A modern entry-level best budget oscilloscope can easily increase the bandwidth from 100 MHz to 529 MHz. How this works and what task the frequency counter has, shows our text.
Anyone working with an oscilloscope every day quickly becomes embarrassed to get more out of their meter. The internet offers plenty of information on how to hack the hardware. A popular source of information is the EEVblog of the Australian David L. Jones. A few days after the Keysight 1000X oscilloscope was released, it was hacked – or rather, its specifications changed. In this specific case, the bandwidth was doubled.
This so-called spec hack could be defined as follows: When an engineer uses sound knowledge of his equipment to achieve equipment performance that exceeds typical equipment expectations. The text should show how to hack the specifications of the frequency counter in an oscilloscope. If you think a 100 MHz oscilloscope could only show signals up to 100 MHz, you’re wrong. The bandwidth of an oscilloscope is not limited by what is on the front of the device.
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What the bandwidth of an oscilloscope says
But first a look at the oscilloscope: A modern oscilloscope today is more than an oscilloscope as it used to be. Since the invention of the Mixed Signal Oscilloscope (MSO) by HP in 1996, oscilloscopes such as the entry-level models of the Keysight InfiniiVision 1000 X series have been equipped with more and more additional measurement functions. In recent years, a built-in hardware frequency counter was added. And to stir the drum: You get the extra function even for free if you register the oscilloscope at Keysight.
To understand how much the upper-frequency limit of the frequency counter can be extended, one needs to look at which the bandwidth specification describes oscilloscope characteristics. An oscilloscope with a bandwidth of 100 MHz means that the oscilloscope will display a value for the amplitude of a 100 MHz sinusoidal input signal that is 3 dB below the actual amplitude value.
For example, with a 100 MHz sinusoidal signal and an amplitude of 1 V, the oscilloscope displays a 100 MHz sinusoidal signal with an amplitude of 707 mV. It may seem crazy that the measured amplitude of a signal is 30 percent less than the actual signal amplitude – but this fact provides some bandwidth margin, especially when taking measurements on digital signals. It should be remembered that a square wave signal is composed of a fundamental frequency and an infinite number of harmonics.
The bandwidth can be outsmarted
Relevant in this context is: The bandwidth determines how much the amplitude of signals beyond the bandwidth limit is displayed attenuated – and not what frequencies are still visible or not. So you can also see signals above the bandwidth limit, but they appear smaller than they are.
In many cases, this can be tolerated during daily work. Instead of steep flanks you can see more or less sanded flanks. Whether that influences the design decisions or not depends on the individual case. But if you want to use the internal frequency counter of an oscilloscope, you can benefit from this effect.
To understand why one can outsmart the oscilloscope bandwidth in frequency measurements, one has to understand the function of the frequency counter. A frequency counter is so called because it counts the number of edges within a certain time. That’s the so-called goal time. From the counted number of edges, it then calculates the frequency according to the following equation: Frequency = number of pulses/gate time.
Edge triggering and extended trigger functions
Regarding hardware, a frequency counter is nothing more than a combination of a comparator that detects signal edges and a microcontroller that counts and displays them. In an oscilloscope, this infrastructure is already built into the trigger system.
The trigger circuit of an oscilloscope contains comparators in the signal path. The comparators are used for edge triggering and extended trigger functions. Therefore, it is not difficult for an oscilloscope developer to integrate a frequency counter into the oscilloscope. Little extra hardware may be required, but the essential functional blocks already exist.
The most important specification of a frequency counter is accuracy. The more accurate the time base, the more accurate the counter. Oscilloscopes already contain a high-precision time base, which can be shared by the frequency counter.
The trigger circuit of an oscilloscope often has its signal path, which filters out the actual signal and suppresses noise and other interfering components. Unlike the oscilloscope’s signal acquisition circuitry, the trigger circuit does not have to reconstruct the input signal with the highest possible accuracy but can be limited to the simpler task of identifying edges. A frequency counter can use the trigger signal path instead of the signal detection path and thus detect the edges with higher accuracy.
- An oscilloscope may display signals or signal edges beyond the bandwidth limit, but the displayed amplitude is less than the actual one.
- Frequency counters must count edges; this works even if the measured amplitude does not correspond to the actual one.
- An oscilloscope has a dedicated signal path optimized for flank identification.
This allows a frequency counter in the oscilloscope to theoretically measure frequencies beyond the bandwidth limit. The question is: which is the highest measurable frequency? It is not difficult for a Keysight employee to answer that question. I connected my 100 MHz 1000X oscilloscope to a 67 GHz performance signal generator (PSG). The PSG delivered a wobbled sinusoidal signal. Now the frequency displays of the PSG and the oscilloscope can be observed.
Bandwidth increased from 100 to 529 MHz
The 100 MHz oscilloscope was able to measure frequencies up to 529 MHz correctly! This is more than five times the scope. One of the most important specifications of a frequency counter is the timebase accuracy. The timebase specifications of an oscilloscope-internal frequency counter are usually the same as those of the oscilloscope. Some oscilloscopes allow you to use an external, high-precision frequency reference. Thus, the timebase accuracy can be further improved, even for frequency measurements.
It’s always fun to discover the hidden abilities of a meter. Sometimes this is an Easter egg hidden in lower levels of the menu (Easter egg), sometimes a measurement that was previously impossible. Those who want to measure more accurately and reliably must have solid basic knowledge. Then you can avoid traps that could lead to troubleshooting in the wrong direction.