 Clifton Laboratories 7236 Clifton Road Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711 E-mail: Jack.Smith@cliftonlaboratories.com |
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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.
BPSK (Binary Phase Shift Keying)
PM 45 Degree Shift (Phase Modulation)
PM 180 Degree Shift (Phase Modulation)
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.
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.
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.
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)
SSB (Single Sideband Modulation)
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.
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
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.
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