OpenBCM V1.07b12 (Linux)

Packet Radio Mailbox



 Login: GUEST

G8MNY  > TECH     12.05.22 09:17z 170 Lines 7417 Bytes #27 (0) @ WW
BID : 21456_GB7CIP
Subj: AF 2 Tone Test Osc Design
Sent: 220512/0915Z @:GB7CIP.#32.GBR.EURO #:21456 [Caterham Surrey GBR]

TWO TONE AUDIO OSCILLATOR                (Updated Jan 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

A two tone non harmonically related sine wave test oscillator is used for
setting up & testing basic linearity of linear amplifiers & RF Tx stages,
using a scope to see the RF waveform directly or AF/DC from a diode detector.

A simple solution may seem to use a DTMF encoder as it is a 2 tone generator.
But for SSB testing the 2 tone levels need to be independently adjustable, to
allow for the variation in mic response, mic amp response, & SSB filter shape
etc. to ensure the exact balance through the RF stages so the desired RF test
signal is obtained. However you may be lucky with the odd pair of DTMF tones
being just right for your rig, so give it a try & see the IN USE bit.


    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 input hookup.

    BALANCE :- A control for setting the relative tone levels
               (as needed in Tx mixer for balance)

    VOLUME  :- A control for setting the AF output level for no Tx clipping.

 RF IMMUNITY:- only 2 power transistor with base stoppers, so very immune.

 쿛ower Sine Osc 1쳐 旼컴컴컴 旼컴커   _/
 읕컴컴컴컴컴컴컴켸 읕  Gain 쳐퀶ain쳐쨈_ 쿗.S.
 旼컴컴컴컴컴컴컴커 旼퀯alance 읕컴켸   \
 쿛ower Sine Osc 2쳐 읕컴컴컴        읕컴컴> Output 

This uses 2 ferrite pot cores with 3 windings on each, 2 make a centre tapped
winding of about 32 mH, & the other winding about 1/4 of the turns giving a low
Z output. This forms the heart of the two power sine wave 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 e.g. harmonics etc. Mine turned out to
be 991 & 1352Hz. Each oscillator level is set by the 47k bias resistor located
at the transistor base, which also makes the transistor stage immune to RF!

                旼컴컴컴컴컴컴컴컴컫컴컴컴컴컴 EARPIECE
               VOL    旼컴컴컴컴컴)컴컴컴컫컴 40R LS
              200                        읕컴 LS
               \/                 쳐컴컴컴컴 JACK
            POT 100     쿗ow Z   
     ((() 991Hz           ((() 1352Hz
 = = = = = = POT        = = = = = = POT
 (((((((((() CORE       (((((((((() CORE
     읕컴)컴컴컴컴컴컴)컴컴좔컴)컴컴컴쩡훟/ o컴쩡330鴛캑<쳐컴o +12V
 쳐캑쳐컴컴                u47                            Charge
  u22                 쳐컴캑쳐컴     +        +           & or run
 쳐컴컴늘컴                          ===      __          External
     u47                             100u   __             DC
        /                    /              __ 9V
 읕47k컴  BFY50       읕47k컴 BFY50            PP3
         \e                    \e              
           읕컴컴컴컴컴컴컴컴컴컴컨컴컴컴좔컴컴컴컨컴컴컴컴컴컴컴o 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 100 does not affect each oscillator frequency & the
outputs are quite good sine waves.

The 200 volume pot set the overall level in use. The internal LS was a old 40
(4T) telephone earpiece that gives high AF level, ideal for putting next to the
Tx's mic.

Construction can be on PCB or ugly, & housed in any plastic or metal box. But
access to the controls is needed when a Tx mic is placed on top of the internal

Using into SSB Tx, the resultant output once balanced & not overdriven it looks
like this on a scope as RF Envelope & as a loaded DC Detector display.

PEAK     .컴.       .컴. Round peak
RF      /::::\     /::::\
       ::::::   :::::: Sharp Null                DETECTOR
      :::::::: ::::::::쿣            PEAK     .컴.       .컴.
     큍::::::::::X::::::::::X                     /    \     /    \
      :::::::: ::::::::                                   
       ::::::   ::::::                                      
        \::::/     \::::/                     읒컴컴컴컴컴좔컴컴컴컴컨
         `컴'       `컴'

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 suitably loaded
diode detector into the scope probe.

Over drive ... shows up a flat 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.

   CLIPPING Over Drive or Under Loaded PA
PEAK     ____       ____ Flat peak
        /::::\     /::::\
       ::::::   :::::: Sharp Null                DETECTOR
      :::::::: ::::::::쿣            PEAK     ____       ____
     큍::::::::::X::::::::::X                     /    \     /    \
      :::::::: ::::::::                                   
       ::::::   ::::::                                      
        \::::/     \::::/                     읒컴컴컴컴컴좔컴컴컴컴컨
         ~~~~       ~~~~

PEAK     .컴.       .컴. Round peak
        /::::\     /::::\
       ::::::   :::::: Wide Null                 DETECTOR
      /::::::::\ /::::::::\V/            PEAK     .컴.        .컴.
     킴::::::::::H::::::::::H                     /    \      /    \
      \::::::::/  \:::::::/ \                                   
       ::::::   ::::::                     /        \  /        \
        \::::/     \::::/                     읒컴컴컴컴컴좋컴컴컴컴컴좔
         `컴'       `컴'

   NO BIAS - Class C PA
PEAK     .컴.       .컴. Round peak
        /::::\     /::::\
       ::::::   :::::: Total Null                DETECTOR
      :::::::: ::::::::쿣             PEAK     .컴.       .컴.
     늙::::::::쳐::::::::쳐                    /    \     /    \
      :::::::: ::::::::                                    
       ::::::   ::::::                                      
        \::::/     \::::/                     읕좔컴컴컴컨컨컴컴컴컴좔
         `컴'       `컴'

   PARASITICS Instability
PEAK     .컴.       .컴. Round peak
       n/::::\    n/::::\n
       ::::::   :::::: Sharp Null                DETECTOR
      :::::::: ::::::::쿣            PEAK     .컴.       .컴.
     큍::::::::::X::::::::::X                    n/    \n   n/    \n
      :::::::: ::::::::                                   
       ::::::   ::::::                                      
       u\::::/u   u\::::/u-Abnormal           읒컴컴컴컴컴좔컴컴컴컴컨
         `컴'       `컴'     Shape

See my Tech Bul "IC 735 No PA Bias Fault", "PA instability in ICOM IC735",
"T500M 12V 500W HF Linear", & "QRO 1kW HF Metered Dummy Load".

Why don't U send an interesting bul?

73 De G8MNY @ GB7CIP

Read previous mail | Read next mail

 16.05.2022 08:55:46zGo back Go up