Learning from the N6QW Direct Conversion Receiver Build

 
Background

I started this project in August 2021 as a "next step" following the 40M Pebble Crusher and 10 Minute Transmitter. I needed to bridge the gap between building a transmitter and receiver with an ultimate goal of combining the two. As one would expect, this project, the N6QW DCR, helped me achieve "receiver" status; but more importantly, it provided me further experience in building. Such an experience consisted of building techniques, testing, and troubleshooting. Yes, I changed horses midstride several times regarding technique. This lengthened the project. But each evolution was well worth it in the end. 

Lastly, couldn't have done this without the help of several experienced builders. Credits at the end of this post.

My build of the N6 QW Direct Conversion Receiver:

Audio Amplifier

The AF amp was my first focus and first construction stage. Why? Well, Nick Tile (G8INE) and Tony Fishpool (G4WIF) recommended such in their notes. I think Pete Juliano (N6QW) did the same.

I built the AF amp first using perf board. I was very excited by the small footprint and such. But in the end, I could not get the stage to function. Most importantly, I found it very difficult to troubleshoot with the maze of connectors under the board. After seeing Paul Taylor (VK3HN) creating etched PCB boards by hand, I felt that I could definitely do the same. He takes a very simple approach that is easily duplicated. 

I found Tony Fishpool and Nick Tile's documentation of Pete Juliano's receiver around the same time that I started etching. I essentially used Tony and Nick's Sprint files for my layout. No need to recreate the wheel. I made some modifications here and there (like adding a voltage regulator to the AF amp). Ultimately, they're designs were very easy to follow.


 

AF Amplifier (Center)

Mixer

The mixer stage was next. Wiring-wise, this was the mist complicated. I'm still a little surprised that I pulled this one off as I used a SMD version of the 1496 Double Balanced Mixer chip. Tiny is an understatement. Like Paul Taylor suggested, I placed the chip on the PCB and made a little tiny dot at each leg of the IC. I then used a very fine Sharpie to hand trace the circuit: 

Mixer Stage Almost Finished (checking off components using Nick & Tony's schematic)

RF Amplifier

I finished the RF amplifier next. After the first few stages, the build started quickening. But I did run into two problems: how do I manage RF and power between the stages? I moved forward while contemplating this ultimately returning the the mixer and RF amp to add SMA connectors to the board. This required a little "manual" etching of the board, but it made for a clean setup.

Regarding power, I used header pins for the +12v power; ground is provided by a really big ground plan on each PCB board and two large boards connected by solder wick. And each stage is attached to the larger PCBs using solder wick. This idea is from Charlie Morris' (ZL2CTM) presentation at the 2021 GQRP convention.

RF Amp Etching

RF Amp Finished

  

VFO

Not without some frustrations, the VFO was a satisfying build: there's just something about lights and digits showing. I attempted to press an old Arduino Uno and I2C backpack into use. I never learned why and what didn't work with that combo. Instead I bought a few Arduino Nanos and several 16x2 LCDs. Based on the pins on the Ardunio and Si5351, I installed several headers on some perf board. It was just easier for working with the pins. I also had some strange stuff going on with the encoder that I solved by both 1) mounting on a board and 2) properly soldering the connections.

VFO Construction

Band Pass Filter

Probably the most unsexy portion of the build, but arguably pretty important. (Spoiler alert: I got the entire rig going, but had this really loud squeal with limited receive. At Pete's suggestion, I found this was a misaligned BPF blocking the receive and cranking up the AF gain.)

I built the BPF using the number of windings on for the appropriate toroid. I also ganged the caps to get me as close as possible to the value Pete gives in his design. I assumed that it worked having no decent way to text it. To be sure, I have a NanoNVA, but adjusting the screen was so darn hard to use. At the suggestion of Tony Fishpool or Nick Tile (I can't remember who), I used NanoVNA Saver vs. fooling with the NanoVNA proper. Found this to be an incredible advancement. 

Here's what the old BPF looked like:

Old BPF peaking at about 6.2 MHz

I confirmed this by bypassing the BPF and finding very little performance increase on receive. 

