For a long time, driving the mini-circuit mixer LO port is a pain. Early attempts was using a transistor crystal oscillator to drive it, and later adding a 3:1 step-down transformer to drive it, and later adding series inductor to filter it. All attempts didn't give satisfied result.
The problem is, when you probe the LO itself, the waveform looks clean, it's a sin wave, but once you connect the LO to the mixer, the waveform become bad, for bad, I means not looks like a sin wave anymore, it more looks like having many harmonics and the harmonics is just looks uncomfortably terrible.
But why this issue stay so long? well, because the mixer seems work fine with those harmonics! It can works to produce the intended mixing product. Many version of remote control receiver was done.
I should try to fix this problem once again.
Waveform of mini-circuit ADE-1 mixer LO port, driven by 74LVC1GX04 crystal driver, at 27MHz. Overshoot is too heavy. Something needs to be done.
Spectrum view in IF port.
I also discovered that the LO harmonics leak through the IF port, the leaks like fireworks. All the way to 1.8GHz.
The breakthrough
By using direct experimental method (blind guess and played around with it until it works), adding a big 470pF capacitor across the LO port to ground give a good result. The waveform looks more sinusoidal and less overshoot. Try adding shunt capacity is not the first time, but last time just not big enough! I guess the mixer LO port can be think of a small value inductor so adding a big cap to resonate out the inductance would do the job.
Shunting a 470pF capacitor on LO port smooth out the overshoot.
The LO harmonics leaks to IF port is significantly attenuated.
Last thing is to check the IF level, if adding the capacitor would make the IF level drop. And it doesn't drop. The conversion loss still keeps in -7dbm.
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