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Monday, 3 March 2014

More Mixer Musings


Following my total confusion over the KISS mixer, I decided to go back to basics and make certain my very little brain:

was up to speed and understanding the diode ring mixer as a starting point.

So, I made one of these:

Please accept my apologies for the simply appalling quality of the image above.

Now, the LO looks like this into 50R:

I make that 1.25V peak to peak which is 5.85dBm.

The RF input looks like this into 50R:

thats a whopping 2.36V peak to peak which is 11.44dBm (that's way too big!).

The output of the mixer looks like this (after a lot of fiddling with the 'scope):

So hopefully it's clear that ever time the LO (we could call it a switching signal) is negative the RF input is inverted on the IF output. If you then apply a filter to the IF output we can find a nice clean signal at the wanted frequency.

Now, if I keep the amplitudes as they are above, this is the output of the diode mixer on the SA:

I've put some markers on some notable signals; the 5.85dBm LO signal is visible at the output port at -32dBm. The RF input signal at 11.44dBm is at the output port at -20dBm and my wanted signals are at 1.7dBm. I also placed a marker on the harmonic at 40MHz - that's at -38.5dBm.

If I now add 30dB of attenuation to the RF signal, the output looks like this:

So now, the 5.85dBm LO signal is visible at the output port at -32dBm (no change), but my RF input signal now at -18.5dBm is visible at the output at -42dBm. My wanted signals are at -23dBm and my random harmonic is at -50dBm.

So, back to the KISS mixer confusion:

Let's look at exactly the same input conditions with the KISS mixer configured as per the schematic:

So, here we have the LO signal at 5.85dBm, the RF at 11.44dBm, with these input signals the bias pot has no effect at all that I can see:

and here, the LO signal remains at 5.85dBm but the RF signal is at -18.5dBm, with these signal levels I can make a slight change to the amplitude of the LO signal in the output with the bias pot - I have set it for minimum LO:

What does all this tell me? Primarily that I need to go out more!

I'm going to let those far cleverer than I draw the conclusions.

Interesting, egh?

1 comment:

  1. When you return from the pub, why don't you try gradually reducing the drive at the LO and see if at some level there is a rapid drop in the LO feedthrough to the IF port. Actually, it might be a good idea to start the other way and slowly raise the LO level from zero and begin with some fresh FETs as I worry that driving the gate into forward conduction may have done something nasty to those devices.