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, I used 2 different oscilloscopes to try and set the frequency to 100Mhz (sig gen reads 80Mhz) and at that frequency it is giving out 140mv measured by connecting directly to the generators high output sockets, at 1Mhz it gives 3v.
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I don't think you are meant to connect the 500R output of this sig gen to a 1M scope via maybe 50cm of 50R coax. If you do it like this the 50R coax is unterminated at the scope end and the system will suffer a lot of frequency droop up towards 100MHz and this is what you are seeing.
If the source impedance of the sig gen is somewhere approaching 500R and the scope input is 1Meg ohm in parallel with 20pF and you use 50cm of (unterminated) coax then theory suggests that the amplitude will drop by about 18dB at 100MHz even if the sig gen emf is consistent in output level. If you then add another 6dB of droop to allow for rolloff in the 66MHz BW scope when fed with a 100MHz signal then you get 24dB droop or a factor of 16 in droop overall at 100MHz.
So a 3V signal appearing on the scope at 1MHz could be expected to fall to 3/16 = 187mV on the scope display when the sig gen is changed to 100MHz if you use the above coax and scope method to measure the sig gen performance. Hope I've got that right, it's late and I'm a bit tired
Try repeating the test with a 50R termination at the scope end of the coax. The level will drop a lot at both 1MHz and 100MHz but you should find there is less droop when comparing 1MHz and 100MHz and maybe this droop will be dominated by the droop in the scope at 100MHz?