Important note: this page has now been updated following the launch of the ADC-216, follow this link for the new audio spectrum analysis page.

This application note looks at the use of Pico ADC converters for Audio Spectrum analysis, topics covered are:

Advantages of Pico Products for Audio Spectrum Analysis

Spectrum analyzers tend to fall into two categories, so called 'swept' spectrum analyzers and FFT based spectrum analyzers. Swept spectrum analyzers work by using one or more notch filters (or mixers) to measure the signal amplitude at a given frequency, by changing (or sweeping) the frequency of this filter a plot of amplitude against frequency can be constructed. Swept spectrum analyzers still have their place in high frequency spectrum analysis, but for audio work they have the disadvantage that the signal must be constant for the whole period of the sweep.

FFT based spectrum analyzers work by digitising the signal of interest using a analog to digital convertor (ADC). The stored values are then processed using the Fast Fourier Transform (FFT) algorithm. The advantage of this method is that the spectrum of one off or short duration events can be captured. For example using PicoScope's trigger capabilities it is possible to capture the spectrum of a single drum beat.

Performing spectrum analysis requires a lot of calculations, some FFT based spectrum analyzers can take several seconds to update a trace. PicoScope uses an optimised, high speed routine for spectrum analysis that results in 'real time' results. Even on a relatively modest computer such as a 33MHz 486 PC the spectrum analyser can still update many times a second.

Pico ADCs Suitable for Audio Spectrum Analysis

Although most of the Pico ADC range can be used for audio spectrum analysis, the ADC100 is the unit of choice. Its 100kHz sampling rate allows spectrum analysis to 50kHz on one channel or 25kHz when both channels are used. The ADC40 and ADC42 only provide spectrum analysis to 10kHz, so can not cover the whole audio range (20Hz to 20kHz). The ADC200 can do spectrum analysis to 50MHz, but has a lower dynamic range caused by its 8 bit convertor (see below).

The dynamic range of the spectrum analyser is the next most important consideration. Most oscilloscopes (whether PC based or benchtop) have a 8 bit resolution (256 steps). This limits spectrum analysis to 48dB of dynamic range (20log256) The ADC40 and ADC200 are both 8 bit devices, although the ADC200s fast sampling rate allows oversampling to improve resolution. Unusually for oscilloscopes, the ADC42 and ADC100 are both 12 bit devices (4096 steps) which gives a theoretical maximum of 72dB of dynamic range, as you can see in the traces below the ADC100 comes very close to this.

Examples using the ADC100

To show the sort of performance you can expect with the ADC100s spectrum analyser we connected it upto a CD player. We chose a 'budget' portable model to show up some of its limitations. One channel of the ADC100 was connected directly to the CD player headphone socket. The PicoScope (for DOS) trace below shows a pure 1kHz tone from a test CD. As expected the result is a sharp peak at 1kHz. The noise floor is at least 70dB below the signal level which is close to the theoretical maximum 72dB dynamic range that can be obtained by the ADC100s 12 bit resolution.

waveform

If we now set the spectrum analyser to look at the whole audible audio range (20Hz to 20kHz) some distortion of the signal becomes apparent. Rather than the >70dB of signal to noise performance we see that we only have about 53dB between the 1kHz signal and the harmonic at 5kHz. (To measure exact values we could have used PicoScope's rulers).

captured signal

Crosstalk is an important performance indicator that can easily be measured with a spectrum analyser. We played a 10kHz sinewave (0dB) on the left channel of the CD player. Ideally no signal would be present in the right channel, on our CD player the crosstalk is visible 60dB down on the signal on the left channel.

crosstalk

An ideal CD player should have a flat frequency response over the whole audio spectrum. The specifications of our CD player stated a 20Hz to 20kHz response within 3dB. We tested this using a sinewave that sweeps from 0 to 20kHz. Plotting such a frequency response is not possible with many FFT spectrum analysers as they take a quick snapshot of the signal then take several seconds processing and displaying the results, the result tends to be that only one frequency peak gets captured during the sweep. PicoScope's data collection and processing is optimised for speed - even on a relatively slow PC (33MHz 386) the spectrum analyser has a near instantaneous 'real time' update rate. The sinewave used for our test takes about 30 seconds to sweep from 20Hz to 20kHz, in this time PicoScope performs 100s of FFTs rather than the 2 or 3 that most FFT spectrum analysers can manage. To display the frequency response as a single line rather than a moving peak, we used PicoScope's peak detect function as shown below. As you can see the -3dB point is not the 20kHz claimed by the data sheet, but is nearer 16kHz.

frequency response