Hi Jim, On 09/08/2018 10:02 PM, Jim Harman wrote: > Magnus said, > > It would be interesting to test the linearity of the TIC separately for > > instance. > > I have done some testing of the TIC. It works quite well for the Arduino > Uno with its 1 V full scale ADC setting, but the exponential shape of the > RC charging from the 5 V is quite evident if you use a processor like the > Micro's 32u4 with a 2.56 V sensitivity. This circuit is also sensitive to > any noise on the 5 V supply. The linearity is not too important for this > application as long as it does not affect the loop gain so much that the > loop becomes unstable.
Agreed. It is expected, but it would be fun to see how well it would do. > However I have modified the circuit to use a simple 2 ma current source as > shown in the attached schematic, with significantly improved linearity. H/T > to Horowitz and Hill's The Art of Electronics for the idea of using an LED > as the voltage reference for the current source. The diode-connected 2N3904 > has less leakage at a small forward voltage than any small-signal diodes I > tested, so the output voltage with a very narrow input pulse is close to > zero. A more serious current source helps. It comes as no big surprise that this has been a topic of interest to several designers, hence there exists a number of patents on it. Different approaches have been used. The HP5335A for instance uses a three transistor setup, where the current source, a resistor, is buffered behind the transistor pair that also switches it in and out. The HP5335A then acts as pulse extender with x200 and a TTL counter to complete the ADC process. Modern designs use a proper ADC instead but the basic problem remains the same. > I could run some tests on my version by setting the oscillator slightly > off-frequency and capturing the resulting sloped TIC output, and would be > happy to share the results if you are interested. Please do. This is one of the methods and probably the easiest to setup for most. Another approach is to use a programmable delay generator with enough resolution, but having one of those around isn't as common as a RF generator which can be detuned with sufficient resolution. As I spend the evening refactoring the lab-bench, the RF generators gets a more prominent placement as well as one of the delay generators. Hope to get the TICs stacked up nicely for ones. Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
