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--- x0x:voltagecontrolledoscillator [2007/04/04 06:08]
hl-sdk
+++ x0x:voltagecontrolledoscillator [2016/01/28 18:05]
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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 SiliconControlledRecifier 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 ====
-<draw name=vcoblock namespace=x0x>
-==== The D/A converter and Resistor Network ====
-The first part of this section is IC9, a 74ac174 Flip Flop, which takes 6 note inputs from Port C of the MicroProcessor 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 resisitor 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.
-==== The Slide Circuit And Buffer ====
-==== Oscillator Drift Compensation and Tuning ====
-==== Anti-Log ====
-==== Switch ====
-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:
-<draw name=vcoswitch namespace=x0x>
-This arrangement is similar to a Silicon Controlled Regulator.
-===  From: Martin Czech martin.czech@intermetall.de, ===
-WorkOnMe:  This needs to be explained a little better and not be so copyright-infringey
-And now to the scr discharger:
-As far as I can see the operation is as follows:
-Q24 is a saturated npn (diode), if a little leakage current flows
-thru Q27, Q24 will establish a temp compensated base potential for
-Q25. This potential is about teh emmitter 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 behaviour 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 enshure
-that the SCR will turn off quite fast, because the emmitter 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.
-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 independend of osc. frequency.
-Could some 303 owner check this ?
-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
-ammount 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 ====

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