INTERMEDIATE LESSON 31logomidi2.gif

                          

LEARNING  OBJECTIVES
Simple frequency calibration.
 10f.1 *Calibrate a variable frequency oscillator (VFO) employing an adjustable tuned circuit. Calibration to show the relevant amateur band edges.
The VFO may form part of the project to satisfy 10e.1, or be part of a previously constructed project or provided by the assessor.

At the test centre you will be provided with a VFO or you will have made one as part of the construction project.
There are a number of ways of setting this up and your instructor will show you how to do this.
Let us assume that you have a VFO that needs to be set up to cover the frequency range 5.0 to 5.3 MHz. The VFO has an adjustable capacitor in parallel with the main tuning capacitor to adjust the frequency of the VFO and another variable capacitor in series with the main capacitor to adjust the frequency range. You have a frequency meter or a receiver which has been calibrated recently.
  1. Set the variable capacitor so that the vanes are fully meshed. This is maximum capacitance.
  2. Adjust the trimming capacitor in parallel with the main tuning capacitor until the frequency meter or receiver reads 5.0 MHz.
  3. Set the variable capacitor to minimum capacitance.
  4. Adjust the capacitor in series with the main tuning capacitor until it reads 5.3 MHz
  5. Repeat steps 1 to 4 until the VFO covers the range 5.0 to 5.3 MHz.
  6. Mark these two points on your VFO tuning scale.
NB
Some VFO circuits can be adjusted using a variable inductor

* Items shown with an asterisk above are assessed as a practical demonstration by the candidate and ‘signed off’ by the assessor on the Record of Achievement.
Note: If the candidate is disabled in any way that reasonably prevents the carrying out of any practical procedure, he or she may talk another person through the task or describe it to the assessor.