Gerhard Fiedler connectionbrazil.com> writes: > Peter P. wrote: > > >>>> buy nH inductor kit > >>> > >>> Yeah. Get the kit where you also get a complimentary bridge in the > >>> desert included. > >> > >> I didn't understand this... I guess you mean it's not worth it? > > > > Yup. It would be a box with pieces of wire and instructions to avoid > > bending them or putting them near any metal or dielectric ... > > I thought about SMD inductors, and about repeatability. Keeping a tight PCB > layout with short traces to reduce dependency on the PCB and using > inductors that come with reasonable tolerances would make it more probable > that the next device works like the one before -- I thought. Did I think > wrong? > > > [...] they cannot predict the real situation. As I said, small details > > can pull the frequency off 1-3%. That's 10 MHz at 433 MHz (way too > > much). > > I have to continue discussing this... When I look at a typical keyfob, they > seem to use standard SMD capacitors and inductors -- with tolerances > probably more than 3% (at least 5%). And yet they seem to work fine, > probably without individual adjustment. From this, I gather that the > frequency of the LC resonance being off 3% can't be that crucial... Again, > am I wrong? There is a difference between the radiating resonant circuit and any other. The radiating circuit is relatively heavily loaded and forced-driven with the SAW setting the frequency, so its Q is low and the caps don't matter so much, it goes more for maximum area to achieve field strength at a distance. That's why no SMD inductor is used for the radiating element (however a correctly chosen stripline should work). Still, there is such a thing as a 'keyfob tune-up' which consists in replacing one of the SMDs with a trimmer and tuning the device for maximum output. Daring people (who have access to a specan) can also modify the tap point for the transistor collector. This could increase the range by 30-50%. The manufacturers design with wide tolerances, and the circuit is far from electrically optimal when designed for manufacturability. I think that you need to understand the tradeoffs between Q, distance, fieldstrength and sensitivity to parts and labor before embarking on this project. Peter P. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist