just use some signal generator with 50Ω output, like Chinese FY6800 or PSG9080.
And use
50Ω attenuator to reduce amplitude to expected level, you can buy these SMA attenuators on aliexpress.
50 uV usually means 50 uVrms, for sine wave it will be 50 * sqrt(2) = 70.71 uVpk * 2 = 141.42 uVpp.
Since Chinese FY6800 or PSG9080 shows unloaded amplitude, you're needs to setup twice higher amplitude to get correct value on the properly loaded output.
So, you're needs to setup 141.42 * 2 = 282.84 uVpp on FY6800. Since it don't allow it, you can use 60 dB attenuator, it will reduce amplitude by 1000 times. So, with 60 dB attenuator you will needs to setup 282.84 * 1000 = 283 mVpp on FY6800.
You can combine two attenuators 40 dB + 20 dB in order to get 60 dB, or combine 30 dB + 30 dB,etc
In order to use SMA attenuators you may need to buy some Chinese
BNC-SMA and SMA-BNC adapters.
If you can get BNC attenuators, you can use it without adapters, but it's more hard to find cheap and good quality BNC attenuators.