[DSP-10] VHF VCO Locking

[Jimmy Oldaker W7CQ]

Hi all

KO7N and I built our DSP-10 from the first TAPR Kitting and had the same 
problems with getting the VCO to LOCK that people are talking about 
now.  Here is what we found.  This is an few paragraphs taken from our web 
site that might help people in setting the LOCKING RANGE of the DSP-10.
................................

  Set the MODE to CW (press ALT-M on the PC Keyboard) and use the keyboard 
to set the Operating Frequency to 147.000 MHz.  Ensure that a jumper is in 
place between pins 1 and 2 of P106, or that an external reference signal is 
being fed to P106 pins 2 and 3.  Adjust the 126- to 128-MHz VCO coil, L102, 
until the tuning voltage measured at the junction of R104 and R105 is about 
3.5 v.

If you have a counter, probe the right side of C127 (where the silk-screen 
legend reads 126 MHz LO) this will show a frequency of 127.335 MHz when 
operating frequency is set to 147.000 MHz and the PLL is locked.  When the 
PLL is locked the incessant beep / beep from the computer every .6 seconds 
will stop.  We found it necessary with the TAPER kits to lower the value of 
C117 to 6.8pf  (from 10pf) to get L102 into the center range of 126 MHz. As 
supplied L102 resonates LOW in frequency even with the core fully out it 
could barely get high enough in frequency to lock at the low end of the 
band before we changing the value of C117.  Now the core is centered in the 
L102.

If you are having problems obtaining full PLL locking range from just below 
144MHz to just above 148MHz you can increase the value of C141.  Just add 
few pf across C141 will do to increase the width of the locking range.  If 
you lower the value of C141 your locking range of the PLL will 
narrow.  With the supplied value of C141 (10pf) our PLL will hold lock from 
143.200 to 148.500 covering the US 2 meter band very nicely.

..................................


This worked for us a year ago and I thought it might help on the new kitting.

The full setup modifications we worked out are found 
in      DSP10_initial_testing.doc     on our shared web site

http://home.teleport.com/~oldaker/dsp10_repository.htm

I mentioned this yesterday but thought an explanation of the problem might 
help.  The whole DSP10 INITIAL TESTING.DOC is compiled with Bob LArkin 
W7PUA initial setup pages with our comments added in RED explaining what we 
found to help us get our DSP10s running.










73's  -  Jimmy  -  W7CQ -  CN83jx

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