ssb+carrier Home transmitter

Dear Radiophiles,

As I have been following this very interesting thread on home am transmitters, I wondered if it would  make sense to have a single-side-band transmitter with full carrier, for home use in the broadcast band?

The expected advantage is a doubling of signal bandwidth passing through the RF/IF circuits of AM radios, from 4-5kHz to 8-10kHz.

Drawbacks, might be that optimum tuning no longer coincides with peak carrier reading at the receiver. If the sideband is upper sideband, then the best tuning should happen 4-5kHz above the peak carrier indication.

Another problem is that if the carrier is tuned too far off center, there will be severe distortion with high modulation, as the signal starts to look like partially suppressed carrier single-side-band. This should be worst for sets with a very narrow RF/IF bandwidth, or for sets that have been aligned for peak gain, instead of wide bandwidth. An AM set that was aligned with a twin hump in the IF frequency response, as is common practice for FM sets, should work well with full carrier single-side-band.

Vestigial lower sidband modulation is used universally for Video broadcast. A good treatment of this topic can be found in:

April 1940 RCA Review pp425-440 "Selective Sideband Transmission in Television" by R. D. Kell and G.L. Fredendall.

The 4.8MB PDF for this article can be downloaded from my personal web site at:

http://home.comcast.net/~jlrmsousa/RCA_Review_1940-V4_April_Vestigial_Sideband.pdf

Please feel free to post this file on your personal page, or upload to RM.

Even after reading this paper, it was not clear to me that non-linear distortion would be entirely absent in demodulation. Distortion can only be avoided if modulation does not exceed 100%. When modulation exceeds 100%, we have partially surpressed carrier, and non-linear envelope detection. Perhaps non-linear distortion can occur when the set is not tuned/aligned correctly.

One way to forestall distortion from carrier attenuation would be to reduce the modulation level at the transmitter, perhaps to 50%, so that 100% modulation is not exceeded when the carrier is attenuated up to -6dB at the slope of the RF/IF response. Another way to put this intuitively, is that the carrier must always be stronger than the sidebands to avoid distortion in evenlope detection.

Regards,

-Joe

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Dear Joe,

not wanting to spoil your efforts I am inclined to say that the task of home brewing a small SSB transmitter with full carrier maybe fine for experts but a bit too demanding for us average radiophiles. You need to supress the unwanted sideband with a suitable filter. I think the desired greater audio bandwidth can be achieved also by detuning (as described by you) a simple A3E transmitter - just modulate it with a wide audio signal.

Regards, Goetz

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Dear Goetz,

Thank you very much for shining the light of simplicity over my musings.

Detuning a full double sidband AM signal will provide the bandwidth extension from 5kHz to 10kHz, just as easily for home use. The ssb approach would only make sense for commercial use in a crowded spectrum.

Detuning should work particularly well with a home transmitter because the signal can be made strong enough by proximity, such that atmospheric noise should not go up with detuning.

Part of my musing was prompted by being unsure wether detuning would result in non-linear distortion. It should not, provided the carrier always remains "larger" than the sidebands.

Regards,

-Joe

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Dear Joe,

an "undistorted" envelope demodulation of SSB or VSB (carrier included in both cases) as it is described in the RCA article is only possible for video signals because the eye is not prone to nonlinear distortion concerning gradation from black to white resp. dark to bright. The VSB video transmission of analogue TV uses this effect.

However, if you analyze the demodulation for a sinusoidal information signal, the nonlinear distortions clearly can be seen. The demodulated signal is no longer sinusoidal if the modulation degree  m is near 1.

Here a phasor diagram for SSB + carrier is shown, and m being the modulation degree.

The phasor diagram for VSB + carrier shows  a similar result. As can be seen, the transition from dark to bright becomes steeper, and the mean of the brightness is changed. However, for a video signal this is of second order. So such a demodulation still can be regarded as "undistorted".

Regards,

Dietmar

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Dear Prof. Rudolph,

Thank you very much for your rigorous clarification. Until now, I had a nagging wonder about the possibility of distortion with envelope detection when one sideband is suppressed partially, or completely.

Your clarification sharpened my curiosity to play around with the signals and see if I could understand the effect from different perspectives.

I plotted the AM and SSB+C signals:

1-Before detection to show ideal AM and SSB+C signals.

2-After envelope detection to show the obvious envelope distortion with SSB+C envelope detection.

3-After synchronous detection (multiply the AM or SSB+C signal by the carrier). Despite the apparent envelope distortion of the synchronously detected signal, none of the distortion products lie in the audio band. I did not illustrate the case of multiplying with a carrier at a different phase, because it has no effect on distortion.

4-Frequency domain spectral plots are included adjacent to corresponding time domain pots to show the original carrier and sidebands, and the distortion products as spectral components.

5-Lastly, I zoomed into the individual RF cycles at 3 different points (0o, 90o, 270o) of the audio modulation cycle. This plot provides a time domain illustration the vector plots you posted.

I used the same modulation factor for AM and SSB+C signals so, in the case of the SSB+C signals, the "m=1" modulation in your illustration corresponds to m=2 (m=200%) in my illustration.

The two attached pdf files include 5 examples of modualtion level at 50%, 100%, 150%, 180% and 200%.

The nominal frequencies that I chose were 400 for the RF carrier and 2 for the modulating audio signal.

One remarkable effect of synchronous demodulation is it's power to ignore out-of-phase components that distort the envelope. This is apparent in the zoomed cycle plots.

Regards,

-Joe

Attachments:

• Comparison of SSBC to AM Modulation=50% 100% 150% (187 KB)
• Comparison of SSBC to AM Modulation=180% 200% (131 KB)

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