[G0HZU_JMR]

I think you need to be wary of the spectral purity of any commercial sig gen that you use for the LO to test a dual gate mosfet mixer. The relatively poor port to port isolation with this type of mixer could cause confusion when you try and measure the mixer noise figure. It usually doesn't really matter how much money you spend on a modern commercial synthesised RF sig gen, the noise floor at about 10MHz away from the carrier is rarely better than -150dBc/Hz. You might find some that can manage -160dBc/Hz but this is rare. Some commercial sig gens can be as poor as -140dBc/Hz.

See below for the noise floor performance of a cheap 134MHz JFET oscillator against a couple of commercial signal generators. The Marconi 2022 and the HP 8648D. You can see that the cheap JFET oscillator is 25-30dB cleaner at a 10MHz offset.

In your case, it will be difficult to do much noise filtering at the LO output because your LO has to tune across about 20MHz. Therefore, if you want to have a decent source to use as a test LO for your mixer development work, I think you will have to make your own oscillator and add a buffer and a booster amp to it. Otherwise, a commercial sig gen is going to be too noisy and it will mask the true noise figure performance of the mosfet mixer.

You can add a narrow BPF at the output of the sig gen to shave off the noise at a 10.7MHz offset, but the BPF would have to be tuned to one part of the FM band only as the FM band is 20MHz wide. I would recommend making a clean VFO to use here. It should be possible to make something with low frequency drift up at 100MHz or so.