[TenTec] AM, was "Just noticed ..."

[Ken Brown]

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
>   
>