![]() Much better than the 7410, but still equal to the rise time of a sine wave at about 55 MHz. Around 20 years further up the technology ladder, and measured as having a rise time of 4.5 nS. Rise time measurement for a 74HC240, 4.1nSĬasting around for higher speed 74 logic variants in a breadboard-friendly DIP package on the bench, the next up was a 74HC240 octal buffer. Still, better than the 2N3904, but surely we can achieve more. This corresponds to the rise time of a sine wave at about 35.2 MHz, but that figure is something of a theoretical upper maximum of the 7410’s performance envelope and the real usable figure would be rather less. Its data sheet quotes a typical low-to-high rise time of 11 nS, perhaps our device was one of the better ones as the ‘scope measured 7.1 nS. ![]() We turned up the only original 74 series device we had to hand, a 7410 3-input NAND gate chip. The archetypal logic gate family is of course the 74 series of TTL devices, of which there are many variants with ever-improving characteristics since the series first saw the light of day in the 1960s. Logic gates are optimised for fast transitions, and should be correspondingly quicker than the 2N3904 we considered earlier. Rise time measurement for a SN7410N, 7.1nS This required some care with respect to lead lengths and ceramic decoupling capacitors to clean up and shorten the transition to this length, it is likely that further measures could shave some more time from this figure. We were fortunate enough to be able to borrow a mercury-wetted relay for this article, and when switching logic level into a 10 K resistor measured through an oscilloscope probe we were able to measure an impressive 4.6 nS rise time. This contact time is well below a nanosecond, which means that the rest of the circuit around the relay and the voltage being switched governs the rise time, so extremely fast times can be achieved. This produces an instantaneous contact, as the mechanism is that of liquid mercury droplets combining with each other rather than spring contacts touching. The mercury-wetted relay is a type of reed relay in which the contacts are coated in mercury by capillary action. But before we make that journey there is a surprising source of very fast rise times that’s not even electronic, it’s mechanical. When it comes to faster transition times, you might expect our path to lead directly to components designed for square wave transitions, such as logic gates. A 4.6 nS rise time from a mercury-wetted relay Of course the 2N3904 is capable of working at much higher frequencies in small-signal mode, but if it has to traverse the entirety of its range you’re stuck at 7.14 MHz. This might sound rather quick, but it corresponds to the rise time of a sine wave just over 7.14 MHz. Thus if you applied a perfect square transition to its base, the corresponding change at its collector would finish happening a maximum of 35 nS later. Taking as an example the 2N3094 popular general purpose transistor, you’ll find it has a quoted maximum rise time of 35nS. If you look at the data sheet for a typical transistor, you will find a section devoted to switching characteristics. That Was Considered Fast, Back In My Day Switching characteristics of a 2N3904, taken from the ON Semiconductor 2N3904 data sheet. So it’s worth taking a look at the rise times you’d expect from everyday circuitry, examining a few techniques for generating rise times that are much faster. When the instrument can happily measure the transition times of all your usual pulse generators, something out of the ordinary is called for. The application that prompted this article was the measurement of oscilloscope bandwidth by looking at how quickly the ‘scope catches up with a pulse that exceeds its bandwidth, for example. ![]() Sometimes though, the rise time of a logic transition is important. The glue logic for your Arduino project can take its time. If we look into the subject a little deeper we learn that what seemed an instantaneous cliff-face is in fact a very steep slope, but when a circuit does its business in milliseconds there is usually no harm in ignoring a transition time measured in nanoseconds. In most cases this assumption is harmless. We’re taught to look at the lines on the screen as idealised, a square wave is truly square, and the transition from low to high voltage and back again is instantaneous. This is a sine wave, they say, this is a sawtooth, this is a square wave, and so on. When we are taught about oscillators as newbie engineers, we are shown a variety of waveforms on an oscilloscope or in a textbook.
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