Comm. Circuits - SSB Question

"webzila" schreef in bericht news: snipped-for-privacy@f14g2000cwb.googlegroups.com...

In an SSB transmitter the carrier and one sideband are suppressed to save power and bandwidth. In a SSB receiver a BFO compensates for the carrier.

petrus bitbyter

webzila posted:

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

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

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.