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G8MNY > TECH 21.07.05 16:31l 103 Lines 5082 Bytes #999 (0) @ WW
BID : 55123_GB7CIP
Read: GUEST OE7FMI
Subj: A Versatile Pulse Tester 3/3
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<7M3TJZ<F6CDD<ON4HU<GB7YFS<GB7CIP
Sent: 050721/0722Z @:GB7CIP.#32.GBR.EU #:55123 [Caterham] $:55123_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Feb 05)
BALANCED LINE TESTING.
This needs a small pulse transformer in an add on box to isolate and match to a
balanced line. A turns ratio of 1:2 will drive the line with about 200ê
floating source. This is quite correct as most balanced lines are around 140ê
at HF, even if used as 600ê at AF. For the transformer I used a small 2 hole
ferrite core testing it with 1/2/3 turns to see which did not reduce the
overall height of the DC pulse (not saturating) & did not cause too much
negative pulse response (L too low), 3 turns was what I ended up with for the
primary & therefore 6 turns for the secondary in slightly thinner enamelled
copper wire. Avoid too many turns as this will reduce the magnetic coupling
increase winding capacitance & cause unwanted ringing etc. The same variable
termination as above can be used to find the Z of any balanced line.
_____ _______
BNC 3 ):( Balanced
Turns):( 6 Turns
______):(_______ Line
Small
2 hole core
You may be surprised just how good ordinary twisted wire is, once it is
properly matched.
CRYSTAL CALIBRATOR.
As the narrow pulse has very high harmonic content strong signals can be heard
into the GHz bands. The use of 250KHz as the pulse rate give 4 markers per MHz.
Do not put the pulse output into a radiating aerial, as it will cause wideband
QRM locally!
SPECTRUM ANALYSER CALIBRATION.
In theory the pulse spectrum is in the form of (sine x)/x [1] this gives an
overall cosign shaped envelope of the markers to the 1st null frequency Fn,
then much weaker repeating « sine wave shape envelopes to infinity.
If the terminated DC pulse height is measured with peak detector (fast diode &
cap, mine was 8V which is 1.28 Watts of peak pulse power!) the true power level
of any of the RF markers is then..
³ Fr
/³\ ³ ³ 2Fr V x T x Fr Sine(Fh/Fn)
³ ³ ³ 3Fr Harmonic Voltage = ---------- x -----------
Harmonic³ ³ ³ ³ 0.707 Fh/Fn
³ ³ ³ ³ 4Fr
Voltages³ ³ ³ ³ ³ Fh = Freq of a Harmonic
³ ³ ³ ³ ³ Fn = Freq Null = 1/T
³ ³ ³ ³ ³ Fr = Repetition Frequency
³ ³ ³ ³ ³ T = Pulse width
³ ³ ³ ³ ³ ³ V = Pulse height
³ ³ ³ ³ ³ ³ . | | .
³ ³ ³ ³ ³ ³ | ³ ³ ³ ³ | . | | .
³ ³ ³ ³ ³ ³ ³ Fh³ ³ ³ ³ ³ ³ ³ ³ . ³ ³ ³ ³ ³ ³ .
³ ³ ³ ³ ³ ³ ³ | ³ ³ ³ ³ ³ ³ ³ ³ | | ³ ³ ³ ³ ³ ³ ³ ³ | .
ÀÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÁÄÁ
Frequency -> Fn 2Fn 3Fn 4Fn 5Fn
A spreadsheet can be loaded with this formula and pulse data, then graphs can
be plotted of the ideal spectrum for comparison. This assumes perfect
terminations and ideal pulse shape etc. but a good starting point.
For UHF-SHF use, surface mount components "chip Rs and Transistor" are soldered
directly across threaded BNC sockets with no wires or tags, this does give
better results but with a lower pulse size of around 4V peak with a sub 0.3 nS
pulse length with 5cm of coax, with no visible spectrum Fn null up to 2GHz.
FILTER PLOTTING.
When this fairly broadband signal source is put via a VHF/UHF filter into a
spectrum analyser with a 300KHz wide IF, the shape of the filter can be seen
immediately and the filter performance easily adjusted. This is normally only
possible with a tracking generator or a high power noise source.
dB Loss dB Loss
0dB_³ _._ _._ 0dB_³ _
³ /' `-' `\ ³ ,'`-' `-'`,
10dB_³ : : 10dB_³ : :
³ | Criticly | ³ | Coupled |
20dB_³ | Coupled | 20dB_³ : 3 pole :
³ | 2 Pole | ³ | VHF Filter |
30dB_³ ,' VHF Filter `. 30dB_³ : :
³_./ \._ ³ _./ \._
40dB ÀÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄ>F 40dB ÀÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄ>F
144 145 146 144 145 146
Although this noise source should not be put into a effective radiating aerial,
handheld ¬ wave & helical whips do immediately give their frequency band away
where they match the RF.
Be aware that this broadband pulse can easily overload wideband equipment
(often only 2 tone calibrated), so a band limiting filter should be used before
testing response of preamps etc.
References:- [1] Rx Calibrator and Tx Mon, by G4COL, Radcom June 1998.
[2] Cable Fault Locator, D.Huddart, Electronics World March 2001.
Why don't U send an interesting bul?
73 de G8MNY @ GB7CIP
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