QAM Quadrature Amplitude Modulation – Analog


In previous posts we looked at AM Amplitude Modulation for analog transmission (Ref.1) and QAM Quadrature Amplitude Modulation for digital transmission (Ref.2). In this post we will examine QAM for analog transmission. We first look at sending two independent tones; one tone on the In Phase I channel and another tone on the Quadrature Q channel. The transmit spectrum shows both tones, however we are able to separate them at the receiver. Finally we transmit two independent WAV files. Again the transmit spectrum consists of the two overlapping signals, but we can again separate them on the receive end. This is somewhat like using vertical & horizontal polarization of a carrier wave to send two separate signals. We are assuming exact frequency and phase recovery at the receiver (non-trivial task!).

QAM Modem Discrete Tones

Fig.1 QAM IQ Modem Discrete Tones 500Hz & 1KHz
Fig.2 Orthogonal Carriers I_coswct & Q_sinwct
Fig.3 DSB Output & Input IQ Tones vs. Output IQ Tones
Fig.4 QAM Modulator Output Spectrum

Figure 1 shows the Scicos block diagram of an IQ modem. The carrier wave is set at 10KHz. The 500Hz tone directly modulates the I_coswct carrier to form a DSB_SC signal. The 1KHz tone directly modulates the Q_sinwct carrier and the two DSB_SC signals are added together. Figure 2 shows the I_coswct & Q_sinwct carriers. Figure 3 shows the transmit DSB_SC sum, the two input tones vs. the same recovered output tones. Figure 4 shows the transmit spectrum showing the USB (10500Hz & 1100Hz) and LSB (9500Hz & 9000Hz) tones with no carrier. On the receive end, the signal is multiplied by the same I & Q carriers and low pass filtered to remove the sum components.

QAM WAV Test Files

Fig.5 Audacity Recorder for I_WAV Test File
Fig.6 Test File I_WAV Low Pass Filtered at 4KHz

The discrete tones do not overlap, so to show that this process can work for two signals that overlap in frequency, I created two separate WAV files using Audacity (Ref.3). I spoke into the laptop mic “I channel” repeatedly for 4 secs recording at fs=22050Hz. Then I repeated this for “Q channel”. I low pass filtered both WAV files at 4KHz at 12db/octave. I used these two WAV files to create the two structures V1 & V2 (4×22050=88200 samples) that I read into the QAM Scicos Model (Ref.4).


Fig.7 QAM IQ Modem WAV Files
Fig.8 Input IQ WAV Files vs. Output IQ WAV Files
FIg.9 QAM Modulator Output Spectrum

Figure 7 shows the same Scicos block diagram as before, but this time the discrete tones are replaced by the two structures V1 & V2 representing the WAV files. Figure 8 shows the DSB_SC output and the I_WAV and Q_WAV inputs vs. the recovered I_WAV & Q_WAV outputs. Figure 9 shows the transmit USB/LSB spectrum.

Fig.10 YouTube Video QAM Quadrature Amplitude Modulation – Analog

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#1. – “AM Modulator”

#2. – “64QAM for LTE/5G”

#3. – “Audacity”

#4. – “Scilab & ScicosLab – WAV Files”

#5. – “HF Radio Telecommunications Learn by Simulation”

By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.