This shows you the differences between two versions of the page.
Next revision | Previous revision Next revision Both sides next revision | ||
x0x:voltagecontrolledoscillator [2007/04/04 06:08] hl-sdk |
x0x:voltagecontrolledoscillator [2007/09/30 18:26] hl-sdk |
||
---|---|---|---|
Line 1: | Line 1: | ||
===== The Voltage Controlled Oscillator ===== | ===== The Voltage Controlled Oscillator ===== | ||
{{template>.templates:schema|section=VCO|inputs=DigitalSequencer: Slide On/Off,Note|outputs=VoltageControlledFilter: Audio Out, MidiAndSync: CV Out}} | {{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 | + | 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://en.wikipedia.org/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 ==== | ==== Block Diagram ==== | ||
- | <draw name=vcoblock namespace=x0x> | + | {{x0x:media:vcoblock.png|}} |
==== The D/A converter and Resistor Network ==== | ==== 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 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 ==== | ==== The Slide Circuit And Buffer ==== | ||
Line 16: | Line 16: | ||
==== Oscillator Drift Compensation and Tuning ==== | ==== Oscillator Drift Compensation and Tuning ==== | ||
==== Anti-Log ==== | ==== Anti-Log ==== | ||
+ | |||
==== Switch ==== | ==== 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: | + | 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: |
- | <draw name=vcoswitch namespace=x0x> | + | {{x0x:media:vcoswitch.png|}} |
- | This arrangement is similar to a Silicon Controlled Regulator. | + | This arrangement is similar to a Silicon Controlled Rectifier. |
=== From: Martin Czech martin.czech@intermetall.de, === | === From: Martin Czech martin.czech@intermetall.de, === | ||
WorkOnMe: This needs to be explained a little better and not be so copyright-infringey | WorkOnMe: This needs to be explained a little better and not be so copyright-infringey | ||
- | And now to the scr discharger: | + | And now to the SCR discharger: |
As far as I can see the operation is as follows: | As far as I can see the operation is as follows: | ||
Q24 is a saturated npn (diode), if a little leakage current flows | Q24 is a saturated npn (diode), if a little leakage current flows | ||
- | thru Q27, Q24 will establish a temp compensated base potential for | + | through Q27, Q24 will establish a temp compensated base potential for |
- | Q25. This potential is about teh emmitter potential of Q24 + 0.6V. This | + | Q25. This potential is about the emitter potential of Q24 + 0.6V. This |
is the reference potential for the whole SCR switch. The temperature | is the reference potential for the whole SCR switch. The temperature | ||
compensation will prevent thermal run away of Q25. If the timing cap | compensation will prevent thermal run away of Q25. If the timing cap | ||
Line 39: | Line 40: | ||
of Q25 gets forward biased Q24 turns on and subsequently Q27, which in | 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 | 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 | + | 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 | 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, | 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 | 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 | + | clamping action of Q24). I think the diode D25 is necessary to ensure |
- | that the SCR will turn off quite fast, because the emmitter potential of | + | 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 | 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 | to not turn off, they are still on at very little currents, so I think | ||
Line 53: | Line 54: | ||
saturated, it takes a lot of time to clear the base zone from all | 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 | carriers. This also causes the flat portion of the saw wave and the | ||
- | time of this flatness should be independend of osc. frequency. | + | time of this flatness should be independent of osc. frequency. |
Could some 303 owner check this ? | Could some 303 owner check this ? | ||
Line 59: | Line 60: | ||
I said that Q27 and Q25 are saturated when the SCR is on, ie. transistor | 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 | 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 | + | 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 | 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 | + | 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 | 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 | that are very sensitive. So the problem of Q27 burnout could be addressed | ||
Line 69: | Line 70: | ||
==== Saw to Square Waveshaper ==== | ==== 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: | ||
+ | {{x0x:waveshaper.png|}} |