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G8MNY > TECH 09.04.26 18:20l 93 Lines 4613 Bytes #10 (0) @ WW
BID : 56786_GB7CIP
Subj: DC current sources for coax/AF.
Path: JH4XSY<F1OYP<F1OYP<PY2BIL<HB9ON<VK7AX<GB7CIP
Sent: 260409/0852Z @:GB7CIP.#32.GBR.EURO #:56786 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated May 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
HIGH Z CURRENT SOURCES.
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
ﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ +ve supply e.g. >+48V @ 10mA
ｳ 0R5 ﾄﾄﾄﾂﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄｿ
+ｳ ｳ ｳ 100R ｳ 100R
100u === ｳ/e 47uFｳ+ ｳ 47uFｳ+ ｳ
ｳ ﾚﾄﾄｴ TIP === ｳ/e === ｳ/e
ｳ ｳ/e ｳ\ ﾃﾄﾄｴ ﾃﾄﾄｴ
ﾃﾄﾄｴ ｳ 1M ｳ\ 1M ｳ\
100K ｳ\ ｳ 47uF+ ｳ ｳ ﾃﾄﾄﾄﾄﾁﾄ> To
ﾀﾄﾄﾄﾄﾁﾄﾄﾄﾄｴ +12V nominal AF <ﾄﾄｴﾃﾄﾄﾁﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄ)ﾄﾄﾄﾄﾄﾄ> Mic
Video + ｳ Kit <ﾄﾄﾄﾄﾄﾄｴﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ
Kit ﾄﾂﾄ <ﾄﾄﾄｴﾃﾄﾄﾁﾄﾄﾄﾄ>DC to camera 47uF +
75R 1000uF & Video from it
ﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ>
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 and 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 and
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.
HIGH Z DC LOADS.
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
Caps.
+ /ｳ
Line + <ﾄﾄ68Rﾄﾂﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄｴﾃﾄ<'bｳ
+ COAX + with DC \ｳ 100K 47uF `\ｳ
<ﾄﾄﾄﾂﾄﾄﾄﾄﾂﾄﾄﾄﾄｴﾃﾄﾄﾄﾄﾄ< Camera ﾃﾄﾄﾄｴ
1K ｳ/ 1000uF Video e/ｳ ｳ +19.5V
ﾃﾄﾄｴ TIP ﾃﾄﾄﾄﾄﾄ)ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄｿ+
ｳ ｳ\e e\ｳ ﾃﾄﾄﾄｿ +ｳ ===
_ｳ_ ﾃﾄﾄﾄﾄﾄ> ﾃﾄﾄﾄｴ _ｳ_20V ｳ ｳ47u
9V /_\' +ｳ100u 8.5V DC DC + /ｳ 100K /_\' + ｳ/ｳ ｳ
ｳ === to Camera Line - <ﾄﾄ68Rﾄﾁﾄﾄﾄﾄﾄﾁﾄﾄﾄ)ﾄﾄﾄﾄｴﾃﾄ< aｳ ｳ
ﾄﾄﾄﾄﾁﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄ> ｳ 47uF -ｳ\ｳ ｳ
C A M E R A S U P P L Y Shield <ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾁﾄ
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 and no transistors, but these circuits are better!
RF APPLICATIONS
For a fixed frequency or band of frequencies (HF) often a choke and capacitors
will do to isolate a DC path from RF.
C C
Tx/Rx o)ﾄﾄﾄﾄｴﾃﾄﾄﾂﾄﾄ0========coax=========0ﾄﾄﾄﾂﾄﾄｴﾃﾄo)Aerial
L) ｳ ｳ L)
Control DC>ﾄﾄﾂﾄﾄﾄﾄﾙ ｳ ｳ ﾀﾄﾄﾄﾄﾄﾂﾄﾄ>Relay/ATU/Preamp
=== C ｳ ｳ === C
ﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄ
C must be big so that Xc < Z (2x 50ohm) e.g. Xc < 10ohm @ the lowest frequency.
L must be large so the Xl > Z (50/2) e.g. Xl > 250ohm @ 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 and 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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