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home | newest check | boards | help index | log | ps | userlogin | send sysop | slog | status forward | bcm news | users | version | remove cookieG8MNY > TECH 23.04.24 15:31l 111 Lines 3942 Bytes #4 (0) @ WW BID : 6065_GB7CIP Read: GUEST Subj: VCR Triplexer to TV Input Path: JH4XSY<N3HYM<W0ARP<LU9DCE<EA2RCF<I0OJJ<GB7CIP Sent: 240423/0630Z @:GB7CIP.#32.GBR.EURO #:6065 [Caterham Surrey GBR] $:6065_GB From: G8MNY@GB7CIP.#32.GBR.EURO To : TECH@WW By G8MNY (Corrected Mar 06) (8 Bit ASCII graphics use code page 437 or 850, Terminal Font) LA6EAH asked for a circuit to feed 3 RF signals from VCR to a TV.. With the systems options below if there is an external aerial, then 2 clear RF channels are needed per modulator, as they are normal AM not VSB. Also check on digital TV signals as these often appear as just high noise on your VCR channel! If you can check on a spectrum analyser, you will clearly see how good or poor the setup is... Strong 10 mVエCar Sound Sound .ウ. Sound 3 mvエ ウ. FM ウ .ウ. ウ 1 mVエ ウ. ウ Nicam Weak Signal Unwanted ウ .ウ. Colウ Unwanted 0.3mVエ ウ. Colウコ _______ Sound ウ/\.. ウ ../\ウ Sound 0.1mVエ.ウ ../\ウコ ウ COFDM ウ 2x ウ ウ ウ 2x 30uVエ ウ ウコ ------ウ ウ------ | Unwantedウ ウ | 10uVウ ウ ウコ ウ2nd ord -20dB growth ウ | L S B ウ ウ | タトツトトトトトトツト トツトトトトトトツトトトトトトトツトトトトトトツト トツトトトトトトツトトトトトトツトトトトトトツトトトトトトツ 8MHz ch 1ch +2 dirty ch! 2 channels + products NORMAL V.S.B. TV C.O.F.D.M. DIGITAL TV D.S.B. (AM) VCR OUTPUT It is easy to put the wide VCR output over a wanted channel, or have a digital channel under the chosen VCR channel seen as noise on the VCR output. Also amplifying a digital signal before the Rx often leads to corruption of the digital signal, as amplifiers have to be VERY LINEAR to handle the weak digital signals in the presence of stronger analogue signals. OPTION 1 The normal way レトトトトトトソ レトトトトトトソ レトトトトトトソ レトトトトソ AERIAL>トエ VCR1 テトトトトトトトトトトトエ VCR2 テトトトトトトトトトトトエ VCR3 テトトトエ TV ウ タトトトトトトル Attenuate タトトトトトトル Attenuate タトトトトトトル タトトトトル 3-6dB 3-6dB Because most VCRs have 3-6dB RF gain in their aerial isolating amplifiers, attenuators are often needed, so that the following VCRs are not overloaded with the sometimes stronger modulator outputs. Some older VCRs have local- distant switches to help improve their sensitivity & through linearity. The attenuators can just be made up of the thin lossy leads you normally get with the VCRs! There are no losses in this system just RF amplifier problems. OPTION 2 The abnormal way レトトトトトトソ レトトトトトトトトソ レトトトトソ AERIAL>トトエ VCR1 テトトエ テトトトエ TV ウ タトトトトトトル ウ ウ タトトトトル レトトトトトトソ ウCOMBINERウ ウ VCR2 テトトエ ウ タトトトトトトル ウ ウ レトトトトトトソ ウ ウ ウ VCR2 テトトエ ウ タトトトトトトル タトトトトトトトトル The combiner can be any of several types:- using 2 x low loss (3.5dB ea) standard splitters, although matching is dubious! 3.5dB loss (o)_________________________ _______ ウ )| FB 7dB loss (o) ウ )| FB 150R )トトトト(o) 150R )トトトトト(o)(o)_ウ____)| 7dB Loss (o)___ウ___)| or a dedicated 3 way resistive splitter eg. 4x 39R in a star network properly matched to 75R. (o)-39Rトソ (o)-39Rトナト39R-(o) (o)-39Rトル ウ ウ___________ウ Port to any port 10dB loss or slightly less loss with a dedicated matching ferrite bead transformer. (o)-47Rトソ レトトトトトト(o (o)-47Rトエ__)| 2t ウ (o)-47Rトル )| 1t ウ Port to common port loss 7.5dB (0.5dB in Xformer) ウ________ウ______ウ OPTION 3 The switch way レトトトトトトソ AERIAL>トトエ VCR1 テトトソ タトトトトトトル ウ レトトトトトトソ o レトトトトソ ウ VCR2 テトトo<トトトエ TV ウ タトトトトトトル o タトトトトル レトトトトトトソ ウ ウ VCR2 テトトル タトトトトトトル Good switches at UHF are rare! Most have 1-2dB insertion loss & only 20dB isolation. Why Don't U send an interesting bul? 73 de John G8MNY @ GB7CIP
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