- #1

Fisher92

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## Homework Statement

Due to its bandwidth and power efficiency, SSBSC is commonly used in Frequency Division Multiplexing (FDM) systems. FDM systems are used to combine a number of separate information channels into a single transmission. Each channel uses a separate carrier frequency and the carriers are spaced evenly in the available bandwidth.

Design an FDM system to suit the following specifications.

• Available bandwidth range 100 to 120 kHz

• Number of separate information channels 4

• Information bandwidth 100 Hz to 4.5 kHz.

• Sideband suppression required in each channel is 40 dB.

In the design you need to consider the following:

• What is the frequency of local oscillator required at each channel?

• What are the parameters (Q, f

_{c}) for bandpass filter required at each channel?

• Provide a diagram of the FDM transmitter. As a minimum, this need to include blocks for the LO, modulator (what type should you use), bandpass filter and amplifiers. Think about what you can use to combine different signals for before transmission.

## Homework Equations

NA/// useful & succinct : http://en.wikipedia.org/wiki/Frequency-division_multiplexing

## The Attempt at a Solution

What is the frequency of local oscillator required at each channel?[/B]

Frequency division multiplexing (FDM) means that the total bandwidth available to the system is divided into a series of nonoverlapping frequency sub-bands that are then assigned to each communicating source and user pair.

I have a 20kHz window and 4.4kHz information signal. So I want:

Carrier frequencies at:

Channel 1: 102.2kHz

Channel 2: 107.2kHz

Channel 3: 112.2kHz

Channel 4: 117.2kHz

?

**What are the parameters (Q, f**

_{c}) for bandpass filter required at each channel?This is essentially generating the SSBSC signal, as i read it?

To suppress the LSB I want a band pass centered in between the carrier of each channel + 1/2 of 2.2kHz? This would five my critical frequencys for the Bandpass filter at fc and fc+2.2kHz?

(Bandpass filter has 2 critical frequencys that dictate the center frequency.)

channel 1:

[tex] f_0=\sqrt(f_c1*f_c2)=\sqrt(102.2k*104.4k)=103.294kHz [/tex]

[tex] Q=\frac{f_0}{BW}=\frac{103.294k}{2.2k}=46.952[/tex]

channel 2:

[tex] f_0=\sqrt(f_c1*f_c2)=\sqrt(107.2k*109.4k)=108.294kHz [/tex]

[tex] Q=\frac{f_0}{BW}=\frac{108.294k}{2.2k}=49.225[/tex]

channel 3:

[tex] f_0=\sqrt(f_c1*f_c2)=\sqrt(112.2k*114.4k)=113.295kHz [/tex]

[tex] Q=\frac{f_0}{BW}=\frac{113.295k}{2.2k}=51.50[/tex]

channel 4:

[tex] f_0=\sqrt(f_c1*f_c2)=\sqrt(117.2k*119.4k)=118.295kHz [/tex]

[tex] Q=\frac{f_0}{BW}=\frac{118.295k}{2.2k}=53.77[/tex]

I want a 40dB suppression outside of the critical frequencies given... there's 3dB suppression at the critical frequencies... I assume 40dB/decade after that which means I need a 2nd order active filter with butterworth response?

**Provide a diagram of the FDM transmitter. As a minimum, this need to include blocks for the LO, modulator (what type should you use), bandpass filter and amplifiers. Think about what you can use to combine different signals for before transmission.**

Stuck here, I think my LO needs to be tuned to the channel frequencies I noted above. I am not certain about what i have said about the bandpass filter - but reasonably so.

-If the bandpass is required to supress the LSB as I have assumed than I can use a run of the mill DSBSC to modulate all the carriers and then just filter out the LSB before amplification??

Thanks for any help.