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Monday, 4 August 2014

Even More KISS Mixer Musings


The last time I fiddled with the KISS mixer from the Minima was back here:

Now, lots of QRPers have fiddled with this since and the musings of all of us have gone into the brain of Ashhar Farhan, VU2ESE and this has popped out as an improved design:

So, here's a first bash at giving it a try.

To test this properly I need to tap into the Si570 LO inside my minima so that the LO levels are as they will be, but for now I am trying something basic using the bench signal generators I have here.

So, for this test my LO will be at 27Mhz and generated by my EMRFD Sig Gen. Here's the signal into a 50R terminated 'scope:

For the RF signal I will use my DDS Sig Gen at 7MHz. Here's that signal into a 50R terminated 'scope:

So the LO signal is connected to the LO port in the schematic above (that's the port on the far left), the RF signal is connected to the RF port (that's the one at the bottom right) and the Spectrum Analyser is connected to the IF port (that's the only one we haven't used yet). Here's a picture of the mixer:

So, this is what we see on the Spectrum Analyser sweeping 0 to 100MHz:

The 10K pot is sat in a central position and I haven't made any attempt to fiddle with it. If I do have a fiddle then I can reduce the signals at marker 1 which is the RF port input at 7Mhz quite significantly (I can also make it much bigger too):

So, in the above image we have the following:

I now need to take my minima to bits and use the Si570 as the LO signal..... i also need to go back and review the results from the last tests on the previous incarnation of this mixer....

A quick check with the IF and RF port connections swapped round suggests a good isolation between the LO and RF ports of something like 45dBm, however the harmonic at 54 MHz is stronger and needs investigation:

If I now increase the LO drive level, thus, by adding a general RF amp to the output of the sig gen:

We now see the following with the initial configuration (Spectrum Analyser connected to IF port), just to keep you on your toes the SA is now sweeping from 0 to 80MHz.:

And with the SA connected to the RF port:

In both of the last two tests the pot in the circuit has allowed me to reduce the 7MHz signal drastically.

Finally, if we swap things about a bit and put the LO at 27MHz amplitude 14dBm and a 20MHz signal amplitude -5dBm into IF Port and SA on RF port we can go from this (just to keep you on your toes the SA is now sweeping from 0 to 80MHz):

to this:

by adjusting the pot in the schematic. It looks like a multi turn pot would be a most excellent idea as the adjustment is very critical but the 27MHz LO signal can be almost nulled out completely as we could with the 7 MHz LO signal in the earlier example.

We need to test this at the LO frequency range of the Minima and with the harmonic and other content of the Si570, but it does rather look like this mixer with a single lpf at 30MHz and just one trap at 20MHz will do just nicely.

Cat's not been helping much:

He did raise an eyebrow due to the excitement when the final SA image with the nice null was saved but soon settled again, he does like an RSGB book as a pillow

Fun though, egh?

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