[NLRS] DEMI transverter LO question..

[W0ZQ at aol.com]

In a message dated 6/20/2004 1:24:10 PM Central  Standard Time, ray at w9ray.org 
writes:
I have a DEMI 902-28 transverter  problem..  I net the LO frequency to 
874.000 but when I shut off the power,  turn it back on immediately, the LO 
Frequency moves...to 874.032 and I have to  retune it every time! it appears to quite 
repeatable...   Any ideas  what the problem is...  



Hmmm, not sure, but the oscillator circuit "likes" 874.032  better.   My 
(minor) experience with DEM LO's is not to tune them to  frequency (in your case 
874.000) put rather to its strongest signal (which is  "close" to its primary 
freq) ... I think on the DEM you tune the  small capacitor for maximum voltage 
on a detector.   By tuning to  the maximum signal I find that the LO is more 
stable, provides a better signal  level to the mixer, and assures startup with 
both low voltage and when cold much  better than when you tune it for an exact 
match (ie; 874.000).   You  can then compensate for frequency with your IF 
rig.   For example, my  2304 DEMI is 2304.100 when my IF rig is 144.123.   
 
If you are to much of a purest to accept this, you can dither with the LO  
tank circuit in and around the xtal.  Try compressing or expanding the  windings 
on the (series ?) coil that is in the tuned circuit with the xtal, or  
perhaps change the fixed capacitor values by a couple of pf.   
 
I just remembered that DEM has a tech note on this subject ..... here is  the 
link
_http://www.downeastmicrowave.com/PDF/Dn016.PDF_ 
(http://www.downeastmicrowave.com/PDF/Dn016.PDF) 
 
73, Jon
W0ZQ