Dual Gate MOSFET Investigations - Intermodulation

A look into the intermod properties of a BF991 amplifier.

You may have seen in my previous post that I have been working on the latest (and hopefully final) major revision of the CC1. Many of the previous decisions on the radio architecture have been thrown out, perhaps most importantly the decision to use a dual-gate MOSFET as the mixer. In the quest for a replacement, I considered using the old standby, a diode ring mixer, but I wanted to be open to other possibilities as well. As shown in that last post, the KISS mixer from Chris Trask seems to have excellent intermod performance with relative simplicity. So the current plan is to try to build an IF chain using the KISS mixer and see if it will work well in the CC1.

Having quantified the performance of the KISS mixer, the current quest is to find an IF amplifier that will provide decent performance at a reasonable current "price". With an IIP3 of approximately +30 dBm (I believe it should be able to get the mixer there with some improvements in components), the limiting factor for IP3 performance in the IF chain will be the IF amplifiers. Consider that my current goals for the CC1 receiver are:

  • Dynamic range of around 100 dB
  • Decent sensitivity (less than -130 dB MDS in 400 Hz bandwidth)
  • Reasonable current consumption for portable use (< 60 mA)

In order to achieve this, I've determined (using the excellent Cascade08 program from W7ZOI's LADPAC software suite) that the IF amp that I choose will need the following characteristics:

  • OIP3 of at least +20 dBm (although higher is better since the amp is the limiting factor)
  • modest gain

The current candidate for the IF topology is similar to the design seen in Figure 6.89 in Experimental Methods in RF Design, with no gain until after the first IF filter. To that end, I've been looking a various amplifier designs to see if I could find something that would fit (or at least come close to) the requirements above. Bipolar amps are nice, but use a lot of current. MMICs were another possibility; the ones I have found do have about +20 dBm OIP3, but with around 20 mA of current draw and approximately 20 dB of gain, which means the IIP3 is not that great. I figured it wouldn't hurt to take a look at the dual-gate MOSFET again, as I know that at least they can use modest current and many have excellent noise figure.

Without getting into the weeds of every detail of the experiment that I tried, I'll just recap the important parts. Initially I used a BF998 with an L-network on gate 1 to transform the 2.2 kΩ input impedance of the amplifier to 50 Ω. A pot was provided to provide variable voltage bias to gate 2. Different permutations of source resistor and gate 2 bias were tried, and the best IIP3 I could get from that amplifier was about -3 dBm (with perhaps 14 dB of gain). OK, but not great. So I decided to give the BF991 a try and see what I could get out of it. Again, I tried many variations of source resistor and gate 2 bias, and was able to find a configuration that is somewhat promising.


You can see in the schematic above that I settled on a source resistor of 100 Ω and "dipped" the gate 2 pot for best IP3, which came out at 5.6 V of bias. I also found in previous trials that leaving the source bypass capacitor out improved the IP3 a few dB and decreased the gain a few dB, which was a worthy improvement. Input and output was matched for 50 Ω. The current consumption was only 4 mA, which is pretty great for an IF amp in a portable radio.


Here is the capture of the OIP3 measurement from my DSA815-TG. Only 10 dB of gain, but that is OK as we wanted modest gain. The IIP3 measured +8 dBm, and when you add in the 10 dB of gain, the OIP3 is +18 dBm, which is pretty close to my original spec, and all for only 4 mA.

This all looks very reasonable. But there's one problem. The good IP3 is highly dependent on VDD and VG2, especially the gate 2 voltage. As this is going to be a production radio, there needs to be a reliable way to set VG2 during calibration, every time. Also it appears that I probably need some way to keep VDD stable over a variety of voltage inputs, such as a LDO voltage regulator (maybe 9 or 10 V would work). But I need as much headway as possible in VDD in order to get the most out of my dual-gate MOSFET amp. In my experience, they don't like being voltage-starved. There also appears to be a bit of dependency on the tuning of the input L-network, although that is not as pronounced as the other effects.

As it stands now, this is a promising candidate for the IF amp, but I'll have to find a way to reduce these dependencies quite a bit in order for it to be viable for a commercial product. That's my next line of inquiry, and I'll be sure to have a follow-up post if I am able to get around the remaining limitations

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.