I think you are making this a bit more complicated than necessary. The
easiest way to keep track of AM and DSB is to look at the signal flow
diagrams. If you feed an audio signal with a DC offset into a mixer,
you get AM. If there is no DC offset, you get DSB.


Tom Holden wrote:
"Michael Black" wrote in message
...

"Robert11" ) writes:
Hi,

Let's say I pick up a regular HF shortwave station on AM.

Should I also be able to hear this same station on USB ?

If so, what would the freq. "offset" that I should increase the tuned
freq.
to ?

Should I expect the USB freq to be clearer, or... ?

Thanks,
B.

An AM signal consists of two sidebands, upper and lower, which are mirror
images of each other, and a carrier (a constant level signal) right in
the middle of the two sidebands.

A single sideband transmitter removes one of the sidebands before the
signal leaves the transmitter. Usually, it removes the carrier too,
though
there are some specific cases where they send the carrier along.

If you tune the AM signal with a sideband receiver, then you are
removing the redundant sideband in the receiver. The narrow filter will
slice off the unwanted sideband. And usually, you
also knock out the carrier, which in effect gives you a single sideband
signal with no carrier inside the receiver. The rest of the receiver
thinks
it is an SSB signal, the same as if that's what had been sent by the
transmitter.

But since two sidebands with exactly the same content were sent, you can
switch the receiver between upper and lower sideband, selecting whichever
of
the two sidebands is in better shape. If the receiver was taking in the
full AM signal, if one of the sidebands had interference, you'd hear it
and be unable to do anything about it.

Michael

Strictly speaking both the "AM" signal and the "SSB" signal are AM
(Amplitude Modulation). The common usage of the term "AM" (other than
referring to the MW broadcast band in North America) means DSB-AM (Double
Side Band Amplitude Modulation) with carrier. Likewise, "SSB" commonly means
SSBSC-AM (Single Side Band Suppressed Carrier Amplitude Modulation). There
is also DSBSC-AM (DSB suppressed carrier is pretty rare on hf but used for
the L-R channel in the multiplex for FM stereo and also written as DSSC) and
SSB with carrier (not sure of the mnemonic). Between full carrier and
suppressed or nearly zero carrier is the reduced carrier mode, i.e., DSBRC
and SSBRC. The Canadian time signal transmissions on 3330 and 7330? kHz are
USBRC-AM (Upper Side....).

To answer "what would the freq. "offset" that I should increase the tuned
freq.?" depends on your receiver design and, in some cases, settings that
you make. For example, my DX-394 (correctly aligned and tuned to the
station's frequency) needs no retuning to switch between AM (DSB), LSB and
USB modes. That's because the microcontroller behind the front panel
controls makes the necessary shifts in the 1st and 2nd Local Oscillators and
the selection or not of one of two Beat Frequency Oscillator frequencies.
This is not true of simpler receiver designs.

Simpler radios may have a variable BFO and no automatic tuning. In this
case, you would tune higher than the station's frequency by approximately
1/2 the passbandwidth of your IF filter to suppress the lower sideband and
adjust the BFO frequency until the beat note from the carrier was near zero
frequency at which the pitch of speech and music should sound right with
least distortion. To select the lower sideband, you would tune lower than
station freq by the same amount and readjust the BFO.

My Kaito-Degen 1103 lies between the simpler radio and the DX-394. While
microprocessor controlled, it does no auto tuning in SSB mode and provides
no direct selection of Lower and Upper sidebands. While it has a pretty
decent (for a cheap radio) narrow filter (narrower than the DX-394's!), when
the radio is tuned to display the station frequency, that filter is centred
on the carrier and equal amounts are passed from both sidebands. The BFO is
adjustable over a range slightly larger than +/-1kHz and the main tuning
tunes in 1 kHz steps so you can get some sideband suppression by tuning 1
kHz higher or lower and adjusting the BFO for zero beat/clearest signal.
Off-tuning by 2kHz would be a better fit to the filter but is beyond the
range of the BFO. Were it to have that range on such a small control, it
would be twice as hard as the already touchiness to get close to zero beat.
The DX-394 does not have an adjustable BFO; without modification, it can be
off zero beat by a few tens of Hz.

Receiving a DSB-AM signal with the radio in SSB mode is also known as ECSS,
Exalted Carrier Selectable Sideband, and can be advantageous in reducing the
effects of fading, as well as interference in the suppressed sideband. But
it requires a continuously variable BFO with stable transmitter frequency
and receiver oscillators to attain near zero beat for best enjoyment. That
leads to automatic means of zero beating - and that's a whole other topic
called Sync-AM (or Synch-AM) - join http://groups.yahoo.com/group/Synch_AM/
for more.

Tom