Alas, I ain't got no spectrum analyzer - woe is me!

[ashart]

Poor me!  I had a spectrum analyzer when I worked, but as a retiree, none is available and I sure do miss that gizmo!

However, I do have a decent scope and I'm quite experienced in viewing sine waves.  Looking at my 3.5 Mhz VFO output with a 450 Mhz. scope, I can see absolutely no distortion, irrespective of the amplitude or sweep speed used to display the sine wave.

What EXPERIENCED opinions are around as to what the maximum distortion percentage might be in a "perfect looking" sine wave, or more directly, how far down (in db) might be the products of distortion?

Tnx es 73.

al hart, w8vr

al@w8vr.org and www.w8vr.org

[K1JJ]

Al,

One of the best specturm analyzers on the planet is a good receiver.

Whether you're listening to an AM or SSB signal for purity - or a VFO, just normalize the signal strength to S9+40 over and tune up and down the band looking for crud, and how far down it is on the S meter.. Check X2, X3, etc for harmonic levels.

Much will depend on how clean the receiver is, but for most ham applications, your ear and S meter will tell you all you need to know.

I have analyzed many AM and ssb signals using a receiver and can easily tell when they are acceptable or not.

A VFO will require a look farther down in db for purity, but try it and see.


For actual db numerical readout, another excellent alternative is to pick up a cheap ($20)  Softrock down converter card and connect it to the output of your 455khz receiver. (Or whatever IF and xtal you choose)  Using a standard computer you will have a real high-tech spectrum analyzer. I do that here, except use an HPSDR software defined receiver and computer.

Good luck..
Fabio, K1JJ

[AB2EZ]

Al

Check out this web site / applet. If you click on "sine" it will display a pure sine wave. Then you can use your mouse to move the sliders to add in small amounts of harmonics. My own experience (based on my own calibrated eyeballs...for better or worse) is that... just looking at the waveform on a scope... it is difficult to "see" the presence of harmonics that are 40dB (or more) down (less than 1% in amplitude) from the fundamental... particularly for the case of the 2nd or 3rd harmonic. The 4th and higher harmonics seem to be easier to spot... even at 40dB down.

http://www.falstad.com/fourier/

Adding to what K1JJ said... if you have a decent receiver, you can inject an attenuated version of the oscillator's output signal into the receiver's antenna connector... and use the S-meter to compare the received signal strength at the fundamental frequency, and various harmonics.

Also, for a few hundred dollars you can purchase a modern digital oscilloscope that will work in conjunction with a personal computer (interfacing via a USB port) that will allow you to display a captured waveform's spectrum (FFT), as well as the time domain waveform(s) of the signals arriving at its input(s).  

Stu

[Opcom]

What EXPERIENCED opinions are around as to what the maximum distortion percentage might be in a "perfect looking" sine wave, or more directly, how far down (in db) might be the products of distortion?

Tnx es 73.

al hart, w8vr

al@w8vr.org and www.w8vr.org

Looking at a sine wave on a scope without a perfect one to compare it with is very subjective. There can easily be 5% distortion before it starts to look funny.

If that is what you will be working with then I suggest displaying the good wave from the generator on one channel and the wave to be observed on the other channel. From there you can superimpose them or you can subtract one from the other and more easily see the distortion.

[KM1H]

The operative word is DECENT. Many of the BA receivers and many riceboxes are already non linear at 40 over in RF, IF and audio stages and Id suggest S9 or just a bit higher to take the receiver distortion out of the picture.  Without an antenna connected the receiver noise floor is well below what would be considered acceptable distortion from an oscillator or transmitter. At least with a non synthesized radio the phase noise contribution is absent.

There are still bargains around in the BA style HP-141T SA systems or even a later 181T version. No man should be without a SA if he is serious about signals.

Carl

[K1JJ]

I'm not seeing this non-linearity with my FT-1000D as a "spec analyzer" receiver.  I can use the HPSDR exciter (about -75db 3rd at 1/2 watt) and starting at S9+40 over, the signal crud drops off below S2 up 3.5kc.    This is a clean example that shows the range.   When analyzing a regular ricebox transmitter, (-30db 3rd) I see it no better than an S9 up 3.5kc using the same S9+40 reference, so the tests seem to jive OK.


