Halved power in SSB

[soundman]

Hello,

Just got a question.
We all know that in a SSB modulator the bandwidth and power of transmission is reduced by half than when using AM-DSB. I read that it is becuase we do'nt have one of sidebands in SSB. But What which is confusing to me is that, the load has a CONSTANT impedance when connected to the the output modulator (whether DSB or SSB is used). Suppose the load is 10 ohms (these values are imaginary an maybe do not exist is real). now for instance if a modulator used as a SSB modulator will produce a maximum of voltage less than when used as DSB modulator for the said load so that the power happens to be halved?
Saying the RMS power of a DSB modulator is 10W for the said load now:
Vrms=(PxR)^1/2 =10V
Now the said modullator would produce halved output power if uses as SSB:
Vrms=(5x10)^1/2 =7.07V

Now it seemsthat removing one sideband causes to output volatge to drope and cause the ouput power to be halved in SSB than DSB ,right?
If you are agreed with me till now, SO plaese is there somebody to explain me why removing one side band causes the output voltage to drope?
And while the load is constant to a SSB transmitter by the said properties or values, do I need to incases the output voltage so that improve and recover the original power than DSB, While the SSB modulator is used as a transmitter, or its better to reduce the load so that get the original power, Or I do not need to do any thing and confused myself by something?

Thanks for any help

 

[Nigel Goodwin]

I'm a bit confused by what you're asking?.

SSB, by only transmitting ONE sideband, increases the power over DSB.

If you reduced the power until it was the same voltage as half of the DSB signal, obviously it would be lower power - but why would you do that?.

 

[crutschow]

What you say is true if you don't adjust any gains. But if you have only one sideband out of the modulator instead of two, then you increase the gain so the transmitter output is again at its maximum power rating. The input signal to an RF transmitter is always adjusted to give the desired maximum power output with whatever modulation scheme you are using, thus all the power that was previously going into two sidebands would now be concentrated in one sideband.

 

[JimB]

Soundman, you seem well and truly confused.

Consider an AM transmitter, 10 watts carrier power output.
When this transmitter is 100% modulated, the total output power becomes 15 watts, 10 watts in the carrier and 2.5 watts in each sideband.
The information contained in each sidband is the same.

Consider and SSB transmitter (single sideband suppressed carrier), 10 watts PEP (Peak Envelope Power) output.
It does not transmit the carrier, the carrier contains no information (except to provide a reference frequency for the sidebands).
It only transmits one sideband, we dond need the other one as it has exactly the same information as the first one.
The bandwidth occupied by the transmission is half that of the AM transmitter (only one sideband).

So the SSB transmitter can transmit 10 watts of "information", compared with the AM transmitter which is only transmitting 2.5 watts of information in the same bandwidth.

I hope that this helps your understanding.
Like Nigel, I am at a bit of a loss to understand what you are asking.

JimB

 

[soundman]

Thanks for all inputs,

I'm a bit confused by what you're asking?.

SSB, by only transmitting ONE sideband, increases the power over DSB.

Yes, That's right, But I want to know what if the load happens to be constant (suppose the load is a resistor instead an attena) then please enlighten me what will happen for the said load? I know that in a SSB transmitter we do not have one sidebandm But is this means that the output voltage of the modulator reduces so that the output power resuces while the load is constant? What about the output amplifier connected to the modulator?

 

[Nigel Goodwin]

Thanks for all inputs,


Yes, That's right, But I want to know what if the load happens to be constant (suppose the load is a resistor instead an attena) then please enlighten me what will happen for the said load? I know that in a SSB transmitter we do not have one sidebandm But is this means that the output voltage of the modulator reduces so that the output power resuces while the load is constant? What about the output amplifier connected to the modulator?

You seem even more confused? - the load IS constant - and as we've already told you, SSB transmitters use the full output power on that single sideband, which is why it goes further.

 

[soundman]

You seem even more confused? - the load IS constant - and as we've already told you, SSB transmitters use the full output power on that single sideband, which is why it goes further.

P=V^2/R
While R is constant the only way the power can increase is increasing the V.
Now please tell me how removing one sideband will increase the output voltage if any?

The second question is how can I measure the output power of such a modulator? I am able to make one at working at 100KHz of carrier...

Thanks

 

[Nigel Goodwin]

P=V^2/R
While R is constant the only way the power can increase is increasing the V.
Now please tell me how removing one sideband will increase the output voltage if any?

Obviously you increase the voltage, that's the whole point of SSB - you use all the power on the one sideband, nothing wasted.

For normal SSB as used by shortwave transceivers it would as simple as turning the transmit audio gain up - but in practice an SSB transceiver would be designed with the correct gain anyway.

The second question is how can I measure the output power of such a modulator? I am able to make one at working at 100KHz of carrier...

SSB is quite difficult to measure, and so is normally specified as PEP (Peak Emitted Power), you could easily read the peaks with a scope.

What EXACTLY are you trying to do?, there's been a lot of SSB threads recently, and all seem to have little understanding of what it is and what it can and can't do.