Introduction
In the previous post we looked at the RTL-SDR for SSB reception on GNU Radio (Ref.1). In this post we look at how to build an SSB modulator using the Hartley Phasing method (Ref.2). The latest version of GNU Radio >=3.8xx is used (Ref.3).
Hartley Phasing SSB Modulator
Classically there have been three different methods of generating SSB. The first method is the filter method where an IF carrier is DSBSC modulated and the LSB or USB is removed using a very selective crystal filter. Both my amateur SSB transceivers (TR4C & K2) use this method. The second method is the Hartley or Phasing method and the final one is the Weaver method. Figure 1 shows the Hartley Phasing type of SSB modulator. The construction is analogous to the Hartley IQ SSB receiver covered in the previous post. Voice signals are first Low Pass Filtered for SSB quality to approx. 300-2700Hz. They are then applied to two balanced I & Q mixers. The audio on the I branch is further phase shifted by 90deg with a Hilbert Transform (Ref.4). In the past, building a Hilbert Transform was extremely difficult. Nowadays DSP techniques can be used.
Hartley SSB Modulator Discrete Tone
Figure 3 shows a phasing modulator built on GNU Radio. The signal source is a discrete tone that can vary from 300Hz to 2700Hz. The carrier for simplicity is chosen at 10KHz. The USB is chosen by adding the two I & Q branches, the LSB by subtracting the Q branch. Figure 4 shows the 90deg phase shift between I & Q information signals. Figure 5 shows the output spectrum showing the USB. Suppression of the LSB is best at 1300Hz >60dB. Suppression decreases for lower audio frequencies.
Hartley SSB Modulator 300-2700Hz Voice
Figure 6 shows a similar GNU Radio modulator as Figure 3, except this time the information source is an audio source block. This takes voice from the laptop audio and is bandpass filtered to 300-2700Hz. Figure 7 shows the I & Q information signals and Figure 8 shows the SSB USB output spectrum.
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References
#1. – “RTL-SDR for SSB on GNU Radio”
https://jeremyclark.ca/wp/telecom/rtl-sdr-for-ssb-on-gnu-radio/
#2. – “The ARRL Handbook for Radio Communications”, American Radio Relay League, 2010, ISBN13: 978-0-87259-144-8, pages 8.5, 13.9
#3. – “GNU Radio Installation”
https://wiki.gnuradio.org/index.php/InstallingGR
#4. – “David Hilbert”
https://en.wikipedia.org/wiki/David_Hilbert
#5. – “HF Radio Telecommunications Learn by Simulation”
https://www.clarktelecommunications.com/simulation.htm
