KA7EXM DSP-10 Info/Exchange
Udiag3, another PLL Tuning Program
This page describes a program written to sweep the 126 MHz PLL across its entire tuning
range. With this program, you should be able to verify PLL lock across the entire
The program is quite simple actually. With some help from Bob, I computed the PLL
values to set the frequency for 122.0 to 128.4 MHz in 0.2 MHz steps. There is a 5
second delay between each step, and the loop repeats forever. Similar to UDIAG2, the
FL1 LED will turn OFF when lock on both the 19.68 MHz and 126 MHz PLLs have lock.
Note a slight blip (100mS) on the LED will occur to tell you a step has occurred.
- Download the ZIP file above. Extract the .dsp and .exe files. Obviously you
only need one of the two (either load the exe or build the .dsp file). Remember, the
.EXE is not a PC executable.
- Use EZFAST.COM in a similar manner as you are used-to by running UDIAG1 & UDIAG2 .
- Examine the PLL Lock status. Remember, both PLLs locking will be indicated by the
LED going OFF.
- Solder a bare wire 1" long at C127. Clip a frequency counter onto this
location. This is the 126 MHz synthesizer output, past 2 stages of gain (should be
very strong there, like +7 dBm or so). While the program is running, you should see
the frequency counter display 124.0, 124.2, 124.4, 124.6 .....
- If you do not see lock across the entire 4+ MHz range, something is wrong in the VCO.
Go back to the PLL Tuneup procedure. Example,
no lock near the higher end of the band. Solution: Make sure you can tune this
frequency in the VCO with the PLL loop broken. You may have to raise the resonant
frequency (at a given bias point on the varactors). A final fine-tune to L102 was
all it took for my unit to secure lock across the entire band.
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