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- | ===== The Voltage Controlled Oscillator ===== | ||
- | {{template>.templates:schema|section=VCO|inputs=DigitalSequencer: Slide On/Off,Note|outputs=VoltageControlledFilter: Audio Out, MidiAndSync: CV Out}} | ||
- | The VCO is comprised of 5 sections. The first of which is the Digital to Analog converter, which takes the digital signals for the note generated by the MicroProcessor and converts them to a voltage. After that the voltage is sent to a buffer and the Slide Circuit. Then it is sent to Q26 which is responsible for (blah... marked as "antilog" on the Tb303 block schematic). Q24 and Q25 act as a special switch, which is based on a [[http://en.wikipedia.org/wiki/Silicon-controlled_rectifier|Silicon Controlled Rectifier]] which generates a Saw Wave. This waveform is then sent out to Q28 a 2SK30 JFET which is used as an operational amplifier. The output is sent to the switch, and also sent to Q8, where the saw wave is shaped into a square and back to the switch. From the switch, the output is sent to the VoltageControlledFilter | ||
- | ==== Block Diagram ==== | ||
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- | {{x0x:media:vcoblock.png|}} | ||
- | |||
- | ==== The D/A converter and Resistor Network ==== | ||
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- | The first part of this section is IC9, a 74ac174 Flip Flop, which takes 6 note inputs from Port C of the microcontroller and a clock signal (called Note Latch). On every clock signal, it samples the binary value from the note inputs, and outputs it to a 200K resistor network where it is added with the 5.333 Volt supply. This resistor network by virtue of math I do not understand takes the binary output of (for example) (010111, middle C) and mangles that into 3Volts. This of course, becomes the control Voltage. | ||
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- | ==== The Slide Circuit And Buffer ==== | ||
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- | |||
- | ==== Oscillator Drift Compensation and Tuning ==== | ||
- | ==== Anti-Log ==== | ||
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- | ==== Switch ==== | ||
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- | Transistors Q24, Q25 and Q27, along with C (... unreadable on the schematic) are responsible for generating the SawWave. The transistors Q25 and Q27 are set up in a special layout like so: | ||
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- | {{x0x:media:vcoswitch.png|}} | ||
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- | This arrangement is similar to a Silicon Controlled Rectifier. | ||
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- | === From: Martin Czech martin.czech@intermetall.de, === | ||
- | WorkOnMe: This needs to be explained a little better and not be so copyright-infringey | ||
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- | And now to the SCR discharger: | ||
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- | As far as I can see the operation is as follows: | ||
- | Q24 is a saturated npn (diode), if a little leakage current flows | ||
- | through Q27, Q24 will establish a temp compensated base potential for | ||
- | Q25. This potential is about the emitter potential of Q24 + 0.6V. This | ||
- | is the reference potential for the whole SCR switch. The temperature | ||
- | compensation will prevent thermal run away of Q25. If the timing cap | ||
- | C33 is discharged by the expo converter such that the emitter junction | ||
- | of Q25 gets forward biased Q24 turns on and subsequently Q27, which in | ||
- | turn raises the base potential of Q25, this is the SCR snapback. The | ||
- | snapback point would depend on the thermal behavior of Q25 if there was | ||
- | not the temp compensation with Q24. If the SCR has triggered the base | ||
- | potential of Q25 rises to 12V (supply) minus the diode voltage of D25, | ||
- | ie. about 11.4V (this is possible because of R101, which disables the | ||
- | clamping action of Q24). I think the diode D25 is necessary to ensure | ||
- | that the SCR will turn off quite fast, because the emitter potential of | ||
- | Q25 has only to rise to 12V - 2x0.6V =10.8V to turn Q25 off. SCRs tend | ||
- | to not turn off, they are still on at very little currents, so I think | ||
- | the additional diode makes the shut off faster and safer. | ||
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- | I think the whole arrangement does only reach to the 12V supply because | ||
- | of the shut off latency of Q25 and Q27, these transistors are heavily | ||
- | saturated, it takes a lot of time to clear the base zone from all | ||
- | carriers. This also causes the flat portion of the saw wave and the | ||
- | time of this flatness should be independent of osc. frequency. | ||
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- | Could some 303 owner check this ? | ||
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- | I said that Q27 and Q25 are saturated when the SCR is on, ie. transistor | ||
- | beta will be very low, maybe 2 or 1.5 or so. This means that a large | ||
- | amount of the transistor current flows through the base, especially in | ||
- | the case of Q27 where there is no base resistance nor diode. If Q27 is | ||
- | not suited for such an application, it will shure burn out. Most good | ||
- | audio transistors with high Ft and high beta have shallow base junctions | ||
- | that are very sensitive. So the problem of Q27 burnout could be addressed | ||
- | with using (perhaps) lower beta, switch type transistor with very high | ||
- | specified max base current. The same applies to Q25 and Q24 (remember | ||
- | tempco compensation, Q25 and Q24 should be the same type) | ||
- | |||
- | ==== Saw to Square Waveshaper ==== |