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Thread: Ts-520 Calibrator Marker Board 25 Khz., Error.

  1. #1

    Join Date
    Dec 2019
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    Kuna, Idaho
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    Default Please delete post

    No follow-up allowed to reply, I will not be posting again...not comfortable with the process.
    Last edited by Mac; Thu 2nd Jan 2020 at 20:20. Reason: Want to delete, not comfortable with the process.

  2. #2

    Join Date
    Nov 2019
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    Ash Fork, Arizona
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    40

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    You have come across a pretty interesting circuit. While it isn't used any more, and you can barely find a reference to the operating mode in any book or document, I still find it interesting. It's not really used any more, mostly because it's a bit unstable and there are better, easier, ways to do the same thing.

    The heart of that circuit is the Astable Multivibrator (Q2 and Q3). The multivibrator is designed to operate naturally at around 25 KHz. It doesn't divide or scale the oscillator frequency, but rather syncs to it. The crystal oscillator (Q1), running at 100 KHz, is coupled to the multivibrator via the 1N60 diode (D1). This coupling provides sync pulses for the multivibrator, which syncs to every fourth clock.

    For low divide ratios, less than 10, this works pretty well. But, as the part values drift over time, the multivibrator could easily obtain sync at a different ratio. You might think that, the natural frequency of the multivibrator would go up, with C6 decreasing, from 39 to 24 pF. But that's part of the instability. Rather than syncing to every second or third cycle, it synced to every tenth.

    The signal from multivibrator doesn't have nice clean edges. But to be rich in harmonics, the rise and fall times need to be fast. So the output transistor (Q4), in a saturated mode, is used to clean up the the multivibrator signal.

    Thanks for the interesting circuit.

    Martin, K7MEM

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