Two-Tone Test oscillators

Discussion in 'Homebrew and Kit Projects' started by WA9SVD, Sep 12, 2020.

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  1. WA9SVD

    WA9SVD Ham Member QRZ Page


    I want to build a two-tone tester for SSB amps. I already KNOW that the tones are supposed to be sine waves, and not harmonically related. I have seen many different circuits, and different frequencies used; I want to use ≈ 800 Hz for the low side, and ≈1800 Hz for the high side, the exact frequencies (+/- about 1-200 Hz) are not really critical. My question is, what level of distortion in the sine waves is considered acceptable for amp testing?
  2. K7JEM

    K7JEM Ham Member QRZ Page

    My guess would be 2% THD or better. This should not be hard to get from sine wave oscillators. .5% would be a good target, if you have a choice.
  3. KL7AJ

    KL7AJ Ham Member QRZ Page

    1% is typical and easy to do with a lamp controlled Wien bridge osculator
  4. G0HZU

    G0HZU QRZ Member

    In my youth I built the two tone oscillator project shown in the ARRL handbook. It was in one of the 1980s editions and I think this oscillator appeared for several years. It used a Wien bridge circuit made from opamps and a couple of lamp bulbs. The performance was very good. I got something like -75dBc harmonics and better than -85dBc IMD levels although I think I fitted a slightly better summing opamp to get this low level of IMD. This level of performance is a bit OTT for ham use but it wasn't difficult to build and the overall performance was very good.
  5. WA9SVD

    WA9SVD Ham Member QRZ Page

    I have that circuit from the 1988 Handbook; however, there are a few problems:

    1. The bulbs specified have no "bulb #," only a RS number

    2. There are no RS stores within 50 miles of where I live

    3. RS (IF it still exists) has a $35.00 minimum for shipping

    4. No time frame or actual availability of parts

    5. $2.00 each for a "grain of wheat" bulb is ridiculous

    6. I hate RS even more now, that they charged me $7.00+ tax for a "no name" coin cell I later found at Fry's for $1.99, with a REAL name brand.

    I severely dislike "single source, no equivalent" parts.

    But thanks.
  6. WB6TIX

    WB6TIX Ham Member QRZ Page

    I don't like single source parts either, but I will give ARRL some slack for things appearing in older publications. Back then RS stores were in many small towns and including RS parts numbers in parts lists was hoped to ease the parts procurement problems. Alas, 32+ years of time passing shows their guess was wrong.

    I dug around and didn't see any bulbs with the 12V 25mA rating, but the nice thing about a lamp controlled wein bridge oscillator is that the lamp is not terribly critical. On the previous page of the handbook a very similar oscillator shows a #327 bulb being used. That bulb has a 24V 40mA filament but doesn't have wire leads so you'd need to solder wires on the lamp and secure it in place. A #1869 bulb, 10V at 14mA and with wire leads should work as well.

    Still, you end up needing a $1 part with the added charges of; $8 shipping, $2 handling, $3 fuel surcharge, $1 environmental fee, $4 packaging material fee, $3 "just because we can" fee,$2 fee to cover the costs of collecting the other fees...and so on..... :)
    Last edited: Sep 13, 2020
  7. KB7WG

    KB7WG Ham Member QRZ Page

    Why not search for a software program, that can generate those tones thru your sound card?
    KD2ACO likes this.
  8. WB6TIX

    WB6TIX Ham Member QRZ Page

    Yes, you could use a sound card to generate the tones. One would need to ensure the tones are clean enough to be useable.
  9. K3RU

    K3RU Ham Member QRZ Page

    "Elecraft 2T-GEN" two tone test generator KIT, $89.95 from DXEngineering.

    I bought one, works well....AFTER I found a bad capacitor supplied in the kit by Elecraft.
  10. N7EKU

    N7EKU Ham Member QRZ Page


    Digikey has MANY small incandescent bulbs. I found some 12V 40ma bulbs for less than a dollar each and you could likely use two in series or one with a resistor to get the action you need. Often the theoretical explanation will give an idea how to make other parts work if you can't find the exact one.

    Digikey has no minimum order, no handling fee, and you can specifiy USPS First Class which is about five bucks or less for something that small.

    But you can also just use a cell phone and an oscillator app playing tones into the microphone or use a plug in the headphone jack to get the tones directly out.

    73, Mark

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