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G8MNY  > TECH     10.07.18 11:03l 93 Lines 4334 Bytes #999 (0) @ WW
BID : 28453_GB7CIP
Read: GUEST
Subj: RF Noise Bridge for LF,MF,& HF
Path: HB9ON<IW8PGT<IR2UBX<IW2OHX<IW0QNL<JH4XSY<JM1YTR<JE7YGF<GB7CIP
Sent: 180710/0852Z @:GB7CIP.#32.GBR.EURO #:28453 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

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

Revisited from an old 2nd hand project.

SCHEMATIC
      ┌──────┐ | ┌─────┐ ┌─────┐ ┌─────┐ ┌──────┐
 ANT>─┤NOISE ├───┤NOISE├─┤NOISE├─┤NOISE├─┤NOISE │
  Rx<─┤BRIDGE│ | │AMP 3│ │AMP 2│ │AMP 1│ │SOURCE│
      └──┬───┘ | └─────┘ └─────┘ └─────┘ └──────┘
         Z   Screen

CIRCUIT
      Small              ┌────┬─330─┬──────────┬──────────┬────┬─o \o── +9V
     Ferrite            680  +│     │          │          │    │ on off
ANT   Ring   Coax        │   ===    1k         1k        470   │
(o────┐    ┌──======─┤├──┤   _│_10u │          │          │    │
_│_    )||(  _│_   │ 10n ├───┐      ├───┐      ├───┐      │    │
Rx   ct)||(        │     │ 100k     │ 100k     │ 100k     │    │+
(o─────)||(───┐ct  │     │   ├──┤├──┤   ├──┤├──┤   ├──┤├──┤   ===
_│_    )||(  _│_   │   RF \│ │ 10n   \│ │ 10n   \│ │ 10n _│_   │100u
       )||(__      │   NPN ├─┘    NPN ├─┘    NPN ├─┘     /_\'  │
  ┌───┘    * nc    │  T3 e/│     T2 e/│     T1 e/│    6V2 │    │
 VR                │     │          │          │          │    │
 100<─┐            │     │          │          │          │    │
 _│_ _│_           └─────┴──────────┴──────────┴──────────┴────┴─────── -ve
      BRIDGE            AMP        AMP        AMP    NOISE SOURCE

THE CIRCUIT
The noise from the zener (use the noisiest U have) is amplified by the 3 RF
amp stages using RF transistors, this provides about 1V peak of noise (as seen
on a scope). The 3 transistors are any good small signal RF NPN types e.g. BF
series. Layout is not too critical, but small is normally best for RF. Feed the
transformer with a coax lead to keep the noise well screened from the Rx port.

THE TRANSFORMER
This is the heart of the bridge, a well made one with give good nulled match
from 100kHz - 50MHz. The transformer windings, layout of the carbon track 100ŕ
VR & connections are ALL critical, if a good 50ŕ non inductive measurement is
going to be any good over all HF bands.

                               Screens
ANT        ___                |       |   ------------
 (o)---- /'___`\...nc         |       |  │            │
SO239  /'/ct  `\`\_____100ŕ   |       |  │            │
      │ │==GND--│ │---->VR    |BATTERY|  │   CIRCUIT  │
 Rx    \ \.___\/,/─┐pri POT   |       |  │   BOARD    │
 (o)====`\.___./\_(')         |       |  │            │
SO239  ct         │ │coax     |       |  │            │
                  NOISE       |       |   -------------

Wind the 4x 4 turns on thin enamelled copper wire in parallel flatly (no
twisting) around a 1-2cm dia ferrite ring. Identify the 4 wires to form the 2
centre taps ct. & clean off the enamel, twist the ct pairs together & solder.

Note the unused connection (nc) provides 180° out of phase noise to cancel the
crosstalk from the primary winding, leave this end a few cm long so it can be
moved around for precision nulling.

Place a piece of tin can (or drill in earth tags on die cast box.) under the
closely located output sockets, transformer, & up to the VR 100ŕ centre slider,
this all helps provide a low ground Z. Keep all connection leads as short as
possible!

TESTING & CALIBRATION
1/ With a Scope or capacitor to Rx check the RF noise increases down the amps.
2/ Using a digital ohm meter measure & make a temporary scale (on paper) for RV
   e.g. stop, 0, 20, 30, 40, 50, 60, 70, 75, 80, 100, stop.
3/ With No ANT or a Shorted ANT, there should be lots of noise on the Rx port.
4/ With a good 50ŕ dummy load the noise should null out with the pot at around
   the 50ŕ mark, check this on all bands. If it varies try moving the nc wire
   end to equalise the crosstalk to the Rx port on the highest bands. Nulls of
   40dB should be possible on most bands!
5/ Modify the scale to precisely agree with the RF impedance matching point.

IN USE
You can use with Rx to match any aerial with a suitable ATU.
It can be used as a noise source for checking filters & Traps etc.

With a spectrum analyser the multiple resonances of any aerial system will be
seen. As a noise source the spectrum response of any filters can be measured.


See also Tech bul "A Versatile Pulse Tester"

  
Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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