# Efficiency of DSBFC Modulator vs. Balanced Modulator: A Comparison

• Fisher92
In summary: I'll try and work from the information he's given me in the lessons and see what happens.Thanks a lot for your help.In summary, the figure shows a balanced modulator with inputs to the top modulator being the information signal and the carrier signal, and inputs to the bottom modulator being the inverse of the information signal and the carrier signal. The output from the bottom modulator is then inverted again and added to the output from the top modulator. The expression for the output signal s(t) can be derived by treating the balanced modulator as a pure multiplier, resulting in s(t)=Ei*cos(2*pi*(fi-fc)t)-cos(2
Yep, I had Pavg = Avg[(Ec^2 sin^2(wc t)] * [1 + avg{m^2 sin^2(wi t)}] but i also tried the integration with yours Pavg = Avg[(Ec^2 cos^2(wc t)] * [1 + avg{m^2 sin^2(wi t)}] directly, with no success.

Fisher92 said:
When I pick up from post 16 I get a big mess with wc and wi on the denominator and a string of sin and cos on the numerator? Can't seem to get what you have for my power?

How could you possibly get a mess in the denominator? P = V^2. Should be no denominator at all except for a constant.

In the next step where its integrated to get the average is when I get a mess. Unless I've missed something with this step I can't get it to come out neatly?

Hey RM, sorry for harassing you about this but I have to submit this in 24hrs and I can't for the life of me figure out how you went from the avg formula in post #16 to the power?

Once I have the power, I think the SNR is given by P/(2N_0*f_IF)?. I can put that in dBm pretty easily.
So using the power from your post 24, I get SNR =(Ec^2 * m^2/4 * G_T * C)/(2N_0*f_IF)?

Thanks

Fisher92 said:
Hey RM, sorry for harassing you about this but I have to submit this in 24hrs and I can't for the life of me figure out how you went from the avg formula in post #16 to the power?

Once I have the power, I think the SNR is given by P/(2N_0*f_IF)?. I can put that in dBm pretty easily.
So using the power from your post 24, I get SNR =(Ec^2 * m^2/4 * G_T * C)/(2N_0*f_IF)?

Thanks

That is correct. Assuming I myself got the power correct, but I'm pretty sure I did.

I really think you can derive the power number yourself. Use avg[f(t)] = ∫f(t)dt/∫dt over an integer number of cycles.

E.g. avg[sin2(wt)] = ∫0Tsin2(ωt)dt/T where T = 2π/ω?

Hey RM, several hours later and still no success with deriving the power expression. Not sure what timezone your in but I'm on the east coast of Australia so it's getting towards bed time here. As I said this assignment is due tomorrow night and the pesky rotation of the Earth means that there is probably only a few hours in the next 12 when we are both awake. -There are still two parts left once I figure out the power derivation:

d. If the total power amplification through the receiver stages is GR find the output power of the receiver.
As you pointed out a few days ago, the power I am trying to find for a. and c. is not actually the power that will be utilised by the superhet.
-It will obviously be limited to the established bandwidth. so -?

e. If the noise power at the output is identical to the noise power at the input of the receiver, find the NF for the receiver
The Noise Figure is essentially the degradation of the SNR throughout the superhet chain. Still not sure where to go from here though?

I may end up not being able to answer d & e, even so I would like to understand it.

I really wan't to figure out the power though as that's let's me answer a and c properly so rehealy hoping you can offer some more pointers when you see this - just can't get it to work out from the Pavg expression for some reason, I'll keep trying though.

Thanks

From trig,
sin^2(2πft) = 1/2 - 1/2 cos(4πft)
The average of cos (4πft) over 1 or more integer periods is zero (1 period = 1/2f).
So the average of sin^2(2πft) = 1/2.

You also need to use the general fact that
avg(a + b) = avg(a) + avg(b) and avg(ab) = avg(a) * avg(b). a and b are assumed independent of each other, which is the case since a = sin(wi t) and b = cos(wc t) here.

Expand [1 + m sin(wi t)]^2] out as 1 + 2m sin(wi t) + m^2 sin^2(wi t), take averages as I've shown, then you wind up with the expression for power as Ec^2 * m^2/4. For your SNR ratio, set m = 1.

I'm in time zone GMT-7 (Arizona USA) all year. (No dst).

Since there is no indication that the receiver itself generates meaningful noise, and since
NF = input SNR/output SNR expressed in dB, what would NF be?

I should point out one other thing: power gain = (voltage gain)^2. So you should recheck if G_T and C were voltage or power gain numbers. If voltage gain you need to use G_T^2 and C^2 and also G_R^2 in your computations (except you don't need G_R at all).

The power gain I gave you was the power at the output of the modulator, before G_T and C.

EDIT: looking at an earlier post of yours, I would assume G_T is a voltage gain and C definitely a power gain.

Last edited:
Thanks RM, very close with the power now. I've got:
$$\frac{(E_c^2*1/2*(1+m^2/2)*G_T^2)}{R_T}$$

The only problem I have is how to get rid of that 1 because I end up with:
$$P=\frac{0.25E_c^2*(m^2+2)*G^2}{R}*C$$

?

Thanks

You also need to use the general fact that
avg(a + b) = avg(a) + avg(b) and avg(ab) = avg(a) * avg(b). a and b are assumed independent of each other, which is the case since a = sin(wi t) and b = cos(wc t) here.

