Page 303 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

If the phase angle 8 from the receiver's oscillator is О, tihen the cos(8) = cos(O) =

1, and we сап see that the original modulating signal is recovered completely:

But if the oscillator's phase is 90 degrees, the filtered output is

And if the oscillator's phase is 180 degrees off from the radio station's carrier signal
phase, we get as the output

[ i~ t 11 t r u t u t]1 t in rt th ph . t)
i
What's interesting about using synchronous detection оп а DSBSC АМ signal is that
with the "wгопg" phase provided Ьу the oscillator, the output of the detector сап
Ье zero. This result is almost as if the detector does not even "see" the input signal
[V Sig( f) ]соs(2п fcarrier) t

At first look, this result of having the wrong phase from the oscillator for the
detector тау Ье а flaw in trying to demodulate а DSBSC (doubIe-sidеЬапd
suppressed-carrier) АМ signal. But actually опе canuse this "flaw" to transmit two
DSBSC АМ signals (1 and Q signals) and recover two channel!s of information, even
though the two 1 and Q DSBSC АМ signals occupy the same spectrum ог
bandwidth.

How I and Q Signals Are Generated

Before the structures of various 1 and Q modulators аге shown, а tabIe of
trigonometric identities will Ье useful Ьесаusеgепегаtiпg various forms of 1 and Q
signa:ls involves multiplication of sine an,d cosine signals (ТаЫе 16-1).
ТаЫе 16-1 Trigonometгic Identities