Discussion in 'General Electronics' started by abhilsah j, Jun 13, 2004.

1. ### abhilsah jGuest

I am trying to develop an intercom that sends out audio signals of 4Kz
bandwidth (three different channels through the same cable/wire.
(Prior to making that, I am actually trying to simulate that on a
computer so as to reduce the cost). I have double side band
suppressed carrier (DSSC) band modulator circuit (MC/LC 1496
modulator), a summer circuit and single side band suppressed carrier
(SSSC) demodulator circuit. But I don’t have the filter circuit
that converts double side band signal to single side band and also I
also don’t have the circuit for filtering out different channels
from the wire. (Since it’s in a simulator, I have quite wide
range of carrier frequencies by simply changing the parameters).
please tell me if i am missing anything.

thank you.

2. ### Jim ThompsonGuest

SSB Mixer = Carrier at 0° mixed with modulation at 0°
ADDED to Carrier at 90° mixed with modulation at 90°

Sign of sum picks sideband.

(I just designed one for a WiFi chip with selectable sidebands at
594MHz or 462MHz;-)

...Jim Thompson

3. ### Tim WescottGuest

You don't need to convert the DSB signals to SSB: Just demodulate one
sideband of the DSB as SSB and ignore the other.

With a SSB demodulator you can just filter the demodulated signal from 0
to 4kHz, this will reject the other signals on the wire.

4. ### JoergGuest

Jim's scheme is one way to do it. However, performing a 90 degree phase shift on an audio signal that ranges from a few hundred Hertz to 4KHz is not trivial. The ARRL handbook, at least older ones, show how to do that with a few dozen R and C. It is called the "phase method" of generating SSB. For a good sideband rejection of 40dB or so you need 1% or 2% parts and with caps that gets expensive. The other option would be a DSP but that becomes a bit elaborate here.

You could also directly filter the DSB signals using switched capacitor filters to pick out a sideband. One steep low pass plus a steep highpass per channel. Gets expensive as well, though.

Why not just take three carrier frequencies and do an FM modulation? The receivers would just lock their PLL onto whichever carrier you select and the audio comes right back.

Regards, Joerg

5. ### Jan PanteltjeGuest

Hi Joerg, you are 'analog', I just found some nice way to make SSB digital,
remember the Weaver method? Never used is as an amateur, but look at this site:
http://www.microtelecom.it/ssbdex/ssbdex-e.htm
And 70 dB or better sideband surpression without any adjustment is not bad at all!
JP

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6. ### JoergGuest

Hi Jan,

Yes, with a DSP or in this case PGA you can do it. I am pretty certain