Expand [1 + m sin(wi t)]^2] out as 1 + 2m sin(wi t) + m^2 sin^2(wi t), take averages as I've shown, then you wind up with the expression for power as Ec^2 * m^2/4.

I think this is where my problem comes in:
avg(1)=1
avg(2m sin(wi t))=0 as the integral of sin(wi t) from 0 to 2pi/w =0
avg m^2 sin^2(wi t) = m^2/2

so avg = 1+m^2/2

Need to get rid of that 1 somehow.

Fisher92 said:
Thanks RM, very close with the power now. I've got:
$$\frac{(E_c^2*1/2*(1+m^2/2)*G_T^2)}{R_T}$$

This is correct.
[quote}

The only problem I have is how to get rid of that 1 because I end up with:
$$P=\frac{0.25E_c^2*(m^2+2)*G^2}{R}*C$$

?
Thanks[/QUOTE]

Why's that a problem? It's correct!

The carrier power is Ec^2/2R and the info power is Ec^2/4R if m=1. This is the power at the modulator output. Multiply by (G_T^2)*C to get the power available at the receiver input.

This power includes the carrier of course. In your SNR calculations you will want to use only the modulated signal (info) power.

Ok, thanks so for the SNR I would essentially have the power above with a (4R) as the denominator of the numerator...and 2fIFNo as the denominator?

I have:
$$SNR=\frac{\frac{E_c^2*1/4*(m^2+2)*G_T^2}{4R_T} *C))}{2f_{IF} N_0}$$
?

Fisher92 said:
Ok, thanks so for the SNR I would essentially have the power above with a (4R) as the denominator of the numerator...and 2fIFNo as the denominator?

For the SNR your numerator is (Ec^2/4R)(G_T^2)C and the denominator is 2No*f_IF.

Fisher92 said:
I have:
$$SNR=\frac{\frac{E_c^2*1/4*(m^2+2)*G_T^2}{4R_T} *C))}{2f_{IF} N_0}$$
?

No, you can't count the carrier power as signal power.

Ok, first mistake is not setting m to 1 - then I need to remove the carrier power. I am going on an eqn I found in the text, Esf=mEc/2 sp Pinfo=(Ec^2/4)/R - Is that how you derived the same?

$$SNR=\frac{\frac{E_c^2 G_T^2}{4R_T}*C}{2f_{IF}*N_0}$$

If the total power amplification through the receiver stages is GR find the output power of the receiver.
As we've established the power at the input is the information or sidebands power
$$P_{out}=\frac{E_c^2 G_T^2*G_R^2}{4R_T}*C$$
Assuming the gains are voltage, & ill make a comment on that

e. If the noise power at the output is identical to the noise power at the input of the receiver, find the NF for the receiver

NF = input SNR/output SNR

If the above, power output is correct, this is easy enough - algebra may simplify but my calculator will figure that out

?

Thanks

Fisher92 said:
$$SNR=\frac{\frac{E_c^2 G_T^2}{4R_T}*C}{2f_{IF}*N_0}$$

If the total power amplification through the receiver stages is GR find the output power of the receiver.
As we've established the power at the input is the information or sidebands power
$$P_{out}=\frac{E_c^2 G_T^2*G_R^2}{4R_T}*C$$
Assuming the gains are voltage, & ill make a comment on that
I think that's good.
e. If the noise power at the output is identical to the noise power at the input of the receiver, find the NF for the receiver

NF = input SNR/output SNR

If the above, power output is correct, this is easy enough - algebra may simplify but my calculator will figure that out

?

Thanks

Shouldn't be too hard! And I speak euphemistically! Let me know what you came up with?

I'm in time zone GMT-7 (Arizona USA) all year. (No dst).

-Gotta love the internet!

For part d, the noise figure, I ended up getting $$N_0=\frac{1}{G_R^2}$$

Probably not right (doesn't make much sense I don't think) but its only one part of a question.

Thanks for all your help with these two questions RM!

Fisher92 said:
-Gotta love the internet!

For part d, the noise figure, I ended up getting $$N_0=\frac{1}{G_R^2}$$

Probably not right (doesn't make much sense I don't think) but its only one part of a question.

Thanks for all your help with these two questions RM!

No, the NF = 0 since the receiver itself does not introduce any noise of its own.

Let me know what if any feedback you get from your instructor for any part or parts of this problem? Especially about the power at the receiver input.

Thanks, will do - the uni has a two week turn around for assignment marking.

Fisher92 said:
Thanks, will do - the uni has a two week turn around for assignment marking.

As the less literate here in Arizona would say - 10-4!

PS where in NZ are you? My in-laws visited NZ some years ago and raved about it.

Central QLD, Australia (3/4 the way up (North is up) on the east coast - the coast is still central for some reason -because everyone in Australia lives on the coast I guess)

Fisher92 said:
Central QLD, Australia (3/4 the way up (North is up) on the east coast - the coast is still central for some reason -because everyone in Australia lives on the coast I guess)

Oops, Australia! Ah well, my in-laws visited Australia also and raved about it too!
Townsville or Airlie Beach perhaps?

I live about 3hrs south of Townsville - Airlie is pretty awesome!

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