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One thing a Z90 can do is provide a spectral signature view of different modulation types. Here are a few samples, generated with an SG-100 function generator, as well as "off-the-air" signals.

You may also wish to view the page Display for a detailed discussion of how the panadapter's settings change the appearance of signals.

BPSK (Binary Phase Shift Keying)

BPSK (binary phase shift keying) with a 50 Hz modulating frequency, with no transition shaping. Note the wide occupied bandwidth. Span: 10 KHz RBW 200 Hz  Video Avg: 1 Vert: 10 dB/Div

FM (Frequency Modulation)

FM (frequency modulation) with a 1 KHz modulating frequency, 5 KHz peak deviation. The occupied bandwidth (20 dB down) is approximately 15 KHz. Span: 50 KHz RBW 1 KHz  Video Avg: 1 Vert: 10 dB/Div

PM 45 Degree Shift (Phase Modulation)

PM (phase modulation) with a 1 KHz modulating frequency, 45 degrees peak deviation. Span: 50 KHz RBW 1 KHz  Video Avg: 1 Vert: 10 dB/Div

PM 180 Degree Shift  (Phase Modulation)

PM (phase modulation) with a 1 KHz modulating frequency, 180 degrees peak deviation. Span: 50 KHz RBW 1 KHz  Video Avg: 1 Vert: 10 dB/Div

Note that phase modulation and FM look quite similar. This is not an accident as phase and frequency are related; frequency is the time derivative of phase, and phase is the integral of frequency.

AM (Amplitude Modulation)

AM (amplitude modulation) with a 1 KHz modulating frequency, 100% modulation. Span: 10 KHz RBW 200 Hz  Video Avg: 1 Vert: 10 dB/Div

At 100% modulation, the total energy in the modulating sidebands is 50% of the carrier, or –3 dB. Since there are two equal sidebands, each individual sideband will be –3 dB with respect to the total, or –6 dB with respect to the carrier.
This is exactly what the SpectraScan shows. You can also see the second harmonic of the 1 KHz modulating waveform at about 35 dB below the fundamental.

FSK (Frequency Shift Keying)

FSK (frequency shift keying) with a 850 Hz shift, 50 Hz modulating frequency. Span: 5 KHz RBW 200 Hz  Video Avg: 1 Vert: 10 dB/Div

The two individual mark and space frequencies, separated by 850 Hz, are clearly seen. Since the SG100 does not shape the mark/space transitions, the resulting spectrum shows many sidebands. 

OOK (On/Off Keying)

Pulse (off/on keying) with 25 ms on, 25 ms off period. Span: 5 KHz RBW 200 Hz  Video Avg: 1 Vert: 10 dB/Div

Again because the SG100 does not shape the rise/fall of the waveform, we see many close-spaced sidebands. The spectrum resembles half of the FSK spectrum, which makes sense, as each FSK tone can be viewed as operating with OOK modulation.

AM Broadcast (Amplitude Modulation)

AM broadcast station 1500 KHz, received by a Racal RA6790/GM, 9.3 KHz bandwidth. Span: 20 KHz RBW 200 Hz  Video Avg: 1 Vert: 10 dB/Div

SSB (Single Sideband Modulation)

SSB (single sideband) amateur radio station 7 MHz band  received by a Racal RA6790/GM, 3.2 KHz bandwidth. (LSB mode) Span: 5 KHz RBW 200 Hz  Video Avg: 1 Vert: 10 dB/Div

To avoid the AGC bringing up the background noise, the example is taken with manual gain control. Also, the RA6790/GM’s IF output is frequency inverted, so the image appears to be USB, although in fact it is LSB.

AM (Amplitude Modulation)

WWV (15 MHz) standard time and frequency radio station received by a Racal RA6790/GM, 9.3 KHz bandwidth. (AM mode) Span: 5 KHz RBW 200 Hz  Video Avg: 1 Vert: 10 dB/Div

WWV, at the time the data sample was taken, was in the 600 Hz modulation mode. The 600 Hz modulating sidebands are clearly seen. They sidebands are approximately 17 dB below the carrier peak, or 14 dB for both sidebands. This corresponds to a modulation percentage of 20%. 

200 KHz View of 40 Meter Amateur Band

40 meter amateur band, from 7.00 MHz to 7.200 MHz. This data is captured with the antenna connected directly to the Z91, and with a “custom IF” of 7100000 Hz programmed in. Span: 200 KHz RBW 1 KHz  Video Avg: 1 Vert: 10 dB/Div

The Z90 and Z91 are sensitive enough to be used directly on an antenna in many instances, but since there is no front end selectivity, images will be received equally with the desired signal. Hence the direct connection mode must be used with some intelligence to discriminate between the desired and image frequencies.