Software Defined Radio
DiRaGen 30 USB
Analog AM Modulation
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Basisband signal generation & spectrum shaping
The combination of the SPARK modulator software and the DiRaGen 30 USB allows besides the generation of DRM signals also the generation of analog AM signals.
Due to the principle of ultra-linear conversion it is possible to generate any digital or analog signal with a maximum bandwidth of 20KHz within the shortwave range.
In the following an example for analog modulation.
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Double-sideband AM modulation with AMSS
Chosen SPARK settings:
Audio Bandwidth: 5000Hz ( 5KHz)
Mod. Degree: 1.0 (100%)
AM-Mode: Double Sideband
Carrier Suppression: OFF
Enable AMSS: ON
Music modulation via chosen MP3 playlist
SPARK delivers the following baseband signal from 0-24KHz with a carrier center frequency of 12KHz:
There is a sharp cut characteristic outside the effective bandwidth (2x audio bandwidth 5KHz = 10KHz modulation bandwidth. Unwanted modulation products are suppressed for more than 80dB outside the channel.
SPARK uses high-order FIR filters ( > 3000) with a sideband suppression > 90dB for spectral shaping.
The same signal on a waterfall diagram:
The very sharp cut off characteristic is also recognizable. Due to spectrum shaping function this analog modulated signals achieves the same spectral purity like a DRM signal.
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Reference output spectrum
In the following we want to compare a standard AM signal to an AM signal generated by SPARK & DiRaGen 30 USB.
For reference purposes a DSB-Signal generated by a standard test generator, with 1KHz internally modulated, modulation degree approx. 100%, output frequency 7MHz:
Symmetrically to the main modulation products the following modulation products drop down exponentially to a maximum level difference of about 40dB related to the main carrier signal within a +/- 5KHz range.
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DiRaGen output spectrum
For comparison purposes the 7MHz output signal of the DiRaGen 30 USB fed with a SPARK-generated baseband signal, chosen modulation degree 100% and also 1KHz modulated:
At first glimpse it is recognizable that the further modulations products besides the main modulation products are suppressed very strong. Outside the defined effective bandwidth (+/-5KHz) there are no further modulation products visible, which corresponds to >70dB attenuation related to the main carrier signal.
This prevents neighbor-channel interference and guarantees also an efficient use of transmitting power.
