Ramp Generator: Oscillator

Background

A while back, I purchased a Topward 8110 Function Generator from eBay at a pretty good price. The unit had a dent in the front panel and was marked as needing repair.   I was able to remove and then flatten the front panel. No other physical damage was found. I did have to replace two driver transistors to get the unit working. But at the end of it all, I was pleased with the unit. Figure 1 shows the front panel.

Figure 1. Topward 8110 Function Generator

One of the reasons I wanted this unit, was that it had a VCF input (Voltage Controlled Frequency). Using a ramp signal, the FG (Function Generator) could be made to sweep a range of frequencies (1000:1 range). Using a scope, it is possible to view a filter response as the frequency is swept through its range.

Some FGs provide this sweep capability as a built-in feature. But to use the Topward 8110 in this way, I needed a ramp generator circuit. So why not build it myself?

Oscillator Design

The Topward FG accepts a range of 0 to +5V in the VCF input. So I decided that the entire circuit would run from a regulated power source of +5V.

The sweep (ramp) frequency was chosen to be under 100 Hz. It didn't need to be high and there was no reason to make it higher. The requirements so far, allowed the use of a NE555 timer and some low voltage rail-to-rail opamps.

The ramp could be generated using opamps but I settled on the 555 timer in order to keep this project simple. This would be the main generator for the whole unit. The one feature of the 555 timer that I liked is that the timing capacitor is charged and discharged within a strict range of 1/3 to 2/3 VCC. Using a +5V supply, this means that the capacitor would operate within a 1.67V to 3.33V range.

The next design problem is that normally a capacitor charges according to an exponential curve. What I needed was a linear ramp. So a constant current source for the charging circuit would be necessary. Figure 2 shows the NE555 circuit.

Figure 2. NE555 Ramp Generator

The LED1 componentwas chosen to have a forward voltage drop of about 1.2 V at about 10 mA. Red LEDs usually have some of the lowest VF values, so a red one was used. In this circuit however, the VR1 turns out to be 3.28V due to the lower current flow (VF=1.72V). It would have been best if VR1 was 2/3VCC + Vbe or higher, but I found that it worked well enough (ideally VR1 >= 3.93V). Voltage at the emitter of Q1 was measured to be 3.38V, with its base at 3.32V.

The PNP transistor Q1, combined with the LED1/R1 divider forms a simple constant current source with the caveat that some loss of linearity occurs at the top of VC1. The remainder of the 555 timer circuit is your standard astable timer configuration. The output on pin 3 will provide a square sync pulse, while the ramp signal of interest forms across C1.

Figure 3. NE555 timer signals C1, and output Pin 3.

From figure 3 you can see the square sync output on the scope channel 2, which varies between ground and about 4.57V. The signal output goes high until the capacitor starts to discharge. At the end of the discharge cycle, the sync output returns high again.

The VC1 is the sought after ramp signal. It rises after the discharge cycle completes. The ramp is very linear except perhaps at the end. Any non-linearity appears unnoticable here. On average, the ramp ranges in voltage from 1.65V to 3.32V before being discharged.

Next Objective

This circuit is not usable yet. We can't feed VC1 into the FG until we:

1. Buffer VC1
2. Level shift it for greater range

The charging circuit would be disrupted if we tried sending VC1 to the FG directly. It needs some kind of a buffer circuit. Further, we desire the full 0 to 5V sweep range, rather than the limited 1/3 to 2/3VCC. This will allow a greater frequency sweep.

In the next instalment, we'll design the level shift and correct the voltage range.

Thanks for reading.