Mixer Melodies. KISS, KISS V2 and Double-KISS

Vk2sja-Photo35

Well its been a while between posts. I got tied up experimenting with Mixers for longer than expected. Here you can see a collection of the various mixers I’ve built over the last few weeks. A Passive Quad J-FET Mixer, Two versions of the J-FET KISS mixer and a couple HC4066 CMOS switching mixers. Many of these went through two or three re-builds while playing around with various configurations.

So why all the different mixers?

Well the story goes something like this…

KISS_V2

Early in the life of Minima some builders started reporting high levels of local oscillator leakage to the RF port in TX mode. You can see this at the 34Mhz point in the spectrum analyser screen shots above. The solution? Farhan then offered the KISS V2 circuit for testing. This circuit gave extremely good adjustable LO suppression but some experimenters, including myself, started seeing higher levels of insertion loss with this mixer. The original KISS mixer had about 7dB insertion loss while the newer KISS V2 was being reported at 10dB+. The first version of KISS V2 I built was up around 14dB. By the third re-build I did get it down to 10dB but couldn’t manage to get it any lower than this.

To me the large local oscillator (LO) leak seems inherent in the design of KISS V1 and other variant mixers based on this pattern. If you look at the circuit schematic you can see that the LO (if nicely formed) will be of equal but opposite magnitude in each side of the input into the mixing transformer. As such these signals should cancel out very nicely at the mixing transformer centre tap. However not so much attenuation is available to the other winding of this transformer. Both the DSA-815 screen shots above are looking at the output from the port on the second mixing transformer winding, not the centre tap port. Hence the high LO level at 34Mhz.

Vk2sja-KISSv3

In all this experimenting I did a lot of Googling slash reading and came across this paper:-

RF Mixer Design

On page 16 I found “Figure 28: Circuit diagram of a FET based switching mixer”

So on a whim I decided to blend this design with the alternate bias method that Joe outlined. This modified circuit can be seen left. I’m now somewhat presumptuously calling this J-KISS version 3. Because that’s easier than typing “Modified version 1 Minima J-KISS mixer with Joe’s floating bias modification and a couple of extra resistors that I added just to see what would happen…”, all the time.

Vk2sja-Photo36

Well it turned out that it worked rather well. In fact for me it worked better and more consistently than any other version that I tried building. Joe’s floating bias had pretty good conversion loss already but adding the resistors dropped the unwanted local oscillator noise almost into the noise floor.

Vk2sja-SA-Shot3

So here is a couple of screen shots of KISS V3 in action. To the left is the mixer in the transmit direction. While on the right is the same mixer running in the receive direction. Note that this clearly shows the difference in the LO leak level depending in direction of signal flow (which port you’re looking at). Which is high here in the receive direction. This characteristic was true of all the J-KISS mixer variants I built.

Vk2sja-SA-Shot4

A couple of important things we should note about this high LO leak level. First, in the original Minima circuit the mixer was connected the other way around. So this higher LO level would be present during transmit and flow through to the the low pass filters. Simply reversing the direction of signal flow through the mixer as done here with J-KISS V3 (and as Farhan intended for J-KISS V2) will now present this higher level to the crystal filter instead. The idea being the the crystal filter will do a better job of filtering it out than the low pass filter. Will this cause any receiver issues? I don’t know yet but at least the transmitted signal should now hopefully be clean and within legal limits for harmonics.

The second important thing of note is that in those J-KISS mixer variants with some sort of manual bias adjustment. Which include both of Farhan’s J-KISS V1 and V2 designs (although the bias arrangement is very different in each). The adjustment will make a significant change to the LO leak level but only on the port connected to the centre tap of the mixing transformer. On the port connected to the second winding I hardly noticed any change at all. Just a few dB at most.

Now my J-KISS V3 mixer was looking pretty good. And while performance is nothing to get too excited about. Let’s face it, a cheap Mini-circuits SBL-1 double balanced diode mixer module would probably run rings around it. But at least my mixer was behaving like a proper mixer should and I can say I built it myself! Oh, and it’s also operating as a switching mixer or “chopper” which is important in the Minima. This version was in some ways the easiest to build and get going because it has no bias adjustment. But herein lies the catch…

Vk2sja-SA-Shot5

For this mixer to work well the J310 FET’s must be very well matched prior to building the unit. So what happens if you don’t? The spectrum analyser screen shot to the left shows exactly the same mixer used above but with a pair of deliberately mismatched J310’s installed. We can see that all the unwanted signal levels jump upwards. The dreaded LO leak level alarmingly so. Interestingly the conversion loss seemed to remain largely unaffected. Compare this screen shot with the one two above.

