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G8MNY > TECH 31.03.04 08:19l 74 Lines 3384 Bytes #999 (0) @ WW
BID : 50764_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: RF Noise Bridge for HF
Path: DB0FHN<DB0FOR<DB0MRW<OK0PKL<OK0PCC<OK0PAD<OK0PPL<DB0RES<ON0AR<IK1ZNW<
7M3TJZ<LZ3NP<VK6ISP<ZL2TZE<GB7YFS<GB7CIP
Sent: 040330/2344Z @:GB7CIP.#32.GBR.EU #:50764 [Caterham] $:50764_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
Revisited from an old 2nd hand project. (new to packet Nov 03)
+ÚÄÄÄÄÂÄÄÄ330êÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÂÄÄÄo \oÄÄÄ +9v
10u === 680ê 1K 1K ³ ³ on off
ferrite ÄÁÄ ³ ³ ³ ³ ³
___ ring coax ÃÄÄÄ¿ ÃÄÄÄ¿ ÃÄÄÄ¿ ³ ³
ANT )||(ÄÄÄÄ=====Ä´ÃÄ´ 100K ³ 100K ³ 100K ³ ³
)||( pri³ 10n ³ ÃÄÄ´ÃÄÄÄ´ ÃÄÄ´ÃÄÄÄ´ ³ ³ ³
Rx Ä--)||(ÄÄÄÄ´ ³ ³ 10n ³ ³ 10n ³ ³ 470ê ³
ct )||( ct ³ BF À\³ ³ BF À\³ ³ BF À\³ ³ ³ ³+
)||(_ ³ NPN ÃÄÙ NPN ÃÄÙ NPN ÃÄÁÄ´ÃÄÄ´ ===
³ nc ³ T3 e/³ T2 e/³ T1 e/³ 10n ³ ³100u
VR ³ ³ ³ ³ ÄÁÄ¿ ³
100ê ³ ³ ³ ³ 6v2 /_\ ³
ÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄ
THE CIRCUIT
The noise from the zener is amplified by the 3 RF amp stages to provide about
1V peak of noise. The 3 transistors are any good RF NPN types. 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 to the Rx port.
THE TRANSFORMER
This is the heart of the bridge, a well made one with give good match from
100KHz - 50MHz. The transformer windings, layout of the carbon track 100ê VR &
connections are ALL critical, if a good 50ê non inductive measurement is going
to be any good over all HF bands.
ANT _____
(o)----/ ___ \...nc
/ /ct \ \_____100ê
³ ³==GND--³ ³--->VR
Rx \ \ ___\/ /Ä¿pri
(o)=== \_____/\_(')
ct ³ ³coax
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 enamel, twist the pairs together & solder.
Note the unused connection (nc) provides 180ø 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 100ê 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 Rx check the RF noise increases down the amps.
2/ Using a digital ohm meter measure & make a temporary scale (on paper) for RV
eg 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 dummy load the noise should null out around the 50ê 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 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 cane be measured.
/QSL
73 de John G8MNY @ GB7CIP
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