Oscilloscope’s Frequency Counter Hacked! (kinda)
2018-08-22 | 8 min read
Hacking the specs
Everyone loves a bargain. And who doesn’t love a hacked oscilloscope? Well, it would be pretty odd for an oscilloscope company to teach you how to hack your own hardware. Besides, that’s already been done (Fig. 1). So, I’m coining a new term: “spec hack.”
Webster’s dictionary will one day define it like this:
Spec hack (/spek’hak/), n. When an engineer uses expert-level knowledge of their test equipment to achieve performance above and beyond typical expectations of said equipment. (Thanks to a spec hack of the flux capacitor, Doc Brown discovered he only needed to go 77 miles per hour to travel through time.)
Today’s spec hack will look at the built-in frequency counter on an InfiniiVision 1000 X-Series oscilloscope.
You may think that a 100 MHz oscilloscope will only let you see signals up to 100 MHz – but that’s not actually true. Why? Oscilloscope bandwidth isn’t as straightforward as the labeled spec.
Oscilloscope Bandwidth brush up
To fully understand how far you can push your frequency counter, you must first understand how your oscilloscope’s bandwidth works. If you are confident that you know all about bandwidth, feel free to skip this next little section. If not, get ready to have your mind blown (or at least maybe learn something new).
Essentially, if you have a 100 MHz oscilloscope bandwidth, it means you can view a sine wave (or frequency components of a non-sine wave) of 100 MHz with ≤ 3 dB of attenuation.
But, here’s the main take away – bandwidth is all about signal attenuation, not just about the frequencies you can or can’t see (Fig. 2).
Usually this won’t matter for your day-to-day oscilloscope usage. You may see round corners on what should be a crisp square wave, but it probably won’t change how you use your scope. But, when you’re using a built-in frequency counter, it can be used to your advantage.
To understand why this effect can be advantageous, you need to understand how a frequency counter works. It’s called a frequency counter because it literally counts. It counts the number of edges found over a specific amount of time, called the gate time. The frequency is calculated like this:
Frequency = Number of pulses/Gate time
From a circuitry perspective, the counter is simply a comparator (to identify signal edges) and a microcontroller to count the output and display the results (Fig. 3) As it turns out, oscilloscopes already have this infrastructure inside their trigger systems.
Why oscilloscopes make great frequency counters
As luck would have it, the trigger circuitry of a scope often has comparators built into the signal path (think “edge trigger”). With some planning, it’s not difficult for oscilloscope designers to include a frequency counter built into the oscilloscope. It may sometimes require extra hardware, but the essentials already exist.
The most important specification of a counter is accuracy - the higher the precision of the time base, the more accurate the counter. Oscilloscopes also have to have a highly accurate time base, so a built in counter can just use the scope’s clock.
Finally, an oscilloscope’s trigger circuitry typically has its own special signal path designed specifically to extract the core signal and block out noise and unwanted frequency components. Unlike the oscilloscope’s acquisition circuitry, the trigger circuitry doesn’t need to recreate the signal with high accuracy, it only needs to do a fantastic job of finding edges. So, a frequency counter can use a scope’s trigger signal path instead of the acquisition signal path and get a higher fidelity edge.
The Spec Hack
Let’s put it all together. So far we’ve learned a few things:
- You can see signals (or signal edges) higher than the bandwidth of your oscilloscope, but it may have an attenuated amplitude.
- Frequency counters just need to count edges; they don’t care very much about the amplitude of the signal.
- Oscilloscopes have a dedicated, specially conditioned signal path dedicated to identifying edges.
So, a frequency counter built into an oscilloscope can measure frequencies higher than the bandwidth of the oscilloscope. The question is, how much higher?
One of the perks of working at Keysight is that that’s an easy question to answer. I pulled out my 100 MHz Keysight 1000 X-Series low-cost oscilloscope and a grossly unnecessary Keysight 67 GHz PSG (because hey, why not?) (Fig. 4) and ran a frequency sweep to see just how fast of a signal the oscilloscope’s frequency counter could measure.
The results blew me away!
The 100 MHz oscilloscope’s counter was able to measure up to 529 MHz! That’s over 5x the bandwidth of the oscilloscope (Fig. 5).
The lesson? Know your equipment!
It’s always fun to find a little, hidden gem in your test equipment. Sometimes it’s an Easter egg mini game hidden away in a secret menu; sometimes it’s a measurement you had no idea you could make.
Having a good understanding of the fundamentals of the equipment you use will not only help you make better, more accurate measurements but also help you avoid any traps that might lead you down the wrong test path.