Next I tried taking out all the silver mica and disc ceramic caps in the LC circuit of the oscillator and replacing them with NP0 ceramic caps. The feedback caps are in the box below the tuning cap, but you can see some of the little NP0s on the outside of the box, connected to a rotary switch. This serves as the equivalent of variable "Bandset" variable cap, with the tuning cap serving as the "Bandspread." I have seven switch positions, each covering about 40 KHz (with some overlap). This gives me all of the phone band and the bottom 30 kHz of the CW band.
Switching to NP0 caps really did the trick. The receiver is now very stable. When I told Farhan about my VFO woes, he mentioned that he'd had very good stability results with surface mount caps. I wonder if this success has more to do with those caps being NP0 than with their surface mount configuration.
Here is a good description of NP0:
NP0 Ceramic Capacitors are single-layer ceramic capacitors made from a mixture of titanates.
A NP0 ceramic capcitor is an ultrastable or temperature compensating capacitor. It is one of the most highly stable capacitors. It has very predictable temperature coefficients (TCs) and, in general, does not age with time.
NP0 stands for negative-positive 0 ppm/°C, meaning that for negative or positive shifts in temperature, the capacitance changes 0 part per million, meaning that it has a flat response across a wide range of temperatures; the capacitance of the NP0 capacitor stays constant (at the same value) despite variations in temperature.
From: http://www.learningaboutelectronics.com/Articles/What-is-a-NPO-ceramic-capacitor
But I think it is a stretch to claim that these marvelous caps do not "age with time!" That would be a really astounding property of the titanium dielectric. That would be a Negative-Positive Zero FLUX capacitor, right?
NP0 stands for negative-positive 0 ppm/°C, meaning that for negative or positive shifts in temperature, the capacitance changes 0 part per million, meaning that it has a flat response across a wide range of temperatures; the capacitance of the NP0 capacitor stays constant (at the same value) despite variations in temperature.
From: http://www.learningaboutelectronics.com/Articles/What-is-a-NPO-ceramic-capacitor
But I think it is a stretch to claim that these marvelous caps do not "age with time!" That would be a really astounding property of the titanium dielectric. That would be a Negative-Positive Zero FLUX capacitor, right?