Here in test image is visibble this self generated 250MHz "spurious" signal.
This image show how it is related to real signals in practical signal levels.
Scope Ch1 10mV/div
Signal adjusted so that using 1MHz this level give around full 10 divisions oscilloscope image.
Oscilloscope display infinite persistence.
Acquire mode normal.
FFT set for 1+dB/div and vertically adjusted bottom to bottom. Horizontally zoomed x2 and shifted 0 near left side.
First sweep from 1MHz to 360MHz
Then stopped and set for 107MHz.
(signal generator (HP8644B) level display -16.3dBm and this have measured that flatness is far better than specs limits) Around 34.24mV rms.
Botom part of image exatly same signal dropped 50dB to -66.3dBm (around 108uV)
(not in this image but 10dB less and still clearly detectable (34.24uV))
Impedance mathing is very poor, only 50ohm terminator with T so freq response suffer some amount about this.
250MHz (ADC circuit produced) spur is marked (also 500M 2. harmonic (and ADC base clock) is there but not in this image). Its level vary some amount due to base noise. (In this image it is more low than typical. This peak level also is different in different individual scopes. It may be even up to near this low cursor level. Specially in these cases fresh calibration may help and also power cycling "trick" after scope is warmed some minutes. Also there may be small differencies between RuiFeng or other brand ADC and scope manufacturing versions - these are so small things... )
If reader is not familiar with dB scales, it is good to realize what these images show.
50dB difference is roughly 1:320 voltage and 60dB means 1:1000 voltage ratio.
Oh - but it have 8 bit ADC.