Matching J310’s and Testing a KISS Mixer

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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.

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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.

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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.

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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).

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Above is the output from the original Minima mixer.

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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

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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.

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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.

Power Supply Completed.

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Well this is how the completed power supply turned out. For those wondering at the over-engineering. Yes, this power supply while very simple is completely over the top for a little Minima. The beauty of the Minima is that it serves as a platform for experimentation.

So I’ve build the supply with both + and -12 volt DC rails. Just in case I might want to play with split power supply audio amplifiers or similar. There is a +9v regulator that feeds from the +12 volt supply. Which in turn feeds to the +5 volt regulator on the logic board. So now the +5 regulator does not get so hot.

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The supply can also provide ~40 volts DC. Which may come in handy for getting higher power from a IRF510 linear. Perhaps…

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To the left is the Arduino Logic PCB mounted above the completed power supply. With the master power LED on the bleeder resistor winking out at us. “Yes I’m working and don’t poke your fingers in here!”.

To the right is the top view of the logic PCB mounted atop the power supply and the entire module bolted back onto the radio chassis.

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Finally another shot of the radio powered up. This time the photo is not as contrived as previous. She is running on her own internal power supply source.

It is interesting to note that this photo was taken with the “Raduino 0.4” sign-on message displayed on the LCD screen permanently. No trick photography nor hung I2C data bus. It just happens from time to time when everything is very stable and the shack has warmed up to a nice cosy constant temperature. As soon as you move the tuning knob it starts displaying the frequency. A quick glance at the source code and I’m convinced that this is normal behaviour. The current Raduino sketch does not display the frequency until there has been some sort of update.

Mostly though when people see the “Raduino 0.4” message forever it means that the I2C data bus is in a hardware fault condition which seems to hang the Arduino Wires Library.