pulsevco
I'm using common voltage controlled oscillator IC – AS3340.
It has some problem with pulse wave output on higher frequencies. The bottom of falling edge looses sharpness the higher I go.
This is how it looks at the limits of audible threshold:
I need some advice on cleaning up this signal so I would get nice and crisp pulse shape all the way.
This is my schematic. I'm measuring it at the PULSE_OUT node, without attaching the amplifier circuit.
Use Power MOSFETs with very low Rdson. Look into an N-channel as well, to reduce Rdson. Don't forget to manage heat dissipation. Even at 90% efficient, you still need to dissipate 80W! Since your microcontroller probably can't drive the FET quickly enough, you probably need a preamp/level shifter stage as well.
See http://www.onsemi.com/PowerSolutions/product.do?id=NVD5890NL as an example.
You don't mention your application, but if you are trying to build an audio amplifer, open loop Class D amplifiers are limited in their THD.
Vishay's application note Faster Switching from Standard Couplers says:
PUSHING THE “SPEED ENVELOPE”
If less than 1 μs toff is required, there are some additional measures that can be taken to further increase the switching time of a standard coupler. […]PHOTODIODE OPERATION
One possibility is to give up current gain for the sake of speed in the extreme, and operate a standard transistor coupler in photodiode mode by only using the base collector pins of a 6 pin coupler. This will vastly increase the switching response of the device but will require an output buffering stage to produce a practical signal level. Also, it is possible to further increase the switching speed of this type of coupler by applying an increasingly large reverse bias voltage on the photodiode detector. The effect is to reduce the junction area and capacitance and increase the frequency response. […] Figure 30 illustrates a photodiode configuration which makes use of a trans-impedance amplifier to provide better gain and noise performance. The key principle of this design is that one can now take advantage of the lower junction capacitance of a simple reverse biased diode rather than the large capacitance of a phototransistor to drastically improve transient performance.
But don't bother with all this complexity; it also says:
At this point, however, economics come into the equation, and a designer needs to consider whether the increased complexity of the design is justified. The other option is to consider one of various parts that are specifically designed for high-speed operation.
For example, the common 6N137 requires 5 mA to switch on, and has a propagation delay of 100 ns, or the TLP2361, 1.3 mA, and 80 ns.
This results in a nice digital signal; you then still need a circuit to make the pulse longer.
Best Answer
As others have said, a Schmitt trigger will clean it up. Reducing the 51k resistor to 10k or so may help as well.
Also consider this waveform distortion is irrelevant for the typical intended electronic music function of the IC. A perfect 18kHz square wave is barely audible under good conditions. The harmonics of the imperfect square are way above audible frequency. The difference between a perfect square and your signal will not be perceptible in the context of electronic music. In an analog synthesizer, achieving perfect waveshape beyond useful audio frequencies isn't necessarily expected, required, practicable, or audible.