40m DC receiver – “It’s alive!”

With a three day Pentecost weekend and my wife away on a camping trip with my son, I had some time to look into the receiver again. With the previous series of tests, I’ve found that product detector was working fine and featured good conversion gain. The LO oscillator injection was flawed because the 74LS132 was behaving incorrectly. I could use my work-horse the PM5134 20MHz function generator. Because the frequency setting is a bit course, I risked the chance of missing a signal in band and thus still not knowing the outcome. Therefor I borrowed the Siglent SDG1032X DDS function generator from my work.

I decided to focus on the hart of the receiver, the product detector, first. I hooked up my random length wire antenna to the gate of the J-FET, the function generator to the control inputs of the switches and a utility bench amplifier to the OpAmp output. (Max gain of 20dB) . I started at 7 Mhz and slowly increased the frequency up to 7.3Mhz. Although I definitely received some signals, it all sounded heavily distorted, except for CW. Connecting a scope to the output of the OpAmp showed some serious clipping. I decided to lower the OpAmp gain by increasing the resistor back again to 2k2. The gain is now Av = 46x = 33dB. This definitely improved the situation, weaker signals were now understandable but strong signals still featured clipping. Therefor I decided to add a RF gain pot to the input of the JFet. After playing around a bit, I’ve found that setting this 1/4 is sufficient in most cases.

With these changes I was able to receive signals all over the 40m band. Within 25 minutes I picked up signals from hams from the UK, Italy, Netherlands and Germany and possible a couple from the USA. By this time it was already 2345h, time to shuffle off.

Next, I’ve added the audio low pass filter to the output of the OpAmp. This definitely improved the audio quality! Much of the annoying high frequency noise was gone. CW signals are now much better to copy for only a smaller part of the band is received. There is still some 50Hz/100Hz hum picked up. Which is almost unavoidable with a receiver lying open on the bench. I realized that there are only low pass filter functions in the gain chain. The corner frequency for the OpAmp is set by Rx and Cx and is set at 0.7Hz… I’ve decided to decrease Cx to 0.22uF, the -3dB point is now set at 330Hz, much better!

Recalculating all RC filter points again, I realized that the input filter to the difference amplifier, also needed to be changed. The current filter point is set by R6+R7 and (C4 + C5//C6) and is only 2.6kHz. Which is fine for CW listening but cuts of a tad too much for voice signals. Therefor C4 was lowered to 4n7, giving a -3dB frequency of 5.1 kHz.

40m DC receiver Rev-C

Next step is to build a local oscillator: one VFO to rule them all!

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