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x0x:voltagecontrolledoscillator [2007/09/30 18:26]
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). ​ Q25 and Q27 act as a special switch, which is based on a [[http://​​wiki/​Silicon-controlled_rectifier|Silicon Controlled Rectifier]] which, when coupled with an integrator, 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 ==== 
-==== 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 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. 
-==== The Slide Circuit And Buffer ==== 
-==== Oscillator Drift Compensation and Tuning ==== 
-==== Anti-Log ==== 
-==== Switch ==== 
-Transistors Q24, Q25 and Q27, along with C33 are responsible for generating the SawWave. ​ The transistors Q25 and Q27 are set up in a special layout like so: 
-This arrangement is similar to a Silicon Controlled Rectifier.  ​ 
-===  From: Martin Czech,​ === 
-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 
-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. 
-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. 
-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 
-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 sure 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 ==== 
-After the saw wave gets passed through the JFET and heads out to the waveform select switch, it heads into a jungle that I can only assume is a waveshaper. Here's a picture of the circuit: ​ 
/home/ladyada/public_html/wiki/data/pages/x0x/voltagecontrolledoscillator.txt · Last modified: 2016/01/28 18:05 (external edit)