As a Multi-Meter gives you one picture, a Scope gives you a more complete picture and I feel the same about an SDR Vs. a dedicated Analyzer.

Welcome to the rathole; you'll find the denizens are friendly. For example: http://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/

Quote
I guess the best way to put it is I feel hindered and handicapped not having one to hand and it kinda puts me off progressing with my experimenting with RF because I know I'll reach a point where I do need the correct test equipment and I can't get it.

See the aphorism in my .sig; it is there because there are direct analogues in electronics.

You might like to consider that spectrum analysers are scalar devices: they only measure amplitude, not phase. That leads to thinking about measuring phase, and hence to vector network analysers (VNAs). Since those aren't very good at measuring dynamically changing systems, you then start to move onto Modulation Domain Analysers and more modern variants. After that you can move on to....

My approach is to find ways of extracting the maximum information using the tools at hand and, only if that is insufficient to determine what equipment I might need to acquire. One example of that is that it is possible to use an SDR dongle as a time-domain reflectometer :)
There are lies, damned lies, statistics - and ADC/DAC specs.
Gliding aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline bd139

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #8 on: November 17, 2017, 10:42:39 PM »
Welcome to the rathole; you'll find the denizens are friendly. For example: http://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/

TEA Disclaimer: you will never leave :)
 

Offline rcowboy78

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #9 on: November 18, 2017, 01:16:29 AM »
Dumble,

I wouldn't consider the FFT on a oscilloscope useless. I've used mine on a couple of projects. For instance, I purchased an old function generator as not working. When I got it, it was a simple power supply issue. Since it was an old Fgen, after I fixed it, I wanted to do a calibration on it. I used the FFT function on my old HP scope to check the harmonics of the sine, triangle, and square wave forms. I didn't use the FFT function to take direct measurements, but what I did do is I used it to lower the second on third harmonics to the lowest points that I could get them in comparison to the main signal. After spending some time with it, I got those signals looking pretty good using this method.

In short, if you really want to get a spectrum analyzer, put your o-scope to work. Buy some old equipment and fix it and resale it. This is how I acquired the equipment that I have. I picked up the function gen for $60 and ended up selling it for $240. If you do this for a couple pieces of equipment, before you know it a spectrum analyzer will be on your desk. It's also a great learning experience.

Just though I would pass my experiences along.

rcowboy
 

Offline 4CX35000

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #10 on: November 24, 2017, 09:33:58 PM »
Thurlby Thandar made a spectrum analyser which used the X/Y setting on a typical oscilloscope. It is not Agilent, R&S standard by any means, but it does give reasonable results if your looking at harmonics and amplitudes for various tests.

Try look out for it on eBay for the TSA250 or the TSA1000. They tend to go for around £50 here in the UK.

Some comments on EEVBlog below.
http://www.eevblog.com/forum/testgear/tsa1000-spectrum-analyser/

TTI datasheet
http://benl.rs-online.com/webdocs/00b6/0900766b800b6c84.pdf
« Last Edit: November 24, 2017, 10:19:57 PM by 4CX35000 »
 

Offline bd139

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #11 on: November 24, 2017, 09:49:30 PM »
Interesting. I have been outbid on three of them so far :)
 

Offline 4CX35000

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #12 on: November 24, 2017, 10:33:39 PM »
There is always the RF Explorer. It does need a computer and some software installing. Never tried one myself.

https://www.seeedstudio.com/RF-Explorer-WSUB1G%2B-p-2986.html

 

Offline G4BXD

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #13 on: November 24, 2017, 11:14:04 PM »
Hi. If your scope has a sawtooth o/p, maybe on the rear, you can use this to sweep a receiver which has a voltage controlled tuning, or maybe a tv tuner unit etc then feed o/p of rx to scope. That way as the trace moves across the screen the vfo in the rx also sweeps across so you have a basic panadapter/spec analyser. I built a small box some time back, 460khz osc, sawtooth from scope tuning it and it was ideal for checking if bandwidths/tuning etc. Great fun to play with.
 

Offline tggzzz

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Re: Using my Oscilloscope for RF work - E.g. Poor Mans Specrum Analyzer?
« Reply #14 on: November 25, 2017, 01:50:33 AM »
I wouldn't consider the FFT on a oscilloscope useless. I've used mine on a couple of projects. For instance, I purchased an old function generator as not working. When I got it, it was a simple power supply issue. Since it was an old Fgen, after I fixed it, I wanted to do a calibration on it. I used the FFT function on my old HP scope to check the harmonics of the sine, triangle, and square wave forms. I didn't use the FFT function to take direct measurements, but what I did do is I used it to lower the second on third harmonics to the lowest points that I could get them in comparison to the main signal. After spending some time with it, I got those signals looking pretty good using this method.

The ADC nonlinearities will manifest themselves as harmonic distortion and spurs. An 8-bit resolution will mean a high noise floor.

A 14 bit ADC will be much more useful than an 8 bit.
There are lies, damned lies, statistics - and ADC/DAC specs.
Gliding aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 


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