Single-Ended Mixers and Reverse Isolation

Progress on CC-Series development proceeds at a reasonably-good clip right now. One of my last big hardware bugs to stamp out is some nasty microphonics that seem to be generated by the combination product detector/BFO. Today, I believe that I made some significant progress towards solving it and wanted to share what I learned.

IF Amp & Product Detector from CC-20 Beta 1

I've done a lot of reading in Experimental Methods in RF Design (EMRFD) about microphonics in DC receivers (read chapter 8!), and the number one cause of it is poor LO-RF port isolation in the mixer. The CC-Series uses a venerable old circuit which hasn't seen much use in a while. A dual-gate MOSFET is pressed into double-duty as a product detector and BFO (see above). Since the dual-gate MOSFET product detector is in a single-ended configuration, it inherently has bad LO-RF isolation. This allows VFO (or BFO in this case) signal to leak out the product detector input, and have a good portion of that signal reflect back into the product detector. So naturally, the CC-20 could be experiencing the microphonics because of this phenomena. One of the solutions mentioned in EMRFD is to put an amp in front of the mixer which has excellent reverse isolation (signals coming into the amp output don't tend to get out of the input, and therefore can't reflect back in again).

I had the suspicion that the common-source JFET amp in front of the product detector might be the culprit. So what's the best type of amp to place in front of a single-ended mixer? The common-gate JFET amp is a good and popular choice. However, VE7BPO notes on a recently published web page that the best commonly found amp configuration for this particular parameter appears to be the cascode (see the bottom of the page).

In order to test this theory, I went to work on a project that I had set aside earier: a direct conversion receiver based on the CC-Series product detector. When there was no preamp in front of it, the microphonics were unbearable. I figured that a good way to test my theory would be to put a cascode amp in front of this mixer and see how much it helped. I decided to put a dual-gate MOSFET preamp in front of it, as this is essentially a cascode amp and it fits with the dual-gate MOSFET product detector. Once the new preamp was added, the change was dramatic. The microphonics were gone.

Next, I decided to be a bit more rigorous in my study and quantify the exact difference between the common-source JFET amp and the dual-gate MOSFET amp. First I breadboarded the common-source JFET amp and ran it through the test procedure in the page linked above (at 18 MHz). The results were atrocious. Only 30 dB of reverse isolation, which is worse than the worst amp listed there (the feedback amp). Next, I dug out an old dual-gate MOSFET amp I had breadboarded for my 2008 investigations and ran it through the same test. As expected, the results were vastly superior: 68 dB of reverse isolation. This lines up nicely with Todd's measured results of >64 dB for the hybrid cascode (I used a spectrum analyzer while he used an oscilloscope, so I was able to get a pretty good measurement down to low signal levels).

So this appears to be strong evidence that the IF amp is the problem. It seems certain that the next version of the CC-Series is going to scrap those awful common-source amps for a much nicer dual-gate MOSFET amp. The lesson to take away from this is that if you are going to use a single-ended mixer for any but the most simplistic applications, it must be fronted with an amplifier with an excellent reverse isolation. While the typical common-gate JFET amp will work OK, for best results it looks like a cascode or dual-gate MOSFET amp is the way to go.

A New Toy for the Shack

AmScope SM-4T

I'm starting to build up the gear for the kitbiz to get off the ground, and the first large investment just arrived at the shack. Thanks to some blog posts from Eldon WA0UWH, I found, who sells all manner of microscopes for low prices. I ended up finding a scope that was very similar to the one that I worked with at Tek; the SM-4T. It's a 7x-45x trinocular stereo zoom scope with a double-arm boom. I also ordered a 80-LED ring light with a variable brightness control. The beast barely fits on my tiny construction bench, but I'll have to deal with the cramped space for now.

Here's some of my impressions after assembling and using it. First off, the base is massively heavy; which is great for stability but no fun when transporting it. I thought the poor UPS drive was going to get a hernia getting it off of his truck. The fit-and-finish is pretty good. Not quite the same as the high-quality brand name microscope at Tek, but I felt like it was good for the price I paid. The manual is terrible. I'm still not sure that they sent me the right manual, since the diagrams in the once I received show a completely different model. But the back of the manual does show the parts for my microscope, so who knows, maybe you just get some generic manual for all of the different models. The Engrish factor is pretty high, as you might expect. Fortunately, I was still able to easily assemble the microscope by referencing the photos on the AmScope website.

The image quality is quite good, although the field of view is a bit smaller than my Tek microscope. The interpupillary distance and diopter setting is fully adjustable. Once I got them set correctly, the microscope was a pleasure to use. The 80-LED light did a good job of providing bright white illumination for the circuit boards that I examined. My first impression is that I've received a good value for my money. AmScope claims that their production line is the same one that manufactures microscope for Zeiss, Leica, Nikon, and Olympus. Based on the quality, that certainly seems like a credible claim. It's a bit spendy for the average homebrewer to purchase, but if you are serious about doing a lot of SMT work, I think that these microscopes would be an excellent choice for the shack.

W0IYH RMS-to-DC Converter

W0IYH RMS-to-DC Converter - CloseupToday I finished construction on the circuitry for the RMS-to-DC converter described on the October 1992 QST article by W0IYH on measuring receiver performance. This is an article that I found on the EMRFD CD when I was searching for information on measuring the noise figure of amplifiers and receivers. I didn't find the article directly, but was directed to it because it was referenced in another paper (on the EMRFD CD) by W0IYH on homebrew noise sources. Thankfully, the authors were kind enough to include both papers on the CD, since the October '92 article gave a lot of the background needed to fully grasp the noise source article. The detector is a fairly simple circuit, since all of the hard work is being done by the Analog Devices AD636JH RMS-to-DC converter. This device measures the true RMS value of DC and AC signals (up to 200 mVrms full scale) by converting the input signal RMS value to an equivalent DC voltage. The rest of the circuit basically consists of op-amps providing plenty of buffering and one stage of gain. The gain stage is present so that you can use a 10:1 scope probe on the input.

W0IYH RMS-to-DC ConverterAs you can see in the photo to the right, there are some external controls for the circuit. At the far left is a pot which controls the signal level from the first buffer into the second buffer (which preceeds the gain stage). This allows you to accurately calibrate the detector to an on-board 200 mV reference. The rotary switch to the upper-right of the pot is the input coupling selector. You can choose from DC, AC, or GND, just like an oscilloscope. In the middle of the photo, you can see the 10:1 Tektronix scope probe that I'm using with this detector. Finally, in the upper-right corner is a rotary switch that allows you to switch in three different levels of signal attenuation: 0, 3, and 10 dB. The 3 dB and 10 dB levels are set with the little blue trim pots to the right of the probe tip. By having this attenuation built-in to the detector, the process of making NF and S/N measurements is a lot simpler.

Unseen in these photos is the simple dual polarity power supply that I built for this detector. Wall current is fed into a 120 V:12 V transformer, which then is recitified by a 1N4007 and regulated by a 10 V zener for both the positive and negative rails. I've still got to get the thing in it's enclosure, but of course I had to smoke test it first before I went any further. After trimming the offsets on the gain amp and the RMS-to-DC converter, the circuit performed pretty much exactly as I expected. No great surprises, fortunately. I've still got a bit of copper clad to trim off of the top edge of the board, then I should be able to get the whole thing mounted in the enclosure and ready to go by this weekend.