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G8MNY  > TECH     30.04.22 09:12z 110 Lines 3925 Bytes #14 (0) @ WW
BID : 1076_GB7CIP
Subj: VCR Triplexer to TV Input
Path: SP7YDD<SR1BSZ<SV1CMG<SV1CMG<GB7COW<GB7CIP
Sent: 220430/0904Z @:GB7CIP.#32.GBR.EURO #:1076 [Caterham Surrey GBR] $:1076_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 mVCar    Sound                                   Sound   ..   Sound
 3 mv .   FM                                              ..    
 1 mV .     Nicam      Weak Signal         Unwanted      .. Col  Unwanted
0.3mV . Col             _______             Sound   /\..  ../\   Sound
0.1mV. ../\             COFDM               2x                   2x
 30uV            ------       ------        |   Unwanted          |
 10uV           2nd ord -20dB growth        |     L S B           |
           
        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
        Ŀ           Ŀ           Ŀ   Ŀ
AERIAL>Ĵ VCR1 Ĵ VCR2 Ĵ VCR3 Ĵ TV 
         Attenuate  Attenuate    
                   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
         Ŀ  Ŀ   Ŀ
AERIAL>Ĵ VCR1 Ĵ        Ĵ TV 
                      
         Ŀ  COMBINER
          VCR2 Ĵ        
                   
         Ŀ          
          VCR2 Ĵ        
           

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)________________________
               _______                )| FB
  7dB loss  (o)      )| FB       150  )(o)
                 150  )(o)(o)____)|
  7dB Loss  (o)______)|


or a dedicated 3 way resistive splitter eg 4x 39 in a star network properly
matched to 75.

 (o)-39Ŀ
 (o)-3939-(o)
 (o)-39    
   _________   Port to any port 10dB loss


or slightly less loss with a dedicated matching ferrite bead transformer.

 (o)-47Ŀ  (o
 (o)-47Ĵ__)| 2t  
 (o)-47  )| 1t   Port to common port loss 7.5dB (0.5dB in Xformer)
   _____________


OPTION 3 The switch way
         Ŀ
AERIAL>Ĵ VCR1 Ŀ
           
         Ŀ  o    Ŀ
          VCR2 o<Ĵ TV 
           o    
         Ŀ  
          VCR2 
         
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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