W1FB DC Receiver experiments 2
On 5/25/2013 in an effort to improve stability and reduce the tuning range, but at the same time restricted by components on hand, I reworked the oscillator circuit as per the calcs shown below. I was able to reduce the tuning rage down to about 80Khz from the previous 500Khz. This makes it much easier to tweek the trimmer cap with a tuning stick. The inductor is now also much more stable due to the increased inductance to 1.85uH from the previous 1.3uH. I also used thicker #22 wire and pulled it tight on the toroid. These two changes have actually made the receiver practical to use in the field.
My aim next is to acquire the right components and rework the design again to the original spec. It seems difficult to obtain NPO caps here in South Africa. Perhaps I will try some SMD components? The inductor should be at least 2.6uH to achieve the recommended L/C ratios.