Why use PLL inside a spectrum analyzer?

[imett]

Hi to everyone
i've a question for a skilled user, i've to design a superheterodine (100MHz 2GHz) receiver with an up conversion at 3GHz ad one down conversion at 50MHz . I've to use two VCO from minicircuits but i've read that it be used with pll to have a stability.
So PLL limited my sweep time,and complited the design and i don't need much precision or stability on the frequency of my VCO's.

I've to use pll when i use a mixer to achieve a low freq vco with high span , example for a sweep output signal 100MHz to 1500MHz RF 1.5GHz-3GHz (variable) LO 1.6GHz (fixed) so IF 0.1-1.5GHz.?
thanks

 

[vfone]

From my understanding you are looking for a dual-conversion superheterodine as in the attached picture, and you want to use just free running VCO’s instead PLLs, because you don’t need high frequency stability.
