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(This experiment was carried out in January 2020)

Probably the simplest UP converter.

Demonstrated here using FT8. I also received SSB and CW stations, but they are apparently threatened with extinction 😁️
All about this below 🎥️.
Here is the circuit:
It could hardly be easier.

A TTL oscillator from the 90s "scrap box", 2 C trimmers, an R and a C in the power supply of the TTL oscillator, and an magnetic loop.

These TTL oscillators are intended for digital applications and therefore generate a nice square-wave signal with all harmonics and side effects, so please don't just build them, but first of all just be amazed.
At least, if you don't use a magnetic loop as an antenna, you should build a low pass between the antenna and this up-converter.

For this "experimental setup"I used a magnetic loop, an RTL-SDR USB stick and the good old Raspberry Pi 3 on which this OpenWebRX version was running.
With the new version of DD5JFK it would of course have looked better today, but that's why I'm not repeating the whole thing again.

First adjust the magnetic loop to the receiving frequency:
The further adjustment of the up-converter was easy, set the trimmers to the lowest C, then carefully stalk ...
The lowest adjustable capacitance should not be above 2.5pF, preferably less, because less is more. The resistor has a value of 100 ohms in this configuration. Of course, this depends on your own experimental setup and the parts used.

In the config_webrx.py only the frequency of the up-converter has to be added to the desired reception QRG.
I caught an oscillator at 33MHz when I reached into the box ...
samp_rate = 1000000 # 1Mhz Bandbreite
center_freq = 40100000 # Oszillator Frequenz 33MHz + 7,1Mhz RX F = 40100000

This setting is also important 😉️     Under no circumstances enter "0" for "auto gain"
rf_gain =44 #manuel 40 #in dB. For an RTL-SDR, rf_gain=0 will set the tuner to auto gain mode, else it will be in manual gain mode.
ppm = 0
further down then:
shown_center_freq = 7100000 #center_freq # you can change this if you use an upconverter

Specify start frequency and modulation as usual ...

The complete structure looked like this:

Here also as audio:

FT8 really always works 😎️

assuming that not all OM's are working at the same time and with QRO, I don't think FT8 is intended for that either.