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Old 10th Oct 2019, 9:23 pm   #1
Steve G4WCS
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Default Two tone oscillator

I want to check the linearity of the Cobra on ssb, and Ive a few linear amplifier refurbs that are untested so need a two tone oscillator. This design seems simple enough and Ive managed to scrape together the components out of the parts drawers (though Ive used a 2N3704). One oscillator done, I’ll make sure its working before I solder up the other.

A bit of theory here https://radiotransmitter.wordpress.c...est-generator/
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Old 14th Oct 2019, 11:45 am   #2
Wendymott
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Default Re: Two tone oscillator

Steve... I had the same problem when making my 40M ssb transceiver. I made a dual audio oscillator, but found that I got results that were very disappointing. I have no idea if it was distorted sinewaves, although they looked ok on the scope, however my monitored waveforms were not as expected.
I bit the bullet and bought a dual audio osc from ebay, and was the end of my problems... from that I have made my multiband SSB transceiver, with no setup problems..
THe amplitudes of both sinewaves should be identical, and the frequencies are.... in my case 750 hz and 1.33 khz... good luck and I hope it works for you.
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Old 14th Oct 2019, 3:52 pm   #3
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Default Re: Two tone oscillator

Steve, on the linked page, the first line talks of 'two wien bridge oscillators' but the circuit diagram shows a variant of the phase shift oscillator, so I'd be a little cautious of other aspects of the design.

Good control of the level is important in low (or even not large) distortion oscillators and this circuit is lacking in anything definite to do the job. It'll be driving the transistor into cut-off round part of the cycle.

Nowadays a bit of work with 'audacity' burned onto a CDrom and played through a portable player has a number of advantages.

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Old 14th Oct 2019, 5:05 pm   #4
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Default Re: Two tone oscillator

The text appears to say that it is a Twin-T oscillator, and the circuit diagram seems to show a Twin-T arrangement. Where did you see mention of Wien bridge? I cannot see a phase shift oscillator.
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Old 14th Oct 2019, 5:21 pm   #5
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Default Re: Two tone oscillator

Quote:
Originally Posted by G8HQP Dave View Post
Where did you see mention of Wien bridge? I cannot see a phase shift oscillator.
Look at the page which opens when you click on the website in post number 1

It talks of wien bridge but shows a phase shift oscillator.

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Old 14th Oct 2019, 9:15 pm   #6
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Default Re: Two tone oscillator

I understand if Steve wants to be hands-on with this but for those of us who are lazier an alternative, virtually off the peg PC-software method is illustrated here:

https://vk6ysf.com/ssb_2tone_test.htm

There is a link to where to get the actual software used at the bottom of that page. Looks like a useful tool, and appears to be free for the 'lite' version.
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Old 15th Oct 2019, 12:33 am   #7
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Default Re: Two tone oscillator

Just looking at the signal on a 'scope won't tell you much unless the TX is really nonlinear.


Surely a spectrum analyser should be used?
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Old 15th Oct 2019, 5:46 am   #8
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Default Re: Two tone oscillator

Your looking at modulation linearity in the two tone test, a spec an won't tell you anything other than how dirty your Tx is overall.
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Old 15th Oct 2019, 11:45 am   #9
Steve G4WCS
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Default Re: Two tone oscillator

Thanks for the replies, for now its kitchen table tinkering/experimentation


Hoping to become custodian of this R&S kit soon which should be useful
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Old 15th Oct 2019, 1:11 pm   #10
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Default Re: Two tone oscillator

That seems to be only a signal generator, so it won't tell you anything about transmitter linearity.

If you're worried about linearity of an SSB transmitter, you need a two-tone generator to drive audio into the transmitter then a high power attenuator and a spectrum analyser to look at the sidebands on the output. You're looking to get the 3rd and 5th order products 30dB below the output fundamental tones, or lower. This isn't going to be visible on a scope.

Without the spectrum analyser, a two-tone audio oscillator with a scope is good for checking the PEP power output.

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Old 15th Oct 2019, 1:38 pm   #11
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Default Re: Two tone oscillator

Quote:
Look at the page which opens when you click on the website in post number 1

It talks of wien bridge but shows a phase shift oscillator.
OK. I though you were referring to the thumbnails he posted. I didn't realise that the link was a different design.
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Old 15th Oct 2019, 9:13 pm   #12
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Default Re: Two tone oscillator

Whatever you do with two-tone tests, always make sure the two tones are not harmonically-related, or close-to-harmonically-related.

