VXO based audio signal generator

Discussion in 'Homebrew and Kit Projects' started by KC0ZJZ, Aug 14, 2008.

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  1. KC0ZJZ

    KC0ZJZ Ham Member QRZ Page

    This is a simple audio signal generator that I built to test some CW filters I was building. Basicly I just beat a couple of rocks together. I figured it would be easier to get a few KHz of tuning range at RF than at baseband. This circuit resolved the two issues that I was having with audio ocillators; narrow tuning range, and drift. It tunes from about 200Hz to about 5KHz. The amplitude variation, while not flat was good enough for my simple tests.

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  2. K7JEM

    K7JEM Ham Member QRZ Page

    There are lots of stable chips that can tune that range, and produce good sine waves. They would probably be easier to build and use than what you've got there.

    But it looks like you did a good job, and it was probably fun doing it.

  3. W4HAY

    W4HAY XML Subscriber QRZ Page

    Nice construction and output!
  4. K5UOS

    K5UOS Ham Member QRZ Page


    That is a very clever way to create an audio signal. Its a direct conversion receiver complete with audio filter. Is the cutoff (Fc) for the filter 11KHz?

    The reason I ask is that you stated the max was 5KHz and a harmonic of 5KHz would be 10KHz....below you filter cutoff.

    Thanks for the idea and it is nicely built.

    73 K5UOS
  5. AC0FP

    AC0FP Ham Member QRZ Page

    Certainly an interesting circuit and I'm sure quite original. Looks like a good job of building also.


  6. KL7AJ

    KL7AJ Ham Member QRZ Page

    I beatum rock together too. Makum big fire. Ugg!

    The Collins PTO got its great stability by running two oscillators and taking the difference....the assumption being that they'd both drift in the same direction, leaving the difference the same. Worked great.

  7. K9STH

    K9STH Ham Member QRZ Page


    Where did you get the "The Collins PTO got its great stability by running two oscillators and taking the difference....the assumption being that they'd both drift in the same direction, leaving the difference the same. Worked great."

    All Collins PTOs are basically single frequency tunable oscillators. They get their excellent stability from using a permability tuned oscillator which uses variable inductance rather than variable capacitance. Most Collins PTOs get their linearity from a series of plates which are adjusted in conjunction with a "cam". Calibration of these was VERY tedious and Collins had very few employees who were qualified to set these plates.

    The primary units which used 2 different PTOs were the 708A-1 and 708A-3 which have PTOs operating in the 1.5 MHz to 2.2 MHz and in the 300 kHz to 400 kHz range. These are "beat together" to get an output from 2.0 MHz to 4.2 MHz. According to Collins specifications either unit can be "set" to the nearest 30 Hz just by reading the dials. The 708A-2 was the power supply for either unit. I have had a 708A-1 (do not have the 708A-2) for quite a number of years. Frankly, I really want to trade this off since I do not use it.

    Depending on the unit, Collins did use a crystal controlled heterodyne oscillator to "beat" with the PTO to produce the desired output frequency. By keeping the PTO output frequency to a relatively low frequency range operating at higher frequencies for the r.f. carrier remains constant rather than any drift being multiplied. This was accomplished in all of the Collins designed receivers as well in many transmitters. But, on many of the older Collins transmitters the PTO frequency was multiplied which did increase the drift the higher you went in frequency. These transmitters include the ART-13, the 32V- series, the KW-1, and so forth.

    However, with the exception of the 708A-1 and 708A-3 Collins unit using PTOs had only a single PTO for frequency excursions and those units, because of their design, were considerably more stable than other tunable oscillators of the era. In fact, even today the Collins PTO units are very stable and compare favorably with "modern" units.

    Glen, K9STH
  8. KC0ZJZ

    KC0ZJZ Ham Member QRZ Page

    Thanks, I wasn't aware of such devices. I've been homebrewing digital circuits for a long time now; but it has been only recently that my interest has turned to radio and analog circuits. It's like starting all over again. Learning new stuff pretty much tops my fun list.

  9. KC0ZJZ

    KC0ZJZ Ham Member QRZ Page

    This circuit is a outgrowth of some earlier experiments with the "Beginers DC receiver" as described in EMRFD. It also provided a test platform for trying out a refinment to the VXO oscillators I was playing with. I also felt that it would give me some hands on with the "premix VFO" technique.

    Yes the audio filter cuttoff is near 11KHz, I have fewer values of the larger sized caps, and I was mostly concerned with knocking down any remaining HF. Good point about the harmonic, thanks.

    Basicly, I'm just getting started building RF circuits, and have much to learn (the best part) about the things I need to deal with.

    I started with a simple regen, then the DC receivers. After that I built a bunch of VFO's of various topologies. Lately I've been playing with some mixers using a dual gate mostfet, a bf998.

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