ICL8038 Precision Waveform Generator/Voltage Controlled Oscillator
An problem with this generators is strong distortion on sine wave output pin 2, so most of the solutions based on this chip has a limited frequency.
Another is that the square wave output from pin 9 is an open collector that needs to be pulled up to the power supply. In some way the quality of other waveforms is dependent on the load on this pin. I excluded the 15M potentiometer indicated in the datasheet on page 7, squeezes the top of the upper half of the signal.
As mentioned, square output pin 9 with open collector, normally would never be able to give a good square wave at these frequencies. Rising edge depends only on the pull-up resistor and its growing very slowly. If we give too strong pull-up resistor, in turn, the trailing edge will be weak because the internal transistor is too heavily loaded.
Here, I fixed the square wave with a comparator so that the slope is pretty steep, steep as applied comparator can give. Shown in the schematic LM339 has 1.3μs response time but it would be good to use even faster model of comparator.
The amplitude of the signal, unfortunately, is not the same for each function, tests have shown that trying to match it with the usual R/R signal dividers will give very distorted waveforms as rounded square and triangle, so i resigned from such divisors.
Sine output on pin 2 as ratio 0.22~ * Vcc
Triangle output on pin 3 as ratio 0.33~ * Vcc
Square output on pin 9 as ratio 0.9~ * Vcc
This however was not a problem since I use the Quad OpAmps TL084 as a mixer anyway. What I cannot avoid at this point is the use of trimmers for signal calibration at 2Vpp.
The physical potentiometers for frequency sweep and duty cycle are respectfully replaced, by the 10KΩ MCP42010 and the 5KΩ MCP4251.
For each VCO both potentiometers of the MCP42010 are used. So there's a MCP42010 for VCO. One is used as the main one and the other is in series for fine tuning. Only a single one MCP4251 potentiometer for duty is used. Half for VCO.
Frequency Modulation and Sweeping
The frequency of the waveform generator is a direct function of the DC voltage at Terminal 8 (measured from V+). By altering this voltage, frequency modulation is performed. For small deviations (e.g. ±10%) the modulating signal can be applied directly to pin 8, merely providing DC decoupling with a capacitor. An external resistor between pins 7 and 8 is not necessary, but it can be used to increase input impedance from about 8kΩ (pins 7 and 8 connected together), to about (R + 8kΩ). For larger FM deviations or for frequency sweeping, the modulating signal is applied between the positive supply voltage and pin 8.
The potential on Pin 8 may be swept down from V+ by (1/3 VSUPPLY - 2V). Then for ALFONSo from +5V (Low Freq) to +3.33V (High Freq).
TL084 High-speed JFET input, quad operational amplifiers
The TL084 is high speed J–FET input quad operational amplifiers. The devices feature high slew rates, low input bias and offset currents, and low offset voltage temperature coefficient.
The TL084 is used to make the harmonics mixer. For each harmonic there is the relative gain adjustment trimmer which allows it to be brought into line with 2vpp.
LM339 Quad Comparators
This quad comparator is used extensively to make CD4066B control lines compatible. The CD4066Bs are used to switch the frequency range, to enable individual harmonics and enable the singles VCO.
The CD4066 must have a dual power supply to fully support the signals produced, and in this mode enabling and disabling of the bilateral switches is done with the +5V/-5V control signals.
Then the +5V/0V MCP23S17 I/O Expander outputs are converted to dual levels using the Quad comparators provided by the LM339.
Currently each VCO needs 8 control GPIOs. For a total of two LM339 per VCO dedicated to this purpose.
The LM339 is also used to purify the square wave output which is suboptimal due to the value of the pull-up resistor. In this case one is used for VCO.
LM393 Dual Comparators
The LM393 is a dual comparator used primarily to produce a +5V/0 signal, derived from the square wave, compatible with the a generic input of the controller.
This allows the controller to calculate the current frequency of the selected VCO.
CD4066B CMOS Quad Bilateral Switch
The CD4066B is a quad bilateral switch intended for the transmission or multiplexing of analog or digital signals.
The CD4066B consists of four bilateral switches, each with independent controls. Both the p and the n devices in a given switch are biased on or off simultaneously by the control signal. The well of the n-channel device on each switch is tied to either the input (when the switch is on) or to VSS (when the switch
is off). This configuration eliminates the variation of the switch-transistor threshold voltage with input signal and, thus, keeps the on-state resistance low over the full operating-signal range.
SPI Devices
The PIC18F4550 has only one MSSP (Master Synchronous Serial Port). While it can be dynamically reset, the open-drain nature of the I2C (Inter Integrated Circuit) protocol makes it incompatible with SPI (Serial Peripheral Interface). So I chose to use only SPI peripherals even with the expenditure of lines for the various CSs (Chip Select).
SPI ICs used :
MCP23S08 High-speed SPI 8-Bit I/O Expander from Microchip
A very convenient prerogative of this series of SPI I/O Expanders is that it has the possibility of setting additional hardware addresses which allow you to select the peripheral with the same CS line.
In the case of the MCP23S08 it has two address pins wired for a total of four I/O expanders per CS.
In the project it is used to drive via SPI the Hitachi HD44780-based LCD in four bit mode, the related handshake and internal LED lighting signals.
In the microcode a specific library has been developed for the coupling of the MCP23S08 and the HD44780.
MCP23S17 High-speed SPI 16-Bit I/O Expander from Microchip
Like its minor brother MCP23S08, the MCP23S17 has the ability to set hardware addresses switching from previous two to three addresses for eight devices on the same CS line.
The MCP23S17 is used to drive the numerous analog bilateral switchs found in mixers. There is one for each two VCO module.
MCP42010 Dual Digital Potentiometer SPI The IC is a double potentiometer with 256 taps 8-bit for each potentiometer. The MCP42XXX series is with the internal register is volatile. So it doesn't keep the last setting. At the moment it is the software that imposes all the default levels on connection. The permanent model is also available in case of future considerations. The MCP42XXX series is available with the values for 10 kΩ, 50 kΩ and 100 kΩ. There is a nominal wiper resistance of 52Ω for the 10 kΩ version, 125Ω for the 50 kΩ and 100 kΩ versions. The channel-to-channel matching variation is less than 1%. The resistance between the wiper and either of the resistor endpoints varies linearly according to the value stored in the data register. At power-up or upon habilitation the RS, all data registers will automatically be loaded with the mid-scale value (80h). The Shutdown pin SHDN, if enabled, leaves the A terminal is opened and the B and W terminals are connected for all potentiometers. A feature of this SPI IC is the presence of SO output pin, multiple devices can be daisy-chained together.
MCP4251 The IC is a double potentiometer with 257 taps 8-bit for each potentiometer. The MCP4251 also has volatile internal register loaded with the mid-scale value (80h). It has a nominal wiper resistance of 75Ω typical for all available values 5kΩ, 10kΩ, 50kΩ, 100 kΩ.