{"id":8799,"date":"2023-10-11T14:35:08","date_gmt":"2023-10-11T18:35:08","guid":{"rendered":"https:\/\/jeremyclark.ca\/wp\/?p=8799"},"modified":"2025-12-10T08:19:14","modified_gmt":"2025-12-10T13:19:14","slug":"plutosdr-for-am-modulation-on-gnu-radio","status":"publish","type":"post","link":"https:\/\/jeremyclark.ca\/wp\/telecom\/plutosdr-for-am-modulation-on-gnu-radio\/","title":{"rendered":"PlutoSDR for AM Modulation on GNU Radio"},"content":{"rendered":"\n

Introduction<\/h2>\n\n\n
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Fig.1 PlutoSDR for AM Modulation <\/figcaption><\/figure>\n<\/div>\n\n\n

In a previous post I looked at using the PlutoSDR as a WSPR beacon (Ref.1<\/a>). The PlutoSDR can be used to demonstrate standard AM Modulation as shown in Figure 1. Transmission is via a shielded sma cable plus 40dB pad to 3 test receivers: A Signal Hound spectrum analyzer, RTL-SDR and PlutoSDR receiver (Ref.2<\/a>). The frequency used is 433.92MHz which is in the PlutoSDR range and is also an ISM band. Remember transmission without the cable requires an appropriate license depending on your country. <\/p>\n\n\n\n

AM Modulation Basics<\/h2>\n\n\n\n
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Fig.2 Basic AM Modulator Fi=1KHz, Fc=10KHz<\/figcaption><\/figure>\n\n\n\n
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Fig.3 AM Modulation m=50% Fc=10KHz, USB=11KHz, LSB=9KHz<\/figcaption><\/figure>\n\n\n\n
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Fig.4 AM Modulation m=100% Fc=10KHz, USB=11KHz, LSB=9KHz<\/figcaption><\/figure>\n\n\n\n

Figure 2 shows a basic AM modulator as previously covered (Ref.3<\/a>). You can clearly see the 1KHz information wave superimposed on the 10KHz carrier wave. Two sidebands are generated at Fc +\/- 1KHz = 11KHz and 9KHz. For a modulation index of 50% sidebands are 12dB down from the carrier and at 100% they are 6dB down. <\/p>\n\n\n\n

PlutoSDR Tx – SpecAnalyzer Rx<\/h2>\n\n\n\n
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Fig.5 PlutoSDR AM Tx with 40dB pad<\/figcaption><\/figure>\n\n\n\n
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Fig.6 SigHound Spectrum m=50% 40dB pad<\/figcaption><\/figure>\n\n\n\n
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Fig.7 SigHound Spectrum m=100% 40dB pad<\/figcaption><\/figure>\n\n\n\n

Figure 5 shows the GNU Radio schematic for the PlutoSDR AM Tx. Note that it is the same as in Figure 2 except that the second signal source and multiplier are replaced by the Soapy Pluto Sink (Fi=10KHz). The varying amplitude must be complex to feed this block, so a float\/complex converter is added and the complex component set to zero. I used the Spectrum analyzer to zero the PlutoSDR on the correct frequency by adding a f_shift of 6KHz in my case (depends on your unit, temperature drift etc). Figure 6 shows sidebands at 12dB down for m=50% and Figure 7 shows sidebands at 6dB down for m=100%. <\/p>\n\n\n\n

PlutoSDR Tx – RTL-SDR Rx<\/h2>\n\n\n\n
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Fig.8 PlutoSDR AM Tx – RTL-SDR Rx 40dB pad<\/figcaption><\/figure>\n\n\n\n

Figure 8 shows reception using the RTL-SDR. Both transmitter and receiver gains were backed off to avoid intermod\/overload. Note the carrier at Fc and USB\/LSB 100KHz away at -12db for m=50% (Fi=100KHz). <\/p>\n\n\n\n

PlutoSDR Tx – PlutoSDR Rx<\/h2>\n\n\n\n
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Fig.9 PlutoSDR AM Tx – Rx Loopback with 40dB pad<\/figcaption><\/figure>\n\n\n\n
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Fig.10 PlutoSDR AM Tx-Rx Loopback with 40dB pad m=50%<\/figcaption><\/figure>\n\n\n\n

Figure 9 shows the GNU Radio schematic for PlutoSDR AM loopback operation. This is tricky even with the pad because the Tx gain & Rx gain settings may cause intermod and distortion. The receiver frequency is set at 433.720MHz 200KHz away from the transmitter so that zero frequency distortion I\/Q imbalance is avoided. Figure 10 shows the transmit and receive spectrum for m=50%.<\/p>\n\n\n\n

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