Sweep control oscillator

The purpose of this device is to help you sweep an audio or rf filter. It outputs a 0..10V sawtooth that can be used to control a sinus/ function generator or an rf generator. Together with a CRO oscilloscope, a millivolt meter and or power meter this can be used to accurately characterize a filter in the frequency. Call it a poor man the start of a poor mans spectrum analyzer.

Block diagram

Circuit description
The whole circuit can be broken down in three distinctive parts. The section around D1, D2, Q1 and IC1 is the 0 to 10V sawtooth generator. The components directly below (P1, P2, A1, A2 and Q2) are the marker section. Finally the components A3-5, P3 and P4 take care of the output scaling.

The sawtooth generator starts with D1, D2, Q1 and R2 which form a constant current source. Because the capacitor C1 is charged at a constant rate, the output will rise linearly. IC1, a classic 555 timer, monitors the voltage level on C1 on pins Trigger and Threshold. If the voltage rises above 2/3 V+ ( = 10V), the internal flip-flop gets set. This causes the output to go low and also the NPN transistor connected to the discharge pin to start discharging capacitor C1. Normally the discharging would stop/ the flip-flop would be reset if the voltage at the trigger pin would fall below 1/3 V+. However, the output of the timer IC is connected to the control via diode D5. The control pin is internally connected to resistor divider which offers the 1/3 and 2/3V+ voltages. If the output goes Low and the discharge pin start discharging the capacitor,the output pulls the control voltage to almost 0V (actually ~0,6V). Hence the voltage at which the 555 resets again is now at circa 0,3V. Thus the sawtooth at C1 rises from almost 0 to 10V and falls then back again to 0.
The repetition rate is set by the charge current, the size of capacitor C1 and the voltage change dV = I*t / C. If we rewrite that we get dV*C/I = t. dV equals to (almost) 10 V. C1 is 470nF. The current in the current source is (Vd1 + Vd2 – Vbe) / R2. If we take Vd1 = Vd2 = Vbe ~ 0,7V we get a current of ~ 1.5mA. If we plug the numbers into the formula we arrive at t= 3.1 msec or repetition frequency of 319 Hz. Which is a nice value to display your trace flicker free on your CRO screen.

Markers are displayed on the CRO screen as brighter dots in the middle of the trace. They are created by momentarily halting the sweep. The longer time spend at position will cause a brighter spot. The circuit features two marker generators. P1, P2, A1, A2, Q1 and the surrounding components take care of this. Since both circuits are identical only one half is discussed here. With variable resistor P1 a voltage can be selected between 0 and 10V. This voltage is connected to the non inverting input of op amp A1. The inverting input is connected to the 0 to 10V ramp off capacitor C1. At the beginning of the ramp the voltage at the non inverting input will be higher. The output of A1 will thus be positive. At the moment the ramp voltage will become slightly higher than the voltage on the non inverting input, the output of the op amp will rapidly swing to the negative supply rail. After C2, which blocks the dc-voltage at the output, a short negative spike will be present. This spike will be passed on to the gate of j-fet Q1. Because of the short negative voltage on it’s gate, Q1 will cease to conduct momentarily. This will halt the current through diodes D1 & D2, effectively stopping the current source around Q1. This will be visible as a short plateau in the ramp. The section around P2, A2 and D4 operate likewise. Diodes D3, D4 function as an analog OR gate that will pass the lowest of both voltages while isolating both marker circuit sections. Diodes D8, D9 make sure that capacitors C2 and C3 are rapidly charged again. This is important if one of the markers is placed closed to the start of the ramp.

The output scaling is handled by op amps A3, A4 and A5, eq it is used to select the start and stop frequency. The operation of this section is best understood with the timing diagrams below :

With P4 you can select the start voltage, and thus the start frequency, at the beginning of the sweep. With P3 you can select the voltage at the end of the sweep and hence the stop frequency. Those are then summed at resistors at R11 and R12. Because the voltage is also halved, the output op amp is set up as an amplifier with a gain of 2x. This brings the maximum voltage back to 10V.

The last section to discuss is the power supply. The 15V used everywhere is being delivered by a standard 7815 integrated voltage regulator. This model is manufactured in the industry standard TO220 housing, which does not even become luke warm. Nevertheless, using an 78L15 is probably not a good idea, the maximum peak current consumed by the NE555s might bring this regulator to its knees. Also the lower thermal resistance of the larger TO220 package ensures a more stable output voltage. Finally, the 2nd NE555 used in this circuit is set-up as a power oscillator with a symmetrical 22kHz square wave as output. The output is then fed to an inverting voltage doubler, providing circa -15V (at no load). Under load the voltages sags to about -8V to -10V. This voltage is used to supply the opamps with a negative power. In this way ordinary (eq. no rail-to-rail input and output) op amps can be used, while the input and output can still go to ground.

Have fun building!
73 de PA3COR

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