So I rebuilt the filter. I started with an extra winding on L1/L2 (24, I think, which should give me about 2.8 uH).  I also removed a few caps and added on some variable caps of unknown range. Truth be told, I just don't trust my meter. Now, Dean Souleles (KK4DAS) suggested that I take a reading on the probes of the inductors and, since I cant tare that reading, account for it in my final read. This seemed to get a lot closer. And that might all be well and good. But I also did this which was both very satisfying and instructive:

• I kept the BPF connected to NanoVNA Saver in a constant sweep from about 6 MHz-7.5 MHz
• I removed windings, one at a time, and resoldered to the filter board
• I monitored the screen and watched the peak move to the right at each change. I then adjusted the var caps to modify the skirts.
• I end up removing about three windings (20 total which ought to give me 2.0 uH according to the chart). 

This was my final result--bam!:

Here's a summary of my adjustments:

Start	Redesign
·        Used T-50-2 (20 turns) vs. T-68-2 (20 turns, 2.28 uH) ·        Fixed caps (10pF for the 7.5pF; 22pF + 47pF + 100pF ganged in the tank circuit)	Changed L1 and L2 to 22 turns then test/cut until peak was in center of 40m band Replaced 10pf cap with 5pf + 0-20 var cap. Replaced 22 pF cap inn tank circuit with 0-20 var cap Replaced the BNC connector with SMA connector

Final Version

Troubleshooting/Evolutions/Lessons

The astute reader will see that this blog's title is "Learning from the N6QW DCR." This was both my purpose of this build and my experience.

After completing the build, these were my results which were "close, but no cigar" on SSB and CW:

The #1 key to fixing this: building by stages and via an al fresco/open design. Again, I used Nick and Tony's tips on troubleshooting (see document 2, Appendix, pg 12). 

Given this noise (above), Pete Juliano suggested:

1. Tune at 100 Hz. I spotted you tuning at 1 kHz and likely your Si5351 is not calibrated and so while you read 1 kHz – that is the command to the Si5351 – the response is OFF because of the calibration. You can tune in the stations better at 100Hz.
2. The sound quality may be improved by doing two things. The first is get a 100nF cap and solder it across the two outer terminals of the volume pot. Next get yourself one of those powered computer speaker systems and run the output through that.
3. The signals actually should be pounding in – what are you using for an antenna?
4. Is the BPF actually passing the 40M band. You might have to squeeze or expand the coil turns. See Bill’s Myth Buster Volume 17
5. Is the PNP Amp working?

First, I fully detached the pot from the board. I was getting weird stuff going on as I pressed down on the pot, as if I had a bad physical and electrical connection. Now, watch what happened when I added a cap across the outer pot terminals:

I tested the RF amp and the mixer--both were doing fine. 

Nick Tile suggested more fixes which I have yet to implement:

The audio noise level only changes in the presence of a strong signal and it sounds to me like the processor working so you may have some noise on the power rails from the Arduino. I put noise filters on the supply lines, I think I used a 1mH choke in series with the line with 100nF to ground at each end on the positive rail into the processor board.

Steve brought something up a few days ago, LM380s can oscillate if pin 6 is grounded, if that is happening, you’ll see it on the o/p if you scope it, and it could be a at a surprisingly high frequency, i decoupled the audio stages pretty heavily on mine and haven’t seen it but it might be a batch/manufacturer dependent issue.

To be sure, there's more that I can do and fiddle with to improve this via trial and error and tribal knowledge. Particularly, I'd like to see if there's a way to more tightly filter out a station that seems to be across all the frequencies within the band pass. Sounds like fun to me.

Lastly, I'd like to give credit and special mention to those who contributed to this project, either knowingly or unknowingly:

• Pete Juliano (N6QW)
• Bill Meara (N2CQR)
• Nick Tile (G8INE)
• Tony Fishpool (G4WIF)
• Paul Taylor (VK3HN)
• Dean Souleles (KK4DAS)
• Charlie Morris (ZL2CTM)