2-stage SSB FM modulator

[Kaleborg]

2-stage SSB AM modulator

Hello,

I have to make a Scicoslab simulation of a 2-stage SSB AM modulator.

My fm=100Hz and my fc=100KHz from there I can calculate the BPF transfer function but I don't how to implement the second stage. What should by my fc? It cant be the same as during stage one. And how to calculate the transfer function of the second BPF. Can somebody shed some light onto the theory behind it or point me to some resources.

Kind regards

 

[Syncopator]

I have to make a Scicoslab simulation of a 2-stage SSB FM modulator.

A what?

You seem to be confused. s.s.b is a form of a.m., not f.m.

 

[Kaleborg]

Re: 2-stage SSB AM modulator

Yes sorry my mistake it should be AM there.

 

[chuckey]

Does BPF mean "band pass filter"?. If you want to generate SSB, you have two problems, 1 is to get rid of the carrier, 2 is to get rid of the unwanted sideband.
There are three ways of doing this:-
1. Filter method, after generating AM, pass the single through a BPF that only allows the wanted sideband through. As speak is taken as 300-3KHz, this means that the filter has to pass Fc +300 and reject Fc and Fc -300 Hz. To make the situation easier, if the AM is generated in a balanced mixer, then there is no Fc in the output, so the filter has to be less steep as it does not have to reject the carrier.
Phasing method. Use use two balanced AM modulators with the audio and the carriers phase shifted before being applied to their respective modulators by +90 degrees for one -90 degrees for the other. adding the two DSB outputs results in the addition of one side band and the cancellation of the other.
The "third method", this is a hybrid of the other two, using a phase shifted pilot carrier and mixer to get the phase shifted audio and phase shifted RF to the final mixer which again is done in two chains which are added to get SSB.
Frank

 

[Kaleborg]

Hi and thanks for the reply.

I understand the theory behind SSB and getting rid of sidebands and all that as well as the frequency discrimination method.
My question is more specific. Its a 2-stage SSB modulation. In there the message signal is modulated twice. And I was hoping that someone could explain how to come up with the math to calculate the frequencies required.

Regards

 

[nomadicon]

This an assignment for EE450?

 

[Kaleborg]

Yes, it is.

 

[FvM]

SSB is just a frequency shift (upper sideband) respectively shift with inversion (lower sideband). In so far, (ideal) double SSB isn't different from single modulation.

 

[chuckey]

Re-read my comments on th phasing method - two modulators*, 4 different sets of phase shifts.
Frank
* do these qualify as two stages?

 

[FvM]

I din't understand the original question related to the two mixers of a quadrature modulator construct. Particularly the poster is mentioning two different carrier frequencies.

 

[Kaleborg]

Hello,

Ok, I am going to explain the problem again.

I have a message frequency fm and a carrier frequency fc. During 2-stage SSB modulation the fm signal is modulated first time by some intermediate carrier frequency (lets call it fi) that is lower than the actual required fc. The product is then fed into a Band Pass Filter. That is the end of stage 1. Now the signal from the BPF is then modulated again, this time it is modulated by the required fc and its output is then fed again into the BPF. This is the second stage. At this point we have the wanted modulated signal.
The question now is: How to come up with the intermediate frequency??

Regards

 

[chuckey]

The IF frequency should be > 10X the modulation frequency and > .1X the final carrier frequency. This allows easy filtering.
Frank