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G8MNY > TECH 02.04.06 13:04l 179 Lines 8113 Bytes #999 (0) @ WW
BID : 24248_GB7CIP
Read: DL1LCA GUEST
Subj: Coded Orthogonal Freq Div Mux
Path: DB0FHN<DB0MRW<OK0PPL<DB0RES<DK0WUE<7M3TJZ<HS1LMV<CX2SA<GB7CIP
Sent: 060402/1046Z @:GB7CIP.#32.GBR.EU #:24248 [Caterham] $:24248_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Mar 06)
(8 Bit ASCII Graphics use code page 437 or 850)
Not knowing too much about it personally, here is some technical thoughts on
transmitting this mode as I see it.
This Transmission mode is used for broadcasting eg. DAB (1536 carriers) DRM &
DTV-B Freeview TV (1707 or 6817 carriers now, incompatabe 8K after 2013?)
It uses multiple carriers all phase modulated eg. Quadrature Phase Shift Keying
³ ³ ³ ³ ³ ³ ³ ³ QPSK 90ø /\ /\ /\
³ ³ ³ ³ ³ ³ ³ ³ on each 0øÄÅÄ180ø | | | | | |
ÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄ>F carrier 270ø \ / \ / \ /
Even carrier spacing Vectors ÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄ
Sidebands nearly touch
dependent on symbol rate (baud)
How electrically efficient is this mode? We know it is the best mode for
bandwidth use, as it can use all the channel space right up to the edge
compared to the less information/bandwidth efficiency of QPSK mode such as
Digital Sat.
_
/~ ~\ |~~~~~~~~~~~~~|
| | ³ COFDM ³
_.-" QPSK "-._ ³ CARRIERS ³
ÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄ ÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄ
Mux Channel Width Mux Channel Width
eg. 30MHz eg. 8MHz
But there are problems, as any hams using PSK31 with just 2 carriers will tell
you, the PA linearity is far more important than for speech amplitude modes let
alone FM.
POWER OF 2 CARRIERS (eg PSK31)
With a PA say capable of 100W RMS/PEP that is 200W RF crest power, then one
would intuitively think it could handle 2 carriers of 50W, but it CANNOT...
CREST CREST V CREST CREST V CREST CREST V
POWER @ 50ê POWER @ 50ê POWER @ 50ê
200 ´ ..-.. Ã 100 200 ´ Ã 100 200 ´ Ã 100
100 ´ .-~ ~-. Ã 70 100 ´ ..-.. Ã 70 100 ´ Ã 70
50 ´/ 100W PEP \Ã 50 50 ´ .-~ ~-. Ã 50 50 ´ ..-.. Ã 50
25 ´ 100W RMS Ã 35 25 ´/ 50W PEP \Ã 35 25 ´ .-~ ~-. Ã 35
12 ´ Ã 25 12 ´ 50W RMS Ã 25 12 ´/ 25W PEP \Ã 25
6 ´ Ã 17 6 ´ Ã 17 6 ´ 25W RMS Ã 17
As you can see from the above a 100W carrier has a crest of 200W & a peak volt
of 100V in 50ê. These are say the Max PA limits in my example.
Now consider the 50W carrier, it peaks 70V & to add another 50W carrier will
need another 70V, for both you need 140V, but that is more than this PA can do.
Only 2x 25W can be fitted in, that is 50V + 50V is OK. eg. 50W mean 100W PEP.
MORE CARRIERS
With 4 equal carriers, the maximum power is again quartered to 6.5W each, or
25W mean, but still 100W PEP (200W crest) needed to handle the in phase peaks
of the 4 carriers.
ÄÄÄ´ Â ÃÄÄÄ ³ /³\
 ³ Á |
³ `\ ³ |
_ ÃÄÄÄ `- Á 100V
/' ³ ³ |
' ÃÄÄÄ Á Á | _
ÃÄÄÄ ³ Â ³ ³ 25V
Á ³ Á \|/ -
42V 50V 17V 100V Peak
17W 25W 2.5W 100W PEP
The carriers can be at any phase angle as that are at different frequencies &
quadrature phase angles, but at some time the vectors will all add up in phase.