Example, don't use 800 and 2400Hz. In one of the 1950s Eimac books there was a series of tables showing the 'best' test frequencies to use so as not to get issues with the non-sinusoidal aspects of one oscillator causing intermodulation with the other oscillator's fundamental.
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Old 8th Jan 2020, 8:11 pm   #13
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Default Re: Two tone oscillator

Picked this up for £10. A vintage “cambridge” unit. Presumably built from a kit.
It has a pp3 battery clip on it but I suspect its designed for a lower voltage, the sine wave is better at 6v than 9v, may run it off two lithium coin cells.
A couple of the caps are cracked, is this an issue ?
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Old 8th Jan 2020, 8:40 pm   #14
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Default Re: Two tone oscillator

It's quite common for the coating to crack at the ends of those "liquorice allsort" or "tropical fish" capacitors.
If the actual terminations are OK then it doesn't matter, other than cosmetically.
I'm not sure those things deserve the silly high prices often asked for them, although they do look nice (if not cracked).
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Old 8th Jan 2020, 10:54 pm   #15
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Default Re: Two tone oscillator

In my youth I made a copy of the two tone oscillator from a 1980s ARRL handbook. This used several dual opamps and a couple of bulbs to make a fairly decent dual Wein oscillator. It had a lowpass filter on the output of each oscillator and then a summing opamp. This provided high isolation between each oscillator and this helped minimise any IMD or harmonic distortion. I used this for many years to help service SSB transmitters.

I quickly learned that the best way to configure one of these 2Tone oscillators is to fit it in a small screened box and to power it from internal (PP3?) batteries. It can then connect to the radio via a regular coiled mic wire. This means the only connection path is directly to the radio mic socket and there is no additional path from the 2Tone oscillator to an external PSU for example. This prevents any earth return paths that can cause spurious problems. The alternative is to try and power the 2Tone oscillator from a bench PSU. It is possible to do this but it is much harder to prevent unwanted RF leakage/loops in the system.

Note that I took all this stuff quite seriously in the 1980s and tried to make a setup that ensured that the radio was by far the weakest link in terms of distortion and spurious, and not the test gear. A poorly setup system that has an RF pickup issue can generate an additional low level AF tone in the radio AF stages because the difference of the final RF frequencies RF2-RF1 gives a second order distortion term that can fall inside the AF passband. This can spoil the waveform on a scope or the response on a spectrum analyser.

In summary, it really is worth the effort to make something decent here otherwise the test setup can sometimes misbehave and make the radio look much worse than it really is...

I dug out my old 2Tone generator and powered it up. It still works! See below for a quick spurious plot taken with a PC soundcard. There is no visible IMD3 distortion and the 2nd harmonic distortion is about 75dB down. This is overkill for testing an SSB transmitter but it really is easy to get performance this good. Also, it's even easier to throw it all away with a poor system setup on the bench.
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Old 9th Jan 2020, 12:49 am   #16
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Default Re: Two tone oscillator

Quote:
Originally Posted by G0HZU_JMR View Post
it really is easy to get performance this good. Also, it's even easier to throw it all away with a poor system setup on the bench.
As said, it's not difficult. I would add that if a simpler oscillator without proper level control (the light bulb thermistors)is used then the performance is very badly compromised. You get equipment that is either significantly better than you need, or significantly worse than you need. There's nothing in-between.

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Old 9th Jan 2020, 4:05 am   #17
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Default Re: Two tone oscillator

I have found building transistor phase shift oscillators, they need a stable power supply and almost always for good linearity and a low distortion sine wave need some un-bypassed emitter degeneration, but not enough so they won't start.

It is a question that often crops up though in some designs; where to get a stable low distortion sine wave from quickly & easily without an elaborate configuration ?

The answer might well be the circuit attached. It was Designed by Mauri Lampi in Victoria AU. It was published in Silicon Chip Oct 2019

It uses a Norton current feedback OP amp, like an LM3900, the non linear function of the diode like inputs makes it possible.

The frequency is Root ( 1/( R1xR2xC1xC2)) all divided by 2pi.

The output amplitude is adjusted with R3.

I have not tried it but it looks like a very satisfactory solution and LM3900's are fun to work with.
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Old 10th Jan 2020, 2:00 am   #18
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Default Re: Two tone oscillator

Has anyone tried the Texas Instruments design for the "Quick sine-wave oscillator" by Walter Bacharowski using the LMV822/932 dual opamps? Unfortunately the spreadsheet calculator for R/C values has disappeared off the face of the www and I got diverted trying to do one in Excel for sensible audio frequencies but had to give up.

Point is, this is completely uncritical of supply voltage and needs no non-ohmic thermal devices. On my to-do list, just to measure the harmonic content with a distortion analyser.

Quick sine-wave oscillator
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