SSB "Q" Channel Communications
Increase Your 40-Meter SSB QSOs
By: L. Harold Allen, W4MMC,
ex-W6IRI, ex-W0OLI, and ex-W4IZH
Some think complexity must be experienced to be appreciated, but I dont. I have identified some of the more complicated supplemental portions of this text that do not need to be understood at this time by starting such portions with "<<" and ending them with ">>". You can be patient and dive into this article right now, or better yet, jump to the end of the text, well at least start at the section titled "Single Sideband "Q" Channel Operation" to begin applying some of my findings sooner. Then you can come back later to review the things you missed and get a better appreciation of what they are all about.
ADVANCED SYNCHRONOUS DEMODULATION FOR "Q" CHANNEL COMMUNICATIONS
The locally generated synthetic r-f carrier [VCO] is phase locked to signals that we do NOT wish to hear, by locking it with the information derived from the symmetrical [mirrored] sidebands of the transmitted signal, instead of its carrier. The quadrature, or "Q", Channel is used for the primary audio output, and the method of operating the system to receive SSB is very different because we do NOT listen to the signal that the receiver is locked-up to! Of course, there are several other frills and refinements, but they can be covered at a later time.
<< If the incoming DSB signals carrier is transmitted, it is disregarded by the automatic phase control of the receiver, provided the outputs of both the in-phase, or "I", demodulator and the quadrature-phase, or "Q", demodulator are ac-coupled to effect an audio band-pass filter to the audio phase detector. If these outputs were dc-coupled the synthetic r-f carrier would lock to the signals original carrier frequency, and would limit the use of many of the features of this advanced synchronous demodulation receiver. >>
<<Advanced synchronous demodulation can take advantage of the diversity feature of all double sideband [DSB] signals, such as amplitude modulation[AM], with or without a carrier, and angle modulation, such as phase modulation [PM] and frequency modulation [FM]. This is accomplished by combining and maximizing the reinforcing information received from both the upper [USB] and lower [LSB] sets of sidebands, by inserting the synthesized r-f carrier at the optimum phase. >>
DOUBLE SIDEBAND OPERATION
It has been demonstrated, proven, and thoroughly documented [in the "Proceedings of the IEEE", many years ago] that the maximum overall communications system efficiency can be obtained by using advanced synchronous demodulation of DSB suppressed carrier signals. << While DSB receivers are more complex than single sideband [SSB] receivers, DSB transmitters are considerably simpler than SSB transmitters, and they allow the use of very simple effective compression and clipping of the modulating audio signal. This type of modulation is legal, and has always been legal, for amateur operations (despite rumors to the contrary). DSB can operate in high-speed vehicles where SSB is very limited by Doppler at UHF. >> In the product demodulators this DSB receiving method uses, the audio outputs resulting from both the upper and lower sets of sidebands combine to reinforce each other in the "I" demodulator to produce a maximum audio output "I" signal when the local reference synthesized r-f carrier [provided by a voltage controlled oscillator] is maintained in the proper zero-degree phase relationship << relative to the vector summation plane, or axis, of the combined sets of received sidebands >>.
Taking another output from the local synthesized r-f oscillator and shifting its phase exactly 90-degrees <<from the above proper phase axis, or plane, >> provides another synthesized carrier reference to be combined with the received DSB signals. In this case, the audio outputs resulting from both the upper and lower sets of sidebands cancel each other as they combine with this 90-degree r-f oscillator signal in the "Q" demodulator to produce an audio output called the "Q" signal. The "Q" signal at this time is at an audio null!
<< It is interesting to note that, when a DSB transmitter modulated with a single audio tone is being received, the total number of audio signals associated with both receiver demodulators would be four if the receiver were not tuned correctly; that is two sets of frequencies at four phases. When tuned correctly, one set cancels out and the other set merges. The resultant is only one tone, and it exists at the "I" channels output. [If you want a good workout, modulate the transmitter with a speech waveform, add an r-f carrier (to make it an AM signal), then de-tune the receiver and do a mathematical analysis on all of these signals. (I am thankful for good computer programs that can do the work for me!)] >>
AUTOMATIC FREQUENCY CONTROL [AFC]
This dc-error component of the output of the audio phase detector can be used, after filtering out the ac-components and integrating, to automatically control the voltage controlled synthetic r-f carrier reference oscillator [VCO] to be at the correct phase [and frequency] to provide the minimum information possible from both sets of sidebands at the "Q" demodulator, and while extracting the maximum information possible at the "I" demodulator. <<A frequency change can not occur without a phase change. >>
<< If DSB sideband symmetry is not maintained, the ac-components at the output of the audio phase detector become larger about a dc-axis which, in turn, becomes smaller for a given local r-f oscillator phase offset. This is because the phase of the "Q" signal is being shifted toward being in quadrature with the "I" signal. When the sidebands of the received r-f signal become completely non-symmetrical, such as exists when receiving a single sideband [SSB] signal, no dc-error voltage is available at this test point to automatically control the local r-f oscillator. However, when the receiver is locked-up to a symmetrical DSB/AM signal, the introduction of an SSB signal, or a CW signal, will not affect the receiver's existing phase-lock because they produce resultant audio that is in quadrature to the "I" channel's audio.>>
<< I have conceived and developed methods for automatic frequency control (AFC) of SSB receivers. However, SSB AFC is a different subject beyond the scope of this article and will not be discussed at this time. But, if you are building a "Q" Channel adapter for your receiver, you could make input circuitry provisions for additional AFC inputs. Also, circuitry should be provided for two audio phase-shift networks and two summing networks [with trim pots], in order to provide "Reject USB" and "Reject LSB" audio outputs. (Please refer to figure 2.) Of course, these could be connected to a "Rejection Switch", which could include these two positions plus "Reject AM" for "Q" channel output, "Reject Nothing" for "I" channel output, and "Reject Everything" to turn off the power. >>
The "I" signal can provide very good sound quality [even for music] from received AM signals. The audio signal, at this point, has not had to undergo the unpleasant distortions normally introduced by SSB transmitters and SSB receivers.