So it can be seen that ten 10V peak carriers can be used on this amp of 100V
max. eg. 10x 1W or 10W mean. (a 10V Peak carrier in 50ê is 1W)
From this the mean power must go down by the number of carriers, & each carrier
power must go down by that number squared.
PA MAX PEP PA MAX PEP
MEAN POWER = -------------- And each CARRIER POWER = -------------- 2
No of CARRIERS (No of Carriers)
BROADCAST
For DAB for example with 1536 carriers, means a 1KW amp could only do 651 mW
mean power, but still peaking 1KW PEP, & each carrier would only have a power
420 uW, YES MICRO WATTS!
For DTV with 6817 carriers spread over 8 MHz, a 33KW PEP amp could only do
4.85 W mean power, & each carrier would be 710 uW, but still 33KW PEP when all
the carriers occasionally are in phase!
So a very large Tx aerial gain & large PA are need to get this Tx mode to give
a respectable ERP.
SPECTRUM GROWTH
With PA mixing due to non linearity the intermodulation product just GROW the
spectrum like this with an amp run to the 1dB (10%) compression point..
|~~~~~~~~~~~~|
-20dB? ³ COFDM ³ -20dB?
ÚÄÄÄÄÄÄÄÄÄÄÄÄ´ CARRIERS ÃÄÄÄÄÄÄÄÄÄÄÄÄ¿
-45dB? ³ 2nd order ³ ³ 2nd order ³ -45dB?
____________³ product ³ ³ product ³___________
ÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÅÄÄ
>-70dB? 2 Channels Adjacent Channel >-70dB?
away QRM channel QRM eg 8MHz
It is very difficult to run a high power RF amplifier efficiently & very
linearly at the same time!
I have been told the broadcast PAs are not run at a linear drive level at all,
but until the spectrum growth/data error rate due to peaks hard clipping, is
troublesome!
RF FILTERING
Of course added RF filters either in PA stage or after a broadband PA, may
reduce the apparent growth.
³ /~~"-"~"-"~~\
³ | |
³ | |
³ | |
³ _./ \._
ÅÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄ
8MHz
With COFDM linear phase or amplitude flatness of the filter is not too
important (this why it is fairly immune to signal path ghosts), as long as each
or most carriers can be decoded in their own narrow bandwidth all is OK.
However signal to noise is important, especially from its own cross modulation
QRM. RF channel filtering only cleans up adjacent channel mixing products, not
in channel products.
Added losses & possible filter flash over must be a problem with peak powers
1000s of times greater than the mean if this approach is used.
AVOIDING IN PHASE
Because there is a serious PA overload problem if all or even most of the
carries add up in phase, the coder software may be designed to alter/restrict
the data that could cause this, but data options & data bandwidth loss or a
time overhead will result if the method is used.
PA EFFICIENCY
It should be clear that amplifying all the carriers in 1 PA is not wise way of
going about multiple carrier transmission. Ideally each carrier could have a
linear PA (needed for QPSK) & a loss free RF directional coupler/adder used.
But the short term cost of this complexity is excessive, & the system
flexibility (no of carriers) will be hardware fixed.
OLD ANALOGUE TV PAs
For main QRO stations these use a separate PA for the FM sound & a Linear PA
for the Video. The video PA is also high level (current) modulated with line
syncs to improve efficiency. The travelling wave type PA tube (several types)
often have annular target anode rings of different HT voltages that greatly
improving overall efficiency.
Using the TV PA efficiency definition of PEP/MEAN DC, efficiency of over 100%
are common on the average picture content (not a black screen)! So you can get
33KW PEP from a 33KW UHF video amp & it only needs 33KW of DC!
With this design of QRO PA, the main problem being the regrowth of unwanted
lower sideband of the TV VSB signal. This is normally dealt with by a water
cooled suckout filter. As the sound carriers are added in with couplers after
the linear PA there is no mixing problems.
ANALOGUE & DIGITAL TV
With these clean QRO analogue Tx the adjacent channel emissions are generally
very low something like > -60dBc. This has allowed the QRP digital TV signals
to exist on adjacent QRO analogue channels providing the Rx is linear. The
reverse is not possible, eg. QRO Digital with QRP analogue adjacent!
Why Don't U send an Interesting Bul?
73 De John, G8MNY @ GB7CIP
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