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G8MNY > TECH 25.08.03 22:39l 80 Lines 3545 Bytes #999 (0) @ WW
BID : 12274_GB7CIP
Read: GUEST DB0FHN OE7FMI
Subj: A 2 Tone AF Test Osc Design
Path: DB0FHN<DB0RGB<DB0MRW<DB0SON<DB0SIF<DB0EA<DB0RES<ON0AR<VK3TE<GB7YFS<
GB7CIP
Sent: 030825/1802Z @:GB7CIP.#32.GBR.EU #:12274 [Caterham] $:12274_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
TWO TONE AUDIO OSCILLATOR
By G8MNY
NEED
A two tone osc is used for setting up & testing basic linearity of linear
amplifiers & any RF stage. A scope connected to the RF output of a linear will
produce the classic bridge rectifier envelope. This can even be done at VHF &
UHF using a suitablly loaded diode probe before the scope.
Over drive shows up a fat topping of the envelope waveform.
Poor bias shows up as poor crossover on the envelope.
Instability at specific power levels shows up as fuzzy bits on the envelope.
FEATURES
POWER :- This unit run from a 9V PP3
On 12V it charges the PP3.
OUTPUTS :- Internal Hi Z LS (EARPIECE) high enough for acoustic coupling.
or jack for mic I/P hookup.
BALANCE :- For setting the relative tone levels
(as needed in Tx mixer for balance)
VOLUME :- For setting the AF O/P level for no Tx clipping .
RF IMMUNITY:- only 2 power transistor with base stoppers, so very immune.
CIRCUIT
This uses 2 ferrite pot cores with 3 windings on each, 2 make a centre tapped
winding of about 44mH, & the other winding about 1/4 the turns giving a low Z
o/p. These form the hart of the two power oscillators.
Each oscillator circuit is a single TO5 (BFY50) transistor connected in Hartley
configuration with a tuning capacitor across each centre tapped winding. The 2
tones must not be mathematically related eg harmonics etc. Mine turned out to
be 991 & 1352Hz. The oscillator level is set by the 47K bias resistor located
at the transistor base which also makes the trasistor stage immune to RF!
----------------------------------- EARPIECE
! ---------- ) ---------------- 40R LS
VOL POT <---! !
200R ---------- ) -------- LS
! !---------JACK
\!/ !
----BALANCE POT---- !
! POT 100R ! !
! ! !
! --------------- ) -------
! ! ! !
((() 991Hz ((() 1352Hz
- - - - - - - - - - - -
((((( ))))) ((((( ))))) ON/OFF
! ! ! ! ! ! /
! --- ) -------- ) ------- ) --------o/ o------330R-!<!--o +12V
! u47 ! ! ! ! !
!---!!----! ! u47 ! ! ! Charge
! u22 ! !---!!----! + ! ! + or run.
!----!!---! ! ! --- ----- External
! .! ! .! === === DC
! ! / ! ! / 100u ! ----- 9V
-47K-!< -47K-!< ! === PP3
! \e ! \e ! !
! ! ! !
---------------------------------------------------o 0V
The output windings feed the balance pot that works like a stereo pot selecting
the proportion of each tone. The tuned power oscillators have enough Q that the
output coupling via the 100R does not affect each oscillator frequency & the
outputs are quite a good sine wave.
The 200R volume pot set the overall level in use. The internal LS was a old 40R
(4T) telephone earpiece that gives high AF level.
Construction can be on PCB or ugly & housed in any plastic or metal box.
Access to the controls is needed when a Tx mic is placed on top of the internal
LS.
/QSL
73 De G8MNY @ GB7CIP
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