The NE602 Direct conversion receiver has been a good project. I’ve learned a lot. This last week of studying the schematic and trying to understand what I can do to peak the performance has been instructional.
Since I was focused on the band spread capacitor I first tried to understand what the spread of the tuning on that capacitor should be versus what it was in my implementation. In the process I manually calculated the max and min value of the schematic. The band spread should have been somewhere between 3.33pF and 8.3pF. A spread of 5pF. As it turns out this is all calculated out in the book. Then I calculated out my values. I have a variable capacitor that ranges from 15-95pF. With a 10pF cap in series that gives me a range of 6pF to 9pF. Only a spread of 3pF. I was right. . . I don’t have enough of a spread to in my band spread capacitor.
What value of capacitor would I need to get my band spread up to the 5pF as suggested in the text. Well I got to bust out my elementary algebra skills and failed dramatically! With the help of my sister-in-law N5LRF (who teaches math) I was able to figure it out.
But what does this mean in reality?
So I decided to calculate out the min/max frequency of the circuit as designed in the schematic and as implemented in my circuit. For this I used an online calculator. But for your reference F=1/(2π√(LC))
As designed the circuit will tune 6.8MHz-7.52MHz. With my implementation it will tune 6.70MHz-7.35MHz.
Hmmm. . . . My LO seems to be right where it should be.
My observation of this circuit in reality is that no matter how I tune the LO, the frequency is unchanged. I clearly have an enormous swing in frequency. I can observe it in the math and I also can observe it on the Oscilloscope. BUT. . . It doesn’t really seem to change the receive station strength. Why is that?
Another set of observations are that when I am measuring with the Oscilloscope I can observe no signal on the LO unless I am transmitting into a dummy load. When I switch the Oscilloscope over to the front end and measure I can observe no signal, whether transmitting into the dummy load or not. Interesting enough when I transmit into the dummy load I also can hear signals on 40M much better. I can still hear the entire band, but I hear them better.
After discussing with my friend Myron (WV0H) I came to understand that when transmitting into the dummy load I am increasing the current flow in my LO. This increased current results in better out of frequency rejection and receiving. The only real solution to this would be to increase the current of LO into the NE602. So the band spread is irrelevant. This receiver is just not very selective. Especially when there are strong adjacent signals. A potential solution would be to have a crystal determine the frequency. I would have a far smaller tuning frequency range, but it would improve the current flow.
I’ve decided that I have learned all that I can possibly learn from this particular endeavor. It’s time to move on to the next project. I’m really thinking about tackling a regenerative receiver and just starting from scratch.
Stay tuned to the site for more updates on building projects and outdoor operating adventures!
I’m a relatively new ham. Got my license, joined a club and I’m having fun! I enjoy CW, pedestrian mobile, backpacking, and I dream of building radios at some point. At present I have a wife and daughters so my building time is limited.