Direct Conversion Receiver - Closing Comments (almost)

I've used this receiver for a while now and in the most part I'm very happy. There are some oddities that a worth mentioning.

No Local Oscillator connected or insufficient level

Without a local oscillator connected or insufficient drive the hex inverter oscillates at around 40MHz. At the same time the receiver audio is very noisy which I find odd. As soon as the local oscillator drive is connected, or increases, the noise suddenly stops when the self oscillation stops. The cause of this has me stumped but in practice it is of no concern.

Dynamic Range

I previously mentioned this and suggested I would use a trimpot to set the audio levels. I made this modification and found it went a long way to easing the adjustment of the audio gain to avoid clipping on strong signals.

However, this did not prove to be the complete panacea for clipping. The problem with setting the gain of the audio chain so you can cope with really strong local signals is you relegate weak signals into the receiver noise.I have concluded this is in due to the way the NE570 operates. I am going to tweak this and report back soon.

In the meantime I hope you find something to build!

Regards
Richard VK6TT


Update: I think I've solved the dynamic range issue. It did turn out that hte strong signal I was trying to listen to was QRO strong. No wonder the NE570 was clipping. I inserted a 36k resistor, though anything from 33k to 47k would probably work, into the signal path connecting to pin 6 on the NE570. See the image below.

 

This allowed the NE570 to cope with this huge QRO signal yet still have enough gain when needed to amplify weak signals.