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Re: [sarex] What am I doing wrong...?]



I don't recall where I got the following text file (probably from someone's Web page).
It looks similar to an earlier posting on this list.
********************************************************

Building the Deviation Scope

     Since this is a homebrewer's project, I won't provide detailed instructions, but this should
     get you going.

        1.Find the discriminator output in the receiver, and attach one end of a 47kohm to
          100kohm resistor to this point -- you may have to experiment to find a value that
          will provide enough signal without loading down the discriminator. Make sure that
          you tap into the discriminator before any coupling capacitor -- you need to have
          DC coupling to make this work. Attach the other end of the resistor to a piece of
          shielded wire, and route it outside the receiver. Attach the shield to ground in the
          receiver.

        2.Set the scope for DC coupling, and attach the probe to the wire. Turn the receiver
          on, and adjust the scope's vertical sensitivity until the noise just about fills the
          screen.

        3.With the transmitter set to low power and feeding a dummy load, and the receiver
          tuned to the transmitter's frequency and also hooked to a dummy load, key the
          radio with no modulation applied.

        4.The trace should now be a single horizontal line. Adjust the scope's vertical position
          control to center the trace on the screen.

        5.Either adjust the transmitter up 5kHz in frequency, or the receiver down 5kHz (do
          one but not both!) and transmit again. The horizontal trace on the scope should
          now be displaced either up or down from the center point. If you can, adjust the
          vertical sensitivity control so that the trace is exactly 5 divisions from the center line.
          If you can't do that, just note as accurately as you can the number of divisions of
          displacement, and in what direction. From this point on, do not touch the vertical
          sensitivity control.

        6.Shift the frequency of the transmitter or receiver 5kHz to the opposite side of the
          desired frequency, key the transmitter, and once again note the amount and
          direction of displacement. The amount should be very similar to the result of the
          previous step, but in the opposite direction; if it is not, the receiver is probably out
          of alignment and you should take care of that before attempting to measure
          deviation.

        7.Divide the displacement you measured in each of the previous two steps by five. The
          result is the divisions per kiloHertz of your measurement system. For example, if the
          5kHz frequency change caused the trace to move exactly five divisions, each division
          equals 1kHz of deviation.

     You've calibrated your system to measure deviation. You can transmit cal tones from the
     TNC as described in the previous section, and measure the amplitude of the recovered
     audio. Applying the kHz/division value you calculated above, you can easily determine the
     deviation of your transmitter, and adjust the TNC's audio level to get the proper value.

     Since this system is DC coupled, it will also show the frequency offset of the transmitter
     as compared to the receiver. This is another valuable piece of test information, but it can
     make measuring deviation more difficult. If you want to look only at deviation and not
     frequency offset, you can switch the scope input to AC coupling when you're making
     measurements.
 

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