Accuracy of the measurement

[Jajaho]

The accuracy of a measurement depends on the combined accuracy of the instruments used as well as on the quality of the setup.
I have found that with my instruments, namely a fully upgraded DS1054Z with 100 MHz bandwidth and 24 Mpts (so technically a DS1104Z) as well as a stock DG1022Z I can achieve the following results up to 10 MHz:

Magnitude/Attenuation, maximum error: better than (pm) 0.3 dB
Phase, maximum error: better than (pm) 0.1 rad

Setup:
1 m RG58 coax from output 1 into a t-splitter mounted directly to ch1 of the oscilloscope and whose other end connects to a 30 cm RG58 coax and into ch2. The cables are from good quality manufacturers but I fished them out of my universities dumpster so there is a good chance that something is wrong with them.

As you can see on the plot above, for frequencies above 10 MHz the magnitude rises up to 1 dB for 25 MHz.
I have jet to figure out what exactly causes this behaviour.