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Friday, 28 February 2014

KISS Mixer Musings


Part of the Minima transceiver I made back here:

was the so called KISS mixer:

Above I have stolen the mixer part from the Minima schematic and labelled the ports. In my test setup I have the LO connected to a test crystal oscillator I had lying about, it's running at 20MHz (ish), the RF port is connected to my bench signal generator and the IF port is connected to the Spectrum Analyser.

Here's what the test setup looks like:

Now, the author of the Minima Farhan gave us some information on the KISS mixer here:

and also a reference here:

Now, here is what I see when I have a play. I have 20MHz LO, 15MHz set into the RF port from my bench sig gen and the spectrum analyser connected to the IF port.

Here's the output spanning 0 through 100MHz with just the LO (20MHz) signal:

This is just my oscillator and it's harmonics - so far so good. Now lets add in the 15MHz signal to the RF port and keep the 100MHz span:

So now we see the 20MHz signal plus harmonics, the 15MHz signal plus harmonics plus all the differences and sums which land at every 5MHz interval. So far so good.

Now, lets zoom in on one of the mixed signals at 35MHz:

Looks good to me.

Now, if I connect my x10 'scope probes to the gates of the FETs, here is what we see:

The 'scope struggles to trigger on anything here, because there is all sorts of frequencies in the signal, however, I think we can safely say that the two signals are the same but out of phase with each other as we would expect.

Now, if I look at the drains (my sources are connected together and the bias supply), we see this:

and if I add the LPF in the scope to bandwidth restrict the inputs:

If we replace the SA on the IF port with a 50R load, then look at the signals on the drains with the SA, here's drain one:

and here is drain two:

Now, in all of the tests above I have tried altering the bias pot and I can see no difference at all under any circumstances. If I look at the voltage at the wiper of the pot I can see 0-5V as expected. However the voltage at the sources read from -0.12 to -0.15V. That isn't what I expected either! If I remove the +5V supply the output doesn't change nor does it if I connect the sources to ground.

The FETs in the KISS mixer effectively replace the diodes in a more traditional diode ring mixer; this chops the RF signal thus providing the IF signal at the centre tap of the RF transformer.

So I replaced the single bias pot with one on each FET, under these circumstances I can alter the amplitude of the unwanted signals in the output but not make any difference to the wanted signals.

In my case the wanted signal is the 20 + 15MHz, here is the mixer output with the two bias pots adjusted to minimise the unwanted signals:

However, if I just remove the 5V line, the bias pots and just ground the two sources on the FETs, here's the output:

And these are the signals on the drains with no bias (the sources grounded):

Any volunteers to explain?


  1. Hello Mark, below the first Rigol screen you write:
    This is just my oscillator and it's harmonics - so far so good.
    This is not good since the 20 MHz signal is only -10 dB down.
    This can be caused by difference in Q1 and Q2.
    The two signals should both be suppressed, since the circuit is balanced.
    I suggest to select two FET's with the same Idss)
    The KISS mixer is a very special mixer, which does'nt have to be terminated with a diplexer and mixer losses are quite low.
    That makes it worthwhile to experiment. hi. 73, Bert

  2. Hello Mark,
    I understand from Minima site, that the fets should work at on/off state, and this require a Vgs from 0 to 5V or close. This almost square ware signal should be seen at fet gates. Maybe you could look if you have enough Vpp on your generator or if the output is a square wave?