40m Direct Conversion Receiver - Final modifications and comments

I made a few more modifications to my receiver and the end result was outstanding!

Firstly, I found the audio AGC prone to distortion on strong signals. The first syllable of every word was a flat spot on the oscilloscope trace till the AGC reduced the gain. The fix was more than a larger capacitor. I found the attack time was too short for voice signals.

The fix was a comparator which independently of the NE570 charged up the rectifier capacitor thereby reducing the gain of the NE570 much faster. See the modified AGC schematic below:

Modified AGC circuit

The final issue was the weak signals seemed to be down in the noise now. Since I had hacked the initial audio stages to use a differential pre-amp, I reasoned I needed a bit more gain. Since I still had an op-amp on the circuit board I simply configured a x10 stage and put it before the low pass filter. Next time I would eliminate the trim pot shown and simply use a 47k resistor. SMR3 might need to be lowered if you don't get enough gain. Since I find putting another 1206 resistor on top of an existing one straightforward I'd build it with a 10k resistor then adjust if necessary SMR3 by putting say a 6k8 resistor on the top if more audio gain is needed.

Modified low level audio stages

So let's see how well the goals for this project were met:

• lots of volume from a speaker since headphones don't work if you're moving about the workshop while you listen to a net,
• Passed - I can easily listen to a net while moving around workshop.
• no audio instability, 
• Partial pass. Micro-phonics mean the speaker has to be mounted separately to the radio. Otherwise, a pass.
• no hum, 
• Passed.
• good AGC so you are not jockeying the volume control, 
• Passed
• good audio filtering
• Passed
• a squelch. 
• Passed.

While I haven't spoken about the squelch circuit I sue that is deliberate on my part.  It was, I believe, a copy of a circuit used in Codan HF transceivers. It works superbly. On voice signals almost on the noise floor it opens and closes between words. I don't suffer any distortion or ill-effects. I'm happy to share this part of the circuit with you via email. Who knows what litigation might result if I simply posted it here?

My intention was to be able to hear S5 to S9+30 signals. I believe I can hear S3 or lower to S9+40 signals but I haven't done any back to back tests with a commercial HF transceiver. Maybe a stereo recording might be worthwhile?

So apart from the micro-phonics I'm absolutely stoked at how well this receiver works. I might build another for 80m at a later date!

73's
Richard

Differential Pre-amp for DC Receivers

I modified my receiver to use a differential audio pre-amp like the Juma does. My initial thoughts are this represents an improvement over the audio pre-amp stages I was using. I used a NE5532 because the calculations I did suggested it would be 10dB better than a TL072 and 13dB better than a LF353.

If I had a better OpAmp to hand I would have used it though my results suggest this would be unnecessary. My calculations showed the NE5532 was more than adequate and if purchased new represented real value. I was fortunate to have recovered almost 100 NE5532's from a few circuit boards I had lying around. So my cost was ham appropriate.

My subjective testing showed this modified receiver was good enough to reveal previously unknown deficiencies in my HP signal generator. I was getting a high pitched tone from the signal generator though this did not prevent me from copying stations on a DX net with ease. However, when I switched to the DDS board I used in my existing 40m transceiver the high pitched tone disappeared and stations from the other side of the world were armchair copy. And I feel this configuration, despite the lash up approach I used, was even more stable at high volumes.

I rate this as a worthwhile improvement. I'm not sure why it appears better than the previous discrete transistor audio preamp I was using. Part of the explanation I put down to the unknown pedigree of the recovered SMD transistors I was using.

Anyway, here are the modified schematics and I heartily recommend this approach to you.

Regards
Richard VK6TT

Audio pre-amp and Low Pass filter

Band pass filter and Product Detector