When receiving AM signals in the presence of selective fading on a normal AM radio [which uses a diode detector] severe audio distortion can result. <<Very unpleasant distortion exists when the received carrier has momentarily arrived at the receiver 90-degrees out of phase relative to the average of the phase summation axis, or plane, of the combined upper and lower sidebands. The result is equivalent, at that moment, to listening to an FM signal on an AM diode detector. >>
By contrast, by using the "I" channel of an advanced synchronous demodulator receiver to listen to the same AM radio signal, selective fading distortion is greatly reduced, and quality music can be enjoyed that would otherwise not be tolerated. The "I" channel receives just about everything. It permits listening to SSB, both LSB, and USB, as well as all forms of DSB, including AM, PM, and FM.
SINGLE SIDEBAND "Q" CHANNEL OPERATION
INCREASE YOUR 40-METER SSB QSOs WITH "Q" CHANNEL COMMUNICATIONS!!!
In other words, if each person in a QSO matched his own transmitters SSB suppressed carrier frequency to the amateur frequency used by an AM broadcast stations carrier, to within about +/- 30 Hertz, then "Q" Channel Communications could be easily achieved.
<<Note: a phase lock to within 20 degrees is required only during reception, not transmission. Setting the SSB transmitters frequency can be done even more accurately by using an option of automatically deriving the SSB transmitters r-f carrier frequency indirectly from the advanced synchronous demodulator adapter. This means that the local SSB transmitters operating frequency can be automatically matched to the frequency being used by the rejected AM signals carrier. As another option, observe that the AM broadcasting stations operating in the 40-meter amateur band are precisely spaced at five kHz intervals. Using an accurate frequency standard, referenced to WWV, to control your SSB transmitter in five kHz steps could allow rapid frequency selection, too.>>
Using "Q" Channel Communications, communications with other SSB stations can now be achieved under conditions that would otherwise be impossible! This is especially useful if the AM broadcast station is being received weakly by both SSB stations [you and the SSB station you are talking to], provided both SSB stations use "Q" Channel reception. However, if the giant AM station is very strong, even 50 dBs of attenuation may not be enough to allow comfortable reception of a SSB station. In that case, if there is not an unused frequency available, then "QSY down exactly 10 kHz" to "share" another amateur frequency being used by another short-wave AM broadcast station that is not as strong. [I used, for the example, 10 kHz, rather than 5 kHz, because the strong stations sidebands are probably very strong, also.] [I have observed that some of the short-wave broadcast stations are using SSB, some without carrier and some with carrier (those with carrier are sometimes referred to as Compatible Single Sideband, or CSSB). You could try using the sideband opposite to theirs, and adjust your receiver to try to reject their sidebands.]
Considering the whopping big signals these giant international short wave AM broadcast stations put into their intended primary coverage area [I have helped design some big 500 kW AM transmitters, but none for 40-meters], I do not think there is any way any amateur SSB transmitter, even with maximum power and an "ideal" antenna, could ever be construed by the FCC [or other regulatory agencies] as intentionally interfering with, or in any way be reducing, the AM stations primary coverage area. I do not understand how these stations are allowed to operate on amateur frequencies and put such large signals into locations that are considerably outside of and beyond what would be a reasonable coverage area for them, even if they were outside of the amateur bands.
Single sideband "Q" Channel Communications is not limited to 40-meters, its also effective on 75-meters or on any other band where short-wave AM broadcast stations are operating on amateur radio frequencies. However, the extreme abuse to the amateur bands by these high-powered short-wave AM broadcast stations is certainly much more apparent in the 40-meter amateur band.
ADDITIONAL OPERATING FACTORS
For those not yet utilizing "Q" Channel reception but are forced to operate close [in frequency] to an AM station, there is an advantage to operating lower in frequency than the AM station and operate on LSB, as is now done traditionally. Similarly, there is an advantage when having to operate higher in frequency than the AM station to operate on USB, which is NOT done traditionally. This makes it a lot easier for SSB receivers to reject some of the AM stations carrier and part of its sidebands.
In order to effectively increase the number of contacts when band conditions are crowded [even during those times when the AM stations are not a problem] SSB stations certainly should NOT be bound to the outdated tradition of using only LSB on 40 and 75 meters. They should be willing to operate on both USB and LSB to reduce the congestion! [Even many CB operators of SSB transceivers learned this lesson a long time ago!]
When you are using "Q" Channel reception you can receive an SSB station and not know if it is LSB or USB without turning off the AFC and de-tuning slightly. << If you hear an amateur using a highly efficient DSB suppressed carrier transmitter with clear and strong speech, while using good speech compression, you will need to use the "I" channel to enjoy it. Otherwise, you will probably not be able to hear it on the "Q" channel. >>
As mentioned in the introduction to this article, I have many other ideas I could possibly share with you in some future articles. Another possibility is to expand on areas discussed in this article. Your comments will be appreciated.
When you get your "Q" Channel receiving adapter working and come across "This is W 4 Mickey Mouse Club calling CQ", please give me a call and tell me how you like it! -30-
Send mail to firstname.lastname@example.org
with questions or comments.
Copyright © 1988-2000 All rights reserved worldwide - antenneX©
Last modified: March 23, 2005