On Jan 17, 2011, at 10:16 AM, Bill, W6WRT wrote:
>> However, this waveshaping also creates a crest factor of 3 dB.
>
> REPLY:
>
> I have always wondered about this. Is the varying amplitude really
> necessarY. What if the carrier was phase-shifted only?
Yes, the amplitude (a co-sinisoidal pattern for the I and Q components) is
absolutely what is keeping the bandwidth of PSK31 narrow.
If you take a carrier and just flip the phase by 180 degrees each time you want
to change a bit (what regular binary PSK does), you will have exactly the same
sin(x)/x keying sideband as you get from RTTY except the baud rate is 31.25
baud instead 45.45 baud for steam RTTY and there is only one carrier, instead
of two in RTTY.
The spectrum of RTTY looks somewhat like the profile of the Golden Gate Bridge:
two carriers, with keying sidebands spreading from both of the carriers.
Imagine the bridge with just a single tower -- that is what a raw PSK signal
looks like.
Which means that a raw PSK signal would be not much that more narrower than an
RTTY signal minus about 170 Hz and squeezed by about 70%.
As with all digital modulation (AFSK RTTY included), you can really narrow down
the bandwidths by using wave shaping (call it envelope modulation if you like,
but for PSK31, it is more like SSB than AM). In the case of PSK31, the
waveshaping is quite extreme, and the bandwidth reduction is also quite
extreme. When a bit changes, the envelope looks like 90 degrees of a sinewave.
The idle Varicode in PSK31 is a phase change 180 at every bit period. This
causes the envelope to look exactly like what an DSB signal (remember the old
two tone generators that we used in early SSB?) that is driven by a 15.625 Hz
audio -- the idle PSK31 signal is so narrow that all it does is look like two
carriers that are 31.25 Hz apart! This is why when it is idling, a good PSK31
signal looks like two lines on a waterfall -- you are seeing the two tones of
the double sideband suppressed carrier two tone test :-).
73
Chen, W7AY
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