Direct Conversion Receivers - Remaining challenges

Well I've just about wrapped up talking about this subject. But before you go away thinking everything is covered there are two challenges that you need to be aware of - Dynamic Range and Microphonics.

Dynamic Range

My agc circuit uses a NE570 and has a dynamic range of 60dB. Which I use to cover signals between approximately S5 and S9+30. Signals stronger than S9+30 will distort without some attenuation. And when there are no signals, or low noise levels, the audio gain is running very high. This  creates the potential for instability and is why I spend so long investigating this area. In the end I tailored SMR9 (the 150k feedback resistor shown below) to suit. I ended up with 100k here and pleasing audio resulted for all stations heard. If I was building this again I would make SMR12 a variable 50k trimpot to ease adjustment to cope with the typical stations being received.

Microphonics

This is a recent problem for me. My previous versions were built in a modular form and I never noticed any microphonics, But the sensitivity of my current version is in part due to a low level of noise in the audio circuits. I also built the entire receiver on one board. Both of these factors have contributed to  microphonics becoming apparent.

After replacing all the surface mount ceramic caps in the low level audio stages with tantalums I still had microphonics. It appears these are originating from a 1206 resistor biasing the base of the transistor in the first audio stage. (refer Figure below) I came to this conclusion after watching the output prior to the agc circuit on the cro. Tapping the board anywhere produced a noticeable voltage spike. But slight pressure on this resistor resulted in a 10 or more fold increase in the spike when the board was tapped. Surrounding parts did not react this way.

Figure 1: Highlighted Microphonic Resistor

I have replaced the resistor but there has been no resolution. Looking at the circuit, above, makes this even more curious. Presumably, the microphonic voltage is being injected into the base and appears at the collector. However, I would expect the 10uF tantalum capacitor to bypass any microphonic noise appearing at the base to ground. My thoughts going forward to fix this are:

1. replace the 10uF tantalum,
2. put another capacitor on the emitter of Q2 in case the noise is being injected into the audio signal path through SMR13,
3. mount the resistor on edge, or
4. use a through hole part (defeat!).

I will let you know what worked.

Regards
Richard VK6TT

Footnote: turns out tantalum was bad. I pulled out my ESR meter and it hardly moved the needle. Replacing it cured the microphonics from this resistor. However, with that sorted I found the sensitivity was dreadful. Which sounds odd. After an hour or so of working back through the audio chain towards the mixer it turned out that the 330nF from the two 100 ohm resistors to ground was noisy. Now this is something I had never come across before but must have been due to leakage current from the 2.5V DC across this capacitor. But from before and after tests this capacitor was defintely the culprit so it was binned and another used. Problem solved.

Replacing all the high capitance surface mount ceramics with tantalums and through hole parts certainly created some headaches but all is good now. In the first 20 minutes of tuning across 40m I logged SSB stations from the east coast of Australia, New Zealand and Indonesia here in Perth on the West coast of Australia.