QRP Labs QCX-mini CW, Руководство по сборке, Страница: 75 / 121

this photograph, 27 little vertical bars are shown, which means the actual measured
amplitude value is 13,824.
If the displayed bar drops below one third of the width of the LCD, then the division factor is
reduced by one and the bar is re-displayed. On the other hand, if the displayed bar
overflows the right edge of the screen, the division factor is increased by one. This simple
method creates an auto-scaling display of the amplitude.
Therefore, to peak the band pass filter trimmer, first adjust the trimmer capacitor while
looking at the division factor in the top right of the LCD. Then carry out the fine adjustment
using the displayed amplitude bar. The peak is quite sharp.
Be wary because there can be more than one peak (more than one response of this simple
band pass filter). So, tune the trimmer capacitor through its whole range, and determine the
maximum scaling factor that you see. In my case here, it is 09. You may see 07, 08 etc, no
problem. Then make the very fine adjustment necessary to peak the amplitude bar.
When you have peaked the response, check carefully that the C1 trimmer capacitor is not
at either end of its range. If it is, this means the resonant circuit is NOT correctly peaked.
You need to adjust the number of turns on the long secondary winding of the T1
transformer. Visual inspection will show clearly whether or not the trimmer capacitor is at
the end of its range.
Perfect!
OK! The “solder blob” on the top plate is somewhere between the 4
o’clock and 8 o’clock position. The plates of the trimmer capacitor are
somewhere nicely in the their range, not at the minimum or maximum
capacitance. You have found the peak response of the BPF and all is
well.
Plates completely closed
Here the fixed and movable sets of capacitor plates are completely
meshed, resulting in the highest capacitance. The “blob” is to the right.
It means MORE inductance is needed. So carefully unsolder one end
of the secondary 3 winding of T1, join a piece of wire, and wind it 5
more turns through the toroid. Then try again. Don’t worry if it looks a
bit messy.
Plates completely open
Here the fixed and movable sets of capacitor plates are completely
unmeshed, resulting in the lowest capacitance. The “blob” is to the left
at the 9 o’clock position. It means LESS inductance is needed. So
carefully unsolder one end of the secondary 3 winding of T1, remove 5
more turns through the toroid, and resolder. Then try again.
QCX-mini assembly Rev 1.05
75