Ken,
I guess it depends on what you assume about the way the signal power is
generated. You assume that the sideband power will be twice as high as
the power in each sideband of a DSB-AM signal. I assumed that the signal
power in the remaining sideband would be the same as it was in the
DSB-AM case, that is that the DSB-AM signal was generated at 100%
modulation and then one sideband was loped off by the filter. Thus the
noise power (by going from 6 kHz to 3 kHz BW) and signal (sideband)
power are both cut in half so the SNR remains the same.
I guess if the transmitter were peak power limited, you could increase
the remaining sideband power by a factor of two and keep the same
carrier, but I suspect that the receiver would have some problems
demodulating without distortion.
Regards, Joel
Ken Brown wrote:
>Hi Joel,
>
>Given an equal noise power distribution across the whole 6 kHz, using a
>3 kHz filter should cut the total received noise power in half. All of
>the TX power (both carrier and sideband) with SSB-AM is inside that 3
>kHz. Same transmit power, half the received noise, resulting in a 3 dB
>improvement in SNR using SSB-AM versus DSB AM.
>
>By not transmitting the carrier, twice as much power can be put into the
>one sideband resulting in an additional 3 dB SNR improvement. Thus a
>total of 6 dB improvement in SNR from full carrier DSB AM to no carrier SSB.
>
>All of the above assumes the same peak power capability of the AM and
>SSB transmitters.
>
>In practice, using the same amount of iron, if built properly an SSB
>transmitter can be made to produce more peak power than an AM
>transmitter, because of the lower duty cycle, not having to generate
>that 100% duty cycle carrier. So, given the same mass transmitter, SSB
>has an additional "virtual" 3 dB advantage over AM.
>
>On the other hand if you use CW confined to a couple of hundred hertz
>bandwidth and power amplifiers running in class C (along with the
>necessary filtering to reduce harmonics to an acceptable level) you have
>an SNR improvement of 10 or 20 dB compared to DSB-AM. This requires a
>skilled operator at the receiving end, which requires effort, and is
>therefore out of style these days.
>
>Ken N6KB
>
>joel hallas wrote:
>
>
>>The classic SSB-AM signal was that generated in the Collins KWS-1 of 1956.
>>
>>I agree, it's not exactly a transceiver, but another example of that
>>kind of signal.
>>
>>If received in a receiver with 3 kHz filter, the SNR of such a SSB-AM
>>signal should be the same as with a DSB-AM signal of the same carrier
>>level in a 6 kHz wide receiver. It also takes up less spectrum.
>>
>>Joel Hallas, W1ZR
>>
>>
>>
>>
>
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