|[AMRadio] Ranger and Scope|
John E. Coleman (ARS WA5BXO)
wa5bxo2006 at pctechref.com
Mon Nov 6 17:14:54 EST 2006
Jack, pardon me if I make statements that you are already familiar
with but I am not familiar with your background and also there may be new
people to this AM tech stuff that are wondering what you talking about. So
The purpose of the Trapezoid pattern is to verify the modulation
linearity against the modulation audio. You must take a sample of the audio
just prior to the modulation circuit (output of modulator on the Ranger or
input to the balanced modulator on a rice box) The audio goes into the
horizontal and the final RF ( I use a sense antenna) into the vertical. If
the modulation characteristics are linier then the sides of the trap pattern
will be straight. This is to say that as the audio voltages vary through
their cycles the RF output voltage should follow them in a linier manner.
Some scope circuits use detected audio. That is audio that has been
detected from the RF output. These will always show linier because they are
comparing the audio detected from the RF envelope against the RF voltage.
It makes a pretty pattern but means nothing.
The SB200 has no audio in it to sample but if you were to build a
detector for the input RF then you could compare it to the output RF to
determine the linearity of the SB200. This could be useful in tuning linear
If you sample the audio from the modulation circuit and compare it
to the linier amps output then you have a complete comparison. If it is not
linier then you will need to back up to the output of the driver and sample
RF there to determine if the non-linier operation is do to trouble in the
modulated device or in the linear amplifier.
In the case of the Ranger you would need to bring out a wire where
the HV is modulated. This is generally at the B+ side of the RF choke in
the final or the top of the modulation transformer secondary. Capacitively
couple the terminal (block the HV) to a resistor divider network to bring
the audio voltage down to a level that the scope can handle. If the
resistors of the divider network are to large in resistance then there will
be phase shifts that may cause the scope pattern to look funny. If the
resistance values are too low then it will load the modulation XFMR more
than you want to. Avoid wire wound resistors as the inductance causes phase
shifts at higher frequencies.
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