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G8MNY  > TECH     25.04.22 06:34z 92 Lines 4597 Bytes #8 (0) @ WW
BID : 734_GB7CIP
Subj: DC current sources for coax/AF.
Sent: 220425/0632Z @:GB7CIP.#32.GBR.EURO #:734 [Caterham Surrey GBR] $:734_GB7C

By G8MNY                                          (Updated May 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

There is a method often used for powering a TV camera over its video coax,
where a choke feed that could be used in RF applications just won't work.
Similar circuits can be used for balance mic lines were lossy resistor feeds
are not wanted.

C A M E R A   F E E D
+ve supply e.g. +24V @ 1A                         M I C   F E E D
컴컴컴쩡컴컴컴컴                           +ve supply e.g. >+48V @ 10mA
              0.5                            컴컫컴컴쩡컴컴컴쩡컴커
     +                                             100        100
100u ===      /e                             47uF+      47uF+   
          旼캑 TIP                              === /e      === /e
        /e  \                                  쳐캑         쳐캑
      쳐캑                                      1M  \       1M  \
    100k \                                47uF+            쳐컴컨> To
      읕컴컨컴컴 +12V nominal          AF <컴늘컴좔컴컨컴컴컴)컴컴컴> Mic
Video         +                       Kit <컴컴컴늘컴컴컴컴컴켸
Kit 컫 <컴캑쳐컨컴컴>DC to camera               47uF +
    75   1000uF     & Video from it

In the camera feed due to the high current a PNP darlington arrangement is used
to amplify the DC through a 100k. The 100k value is dependent on the camera
DC load & adjusted to get the working voltage e.g. 12V to the camera. The video
can pass unimpaired as the 100uF decouples all the video frequencies from the
current source, effectively making the power feed look like a resistor of
100k/(HFE1*HFE2) + a huge AC choke where Xl = (HFE1*HFE2)/Xc. The emitter R
increases the voltage swing for any current changed making it more linear &
maintaining a high collector Z (constant current).

The Mic circuit is much the same but no darlington needed as lower current, but
low noise transistors needed for a balanced or single ended line. Power rail
hum decoupling is not really needed in either of these circuits.

To extract the line DC a reverse circuit is used normally with a regulator.
Some regulator IC can be used, providing they are not unstable with no input
                                                                 +     /
                                     Line + <컴68鴛쩡컴컴쩡컴컴컴캑쳐<'b
+ COAX       +                       with DC        \ 100k     47uF  `\
<컴컫컴컴쩡컴캑쳐컴컴< Camera                        쳐컴
   1k  /    1000uF   Video                        e/        +19.5V
    쳐캑 TIP                                       쳐컴컴)컴컴컴컴컴컴쩡컴+
      \e                                         e\   쳐컴       +  ===
   __   쳐컴컴>                                     쳐컴  __20V       47u
9V /_\' +100u   8.5V DC             DC +           / 100k /_\' +    / 
       ===      to Camera           Line - <컴68鴛좔컴컴좔컴)컴컴늘< a 
컴컴좔컴컨컴컴>                                               47uF -\ 
C A M E R A   S U P P L Y           Shield  <컴컴컴컴컴컴컴컴좔컴컴컴컨컴컨
                                          M I C   A M P    S U P P L Y

For mic preamps just 2 Rs are often used to feed the decoupled rail for the
preamp ICs & no transistors, but these circuits are better!

For a fixed frequency or band of frequencies (HF) often a choke & capacitors
will do to isolate a DC path from RF.
              C                                  C
  Tx/Rx o)컴컴늘컴쩡0========coax=========0컴컫컴늘훟)Aerial
                 L)                         L)   
Control DC>컴쩡컴켸                          읕컴컴쩡>Relay/ATU/Preamp   
            === C                                 === C 
          컴컨컴컴컴켸                     읕컴컴컴컴좔컴컴             

C must be big so that Xc < Z (2x 50) e.g. Xc < 10 @ the lowest frequency.

L must be large so the Xl > Z (50/2) e.g. Xl > 250 @ the lowest frequency.
Also L must not reduce inductance @ DC load (e.g. ferrite/iron dust may do),
and the L must not over heat carrying the DC or the RF & must not have multi
wavelength resonances in it's winding! For more relay options a +/- supply
can control 2 with steering diodes etc.      

For VHF & UHF the C & L are quite small, but must be layed out leadless to
reduce the RF loss. 

See also "Phantom Bal Mic Preamp+Gun Mic" bul.

Why don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

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