Frank has mentioned using a lower signal level before too, but I've not run into the problem.  But it is a good idea to use as low a signal as possible, of course. Other BA receivers may not be so forgiving.


T

[WQ9E]

You can set up an operational amplifier in "differentiator mode" to accentuate any distortion in the sine wave and this will make the display a bit easier to interpret.  You will still need a comparison reference.  

Many years ago I used a Tektronix Type O operational amp in a 545B mainframe set up to differentiate while repairing a very vintage HP-200 audio oscillator.  It made slight sine wave distortion very obvious on the scope display.

Although more difficult to use than modern spectrum analyzers, I have seen the old Tektronix 1L series units available for very low cost ($40 for the pair at one last year) at hamfests.  They require a 500 series scope (with plug in vertical compartment) or with the Tektronix 132 plug-in power supply they can be used with any scope that provides a 100 or 150 volt sawtooth waveform from the time base.  For a real vintage Tektronix setup a type 162 function generator with its 160 series power supply will provide the correct sawtooth waveform for both the analyzer and the scope allowing use with scopes that use/provide a lower voltage sawtooth waveform.  Most useful are the 1L10 (1 to 36 Mhz. center frequency) and 1L20 (10-4,200 Mhz center).   The 1L10 provides good single signal analysis and is very useful for aligning phasing type SSB transmitters while the 1L20 is useful for measuring harmonic and spurious output over a broad spectrum.    These analyzers do require more user knowledge since they are sweep tuned superhet receivers and thus have images but these are very easy to identify and the 1L series still does a great job as long as the user is knowledgeable.

I currently use a 7L13 spectrum analyzer with my 7000 series scopes but I got a lot of use from the older 1L units prior to acquiring a 7L13 at a reasonable price.

[flintstone mop]

Hi Al
Are you looking for a piece of test equipment or would be happy seeing a Spectrum type of display from an SDR??

I think you would be disappointed with a digital O scope or Spec Anny. And many here see used goodies available.
Fred

[K5UJ]

what do you think about this one:

http://cgi.ebay.com/HP-3582A-SPECTRUM-ANALYZER-/150615618928

not me, I have other priorities right now.

[flintstone mop]

Hey Rob
She is purty.....the ebay item......Spec Anny. BUT the seller is selling "as-is". And no returns. He made a statement that there's a 99.9% chance it might work but selling "as-is". It's worth the "gamble". HP is good stuff. The only hitch would be the CRT.
Fred

[w3jn]

That 3582A only goes up to 100 KHz or so anyway.

Here's an inexpensive general coverage SDR that might work well enough.  A hundred bux... http://cgi.ebay.com/Soft66LC2-case-Software-defined-Radio-/190545512457?pt=LH_DefaultDomain_0&hash=item2c5d65c809

[KM1H]

But it doesnt cover 160.....boo!

[K5UJ]

That 3582A only goes up to 100 KHz or so anyway.

Sorry about that.  I guess I thought anything from HP would at least go up through HF. 

[k4kyv]

One of the best specturm analyzers on the planet is a good receiver.

That's all a SA is anyway, at least the analogue version. Just a receiver with the output connected to a display, whose sweep is synchronised to vary in step with frequency modulation applied to the HF oscillator.

I did spend some time at the Flex Radio booth at Dayton, and one of the reps demonstrated the waterfall display and showed me how it worked. I had never quite figured out exactly what they were supposed to do, from the still pictures in the ads.

[KM1H]

I still cant make sense of a waterfall display when trying to decode a QRSS CW signal.

My eyes and brain are out of phase

[W4AMV]

Here is an idea... that I have used at audio and RF when required. The use of a receiver is an excellent method and of-course the analyzer is just that. However, both share a common problem, they are subject to overload and in the final scenario, can regenerate the 2nd, etc... harmonics. Their input stage can overload. So in measurement of PA distortion, we band reject the fundamental. Then pass the remaining signal onto the receiver and measure the remaining tone levels relative to the fundamental. So, build a 3 MHz bandstop filter, pass the remaining signal on to the O-scope, and for first pass estimates, measure the remaining voltage peak level (assume worse case). Convert the measured peak or peak -to -peak and convert to rms and use the THD equation to get the result. Comments?