So building this mixer means that you must match your FET’s. Personally having now been through the FET matching exercise I don’t find the task too onerous. No more difficult than matching the diodes we typically use in other mixers. A procedure which we Radio Amateurs take for granted. And probably a lot easier than profiling/matching crystals for ladder filters.

It occurs to me that the FET matching process only really requires an accurate digital multimeter. While the adjustment of a mixer balancing pot ideally needs a Spectrum Analyser. Admittedly you could probably make do with a general coverage receiver listening to the LO frequency via a direct cable connect to the mixer with some in-line attenuation. The multimeter seems the somewhat simpler though. Anyway it could be that for the average home builder pre-matching the FET’s used to build a mixer may be in fact be easier than adjusting a balance control setting after the mixer is built. Perhaps, maybe…

Vk2sja-Photo37

So finally I tired of playing with mixers. There is much, much more that I have not tried and should have. Looking back there are experiments which need to be re-done because of flaws in my methodology or understanding at that time. Please understand that what I present here are just my experiences. While I stumble around learning new skills. This is what happened to me. With the mixers that “I” built! I’m no Electrical Engineer or RF Design Engineer. Just a hobbyist Amateur Radio enthusiast. So your mileage, as they say, may vary – significantly! But if this project was ever going to reach an operational state. Then progress had to be made at some point. At a later date since this build is a modular scratch build with no PCB to lock me into a particular design. It will be easy enough to swap out the mixer module with an improved version.

So enough was enough and I installed my modified J-KISS in the radio. As seen here from the underside. No connection to either the IF or RF ports as yet. The coaxial cable from the LO port can just be seen sneaking its way out and immediately up to the Si570/logic PCB topside.

Double_KISS

On a final note this section would not be complete without mention of the “Double KISS”. Farhan released this circuit to the Minima group for experimentation. Called the “Double-KISS”, no doubt because it is doubly balanced instead of singly balanced like the original Minima JFET KISS… I think? This mixer uses a modern high speed CMOS bus switch at its heart. Which brings it much closer to the original KISS design detailed in the “Mixer Musings and the KISS Mixer” paper by Chris Trask/N7ZWY. Which is where all this mixing business started in the first place.

So a lot of experimental energy is now being invested in this direction. This style mixer promises exceptional performance. The only downside being the somewhat exotic part required. They are not particularly expensive, just not available from your local electronics store. So the Minima continues to evolve as time goes by. For myself I have ordered some suitable high-speed CMOS bus switches for experimentation but at this stage plan on completing the radio close to the original design. I can then evaluate its performance before making any changes like a mixer substitution.

Well that’s all for the moment. Next up I’ll back-track a little and talk about JFET matching in more detail. Then we shall tackle the 20Mhz Crystal Ladder Filter.

73, Steve. VK2SJA

Matching J310’s and Testing a KISS Mixer

Vk2sja-Photo32

Eventually a bunch of J310’s that I ordered arrived. So I proceeded to pull out 10 units at random to match.

I built another tiny little test board to measure IDSS. The circuit is so ridiculously simple that it was hardly worth the effort of building a board. It only grounds the gate to the source. You measure the current flowing into the drain. But it was handy because of the test socket. It made testing a whole group reasonably quick.

I found measuring IDSS this way on a J310 to be a moving target. The current would continue to fall off the longer I let it flow. Internal JFET heating I assume. I got sick of waiting for it to stabilise by about the 30 second mark. So in the end I plugged them in, counted to 10 seconds and then recorded the value shown on the DMM at that time. Not an exacting science but I was only interested in the relative IDSS value between FET’s.

I also matched each of the J310’s to the transistor I had designated unit #1. I found that two transistors with a very similar IDSS would also present very good balance in the bridge. Somewhat predictable I know. But always nice to have something you only suspect as being true proved by experimental method.

In the end I selected two FET’s with not quite the highest value of IDSS because they were better matched in the bridge. This matched pair recorded only 0.002v using the bridge as the picture above shows. Sorry about the quality of these images. They are a bit dark. More lighting practice required.

Vk2sja-Photo33

So here is my version of the KISS mixer. Above is the Minima KISS Mixer as per Farhan’s original circuit. I used FT37-50 toroids instead of FT37-43’s because I didn’t have any of the smaller ones on hand. I used thicker wire 0.5mm to match. Apart from that and my unusual choice of RF connector it is built as per the original.

This unit has the adjustable 0-5v DC bias which is why the power leads are connected. On my version of the KISS mixer I found that the bias adjustment seemed to have very little effect on the mixer output levels. It had some, just not a significant amount. LO leakage only varied by about 1dB and the wanted RF product by only 0.5dB over the entire bias range. One of my next experiments is to see if the adjustment range actually increases if your FET’s are poorly matched.

Vk2sja-Photo34

This next picture is a modified version of the KISS mixer. Very early on in the life of the Minima Freelists mailing group Joe W3JDR suggested an alternate bias method. This method does not require the 5 volt bias regulator. It directly grounds both FET sources and connects the centre tap of the LO Gate transformer not directly to ground but instead floats that point by connection to ground via a parallel 200K Ohm resistor and a 0.1uF capacitor. You can read Joe’s original post about it here.

Vk2sja-Circuit3

Vk2sja-Circuit4

So do they work? Yes, I’m very pleased to say. They both do.

So what did each version perform like?

See for yourself.

These Spectrum Analyser screen shots were taken with the LO from the actual Si570 at +14dBm and the 20Mhz IF at -10dBm into the mixer. The Rigol DSA-815 is looking at the RF port (simulated TX mode).

Vk2sja-SA-Shot1

Above is the output from the original Minima mixer.

Vk2sja-SA-Shot2

While this last image is the output from the modified mixer.

Next up. The Minima Low Pass Filters and some more mixer testing…

73, Steve. VK2SJA

Matching FET’s

Vk2sja-Photo30

So the next thing on the building agenda was yet another piece of test gear. I want to start by building the Minima KISS mixer. The focus of so much attention recently as some builders were having trouble with it. And because the LO to RF port isolation was being reported as not being high enough. Having read as much as I can understand about the topic (very little but I’m learning). I’ve become convinced that success with a mixer made from two FET devices probably entails careful “matching” of the FET’s. After all, we match the diodes we use in the simple BFO mixer. So why not the FET’s?

From “Mixer Musings and the KISS Mixer by Chris Trask / N7ZWY:”

“…but that the balance of such a mixer making use of discrete transistors will be poorer than the balance of a diode mixer because of the difficulty of matching the rather complex transistor parameters over the operating range (10).” 10. HF Radio Systems & Circuits, 2nd ed. Noble Publishing Co., Atlanta, Georgie, 1998

Where the transistors mentioned above are VHF FET’s. So the problem would seem well known.

Having decided that matching FET’s is probably important, then how does one go about doing it? A quick scout around the Internet, once again led me back to the “QRP & SWL Homebuilder” website and a section called “FET Matching” (scroll down).

If by some miracle or bizarre twist of fate you are reading this and have have not already discovered “The Popcorn website”, then do yourself a favour and head on over. You’re in for a real treat.

The photo left shows my bridge variant. Setup with shorted links in the sockets where the FET’s are normally placed. This is so you can adjust the bridge balance for exactly zero volts.

Vk2sja-Photo31

So the afternoon finished with using the bridge to match some FET’s.

I was going to match a whole bunch of J310’s. Until I discovered that I didn’t have any J310’s that is! I could have sworn black and blue that I had a whole heap. Apparently not.

So I went and matched a whole bunch of Audio FET’s just for the fun of it and by way of practice. Yep, FET’s even taken from the same cardboard parts strip are all over the place!! Best match from matching 9 samples to one control was 0.010 volts. The photo opposite shows 0.313 volts but most pairings were much higher than this.

Now it should be noted that the general consensus is that for best results in a KISS mixer we should be matching the units for highest IDSS. The bridge I’ve built tells me that both FET’s are the ‘same’ but doesn’t say much about IDSS. So that’s the next experiment. Find one J310 FET with a high IDSS and then find him a partner.

Next up.. More FET matching/selection and building a KISS.