By G8MNY (Updated Dec 04) LIMITING As FM must not be over deviated to keep the bandwidth down, a limiter is used, this is unlike a simple clipper used on comms Tx that lets the signal distort. Broadcast limiters have fast attack to cope with the spikiest peak, & several decay time constants are used to mask the limiter breathing effects. Complex limiters may also treat the treble separately with separate faster time constants, as the treble content will be a predominant part of the pre- emphasised signal. To maintain the stereo image both left & right gains must be tracked together! The result is a signal that has it's ñ peak value accurately limited, but sounds perfect! With a good limiter you should not be able to tell the difference between a live studio feed & off air. The peak values result in the actual FM deviation, which ensures the correct overall bandwidth. ³ ³ Peaky pulse +³ ³³ ³_.³ ³ /'\ 0ÅÄÄÄÄÄÅÄÄÁÄÄÅÄÄÄÄÄÄ. ³ '|,^| ³ -³ U\/ Multiple CR 0dB³-. Recovery . ³ ³ ..-'\/\ _ _.' GAIN ³ ³|' `' \| -20dB³ ' Fast attack ÀÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄ>Time 2 4 8 10s BAD LF RESPONSE AFTER LIMITER It is also important that there is no phase distortion between the limiter & the Tx over the frequencies to be transmitted. If there is the limited signal can actually get larger.... No Phase error @ 20Hz +110kHz_³ ._ With Poor LF Phase +100%_³ _ __________ _ _ _ _ _ 140%_³_ _³ `-._ _ _ _ _ _ _ _³ _ +75kHz³ ³ ³ 100% ³ ³ `-._ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³-._³ ³ _³ -100%_³___³ _ _ _ _ _³__________³ _ -100%_³_ _ _ _ _ _ _³ _.-'_ _ -75kHz³ -140%_³ ³_.-' Audio with large 20Hz -110kHz³ ' content limited to just fit -3dB @ 20Hz inside this 20Hz envelope. Tx Peaks now seriously over modulated The same problem occurs in AM Tx, with the poor LF phase response on high level Modulation Transformers cause unexpected clipping! Poor LF phase causes over modulation on some programme material containing deep LF. This is one reason why loud LF is often cut on broadcast limiters. In practice FM Tx may have another hard limiter to protect it from accidental over modulation. STEREO MULTIPLEX The system is designed to be fully compatible with Mono FM radio Rx & without too much increase in bandwidth. To make stereo radio signal a sample of the left & right channels is transmitted every 38kHz. A switch samples the left or right @ 76kHz, to keep the Rx in step a locked 19kHz pilot tone is sent at -20dB (10%) below peak level. 76KHz Left ÄÄo Switch \,_____________________ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿ RightÄÄo /|\ ³ Add ÃÄÄÄÄ´ 53kHz ÃÄÄÄÄ´ Add ÃÄÄ> to Tx ³ ÚÄÄ¿ ÚÄÄÄÄÄÄ¿ ÀÄÄÂÄÄÙ ³Low Pass³ ÀÄÄÂÄÄÙ 76KHz >ÄÄÁÄÄÄ´ö4ÃÄÄ´ 19kHzÃÄÄÄÄÄ´-20dB ³ Filter ³ ³-30dB ÀÄÄÙ ³Filter³ ³ ÀÄÄÄÄÄÄÄÄÙ ÚÄÄÄÄÁÄÄÄÄ¿ ÀÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´RDS 57KHz³ PLL ³ UNIT ³ ÀÄÄÄÄÄÄÄÄÄÙ Left_ Right MUX _ /' `\ ÄÄÄÄ¿ ÚÄÄÄÄÄ - -/-'- ¿` \ Ú - - - - - - ³ ³ ³ ³ ³ ³ ³ ³ ³ - - - ³ - - - ³ - ³ - - ³ ³ ³ ³³ ³ ³ ³ ³ ³ ³ ³³ ³ \,_,/ ÀÄÄÄÄÄÙ À Ä Ä Ä Ù \ , _ , / O/P after switch (Expanded for diagram) RDS Data This is data phase modulated on to a 57kHz carrier that is phase locked to the 19kHz & added at 2-3% to the MUX O/P. The data is actually QPSK @ 1187.5Hz (76kHz/64) which occupies about 1kHz bandwidth. It contains; station ID, Tx ID, Network info, Time, Traffic Flag, Programme type, Current Programme data, etc. MUX SPECTRUM Baseband spectrum __ __ __ 0dB_³ _____...---''' ³ P ³ ```---..._____ _____...---''' ³ ³³ ³ I ³ ³ ³ ³ ³³ ³ L ³ Difference ³ ³ Difference ³ ³³ L+R ³ O ³ Signal ³ ³ Signal ³ -20dB_³³ MONO ³ T ³ DSB L-R ³ ³ DSB L-R ³ RDS -30dB_³³ ³ ³ ³ ³ ³ ³ ÀÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÅÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÂÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÐÄÄ 30Hz 15 19 23kHz 38kHz 53 57kHz The increase in baseband bandwidth from 15kHz for mono to 53kHz for stereo causes a 20dB loss in signal to noise, as it adds in the very poor signal to noise of the high frequency stereo difference signal that you Rx on FM systems. The The resultant noise in stereo image appears as noise from behind you. TX SPECTRUM Bessel Functions can shows the FM sideband harmonic level, for any particular modulation index. For mono the Mod index is Dev/Mod = 75kHz/15kHz = 5, but this analysis is less useful for very complex signals. This is where Carson's rule for minimum bandwidth can give clearer indication. Bandwidth = 2x Peak Dev + 2x Highest Mod Freq This gives the bandwidth of sidebands needed for no distortion. But it does not take into account that the levels of the highest modulation frequency are only 3% (-30dB) of the peak deviation, with resultantly weak sidebands. Deviation Mod <-2x 75kHz -> Mod 0dB_³ _ <57.5>ÚÄÄÄÄÄÄÄÄÄÄÄÄ¿<57.5> ³ / \ ______³ ³______ -20dB-³ ³ ³ ³ ³ _³ _/ \_ ÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄ -40dB ÀÄÄÄÁÄÄÂÄÄÄÂÄÄÁÄÄÄ < - - - - - 265kHz - - - - > 100kHz Theoretical Full Bandwidth More Typical bandwidth of RDS Stereo Broadcast signal as seen on spectrum analyser for ZERO distortion. under heavy modulation. TX RF Harmonics & Mixes These should all be > -60dBc, so added filters are normal. On multiple Tx sites there is a risk of PA mixing, where RF from a nearby Tx can be Rx at the Tx PA at enough strength to cause a Mix. A narrow resonant filter or directional coupler (Circulator) in the Tx feed can protect the Tx from these. Why Don't U send an interesting bul? 73 de John G8MNY @ GB7CIP