Comm. Circuits - SSB Question

[webzila]

Can someone tell me what must be done in the SSB transmitter and in a
SSB receiver to compensate for the lack of a full carrier in SSB
transmission?
Any advise regarding this would be appreciated.

Thank you

 

[Dbowey]

webzila posted:

Can someone tell me what must be done in the
SSB transmitter and in a SSB receiver to
compensate for the lack of a full carrier in SSB
transmission?

I'm not sure what you mean by compensate, but....

There are several methods that can be employed in the transmitter and receiver
to generate and receive SSB.
Here's one very basic one. You must first understand that the ONLY purpose of
a transmitted carrier is to provide a reference frequency for recovering the
sideband information. The Carrier has no other function. We can eliminate it
at the transmitter and insert a new reference signal at the receiver. Also,
both sidebands contain identical information so, except for operating rules, it
doesn't matter which one you use.

Transmitter:
1. Generate a continuous wave carrrier signal at some frequency.
2. Amplify a microphone's signal to a "usable" amplitude.
3. Connect the "carrier" and the voice signal to a balanced modulator. This
will remove the carrier, and leave the upper and lower sidebands.
4. Send the USB/LSB signal into a filter designed to remove either the USB or
LSB portion of the signal.
5. Send the remaining signal into another RF mixer (multiplier) which is being
driven by another RF signal selected so the sum or difference between the two
signals is on the frequency on which you want to transmit AND which provides
the sideband orientation you want.
6. Include buffers and amplifiers as required.

Receiver:
1. Build a receiver using the demodulator described below.
2. Take the signal from the last I.F. amplifier and connect it to the signal
input of a Product Detector.
3. Build a "beat oscillator" operating at the I.F. center frequency and
variable about+ and - 6 kHz. Connect it to the carrier input of the product
detector.
4. The Beat Oscillator is the reference frequency generator replacing the
missing carrier..

Tune the receiver to the SSB signal. Turn on the Beat Oscillator and tune it
across it's range. As you do this you will hear the transmitted voice at a
strange pitch. Continue to change the Beat Oscillator frequency until the
pitch is normal.

That's it.

Don

 

[Michael Black]

"webzila" (aeroman10@yahoo.com) writes:

Can someone tell me what must be done in the SSB transmitter and in a
SSB receiver to compensate for the lack of a full carrier in SSB
transmission?
Any advise regarding this would be appreciated.

Thank you

Is this a trick question? Are you talking about how it's demodulated,

or are you looking for a perceived need for making sure the receiver
is on the exact frequency when there's no carrier to tune to?

Since other's have explained the former, just for fun I'll take a stab
at the latter.

Much of the time, you don't. A slightly mistuned receiver simply sounds
a tad odd, since the recovered audio will not be exactly where you expect
it. If you've mistuned by 10Hz, that 1000Hz tone will be 1010Hz. Since
very little transmitted by SSB is more complicated than voice, one can
usually live with it. Or looking at it another way, SSB is not used to
transmit music since the mistuning will make the music sound awufl.

If you want exact, you start with a crystal controlled transmitter,
and a crystal controlled receiver, and in some cases the crystals go
into a crystal oven.

I don't know if there are situations where a transmitter is sending
data on one sideband and voice on the matching other sideband, but
the data channel would provide a means of proper tuning since you would
know what's being sent on the data channel.

Some attempts at broadcasting with SSB have cheated, either using
a full carrier or an attenuated carrier. But then it's single sideband
with a full or partial carrier.

Fifteen to twenty years ago there was talk of using a similar scheme
for two-way SSB, particularly at VHF and UHF. I have no idea if this
ever saw much application, but it was done by inserting a subaudible tone,
in effect a carrier, albeit not where the actual carrier is. Filter it out,
and you've got the marker for proper tuning. But again, it's not pure
single sideband without carrier.

Realistically, the one useful purpose the carrier serves is to ensure
that the signal is tuned properly at the receiver.

Of course, if one can make do with double-sideband with a suppressed
carrier, the receiver does know where to put the reinserted carrier.
It's always half-way between the sidebands, and since the upper and lower
sidebands are mirror images of each other, that does point to where
the mid-point is.

Michael

 

[Bob Myers]

"Michael Black" <et472@FreeNet.Carleton.CA> wrote in message
news:cm44d6$6qc$1@freenet9.carleton.ca...

Some attempts at broadcasting with SSB have cheated, either using
a full carrier or an attenuated carrier. But then it's single sideband
with a full or partial carrier.

And, just to add a further possibly interesting aside, that's
just what TV does. The luminance ("black and white")
portion of the TV signal is transmitted via "vestigial
sideband" (VSB) transmission, a form of AM in which the
full carrier is retained but one sideband (in this case, the
lower one) is significantly reduced. For TV, the lower
sideband exists only in a greatly reduced form and then
only for the lower video frequencies (i.e., those nearest
the carrier). This means that demodulation in the receiver
is pretty much straightforward AM, with the exception that
there has to be some filtering applied to decrease the
contribution of the lower frequency portion of the upper
sideband, to compensate for the fact that there's also some
LF contribution from the lower sideband.

The chroma information uses an interesting variant of AM
known as "quadrature modulation," in which the two
sidebands around the chroma subcarrier frequency are both
present but carry DIFFERENT information. In this case,
the location of the subcarrier is given by the subcarrier-
frequency "burst" that's included during the horizontal
blanking period.

Bob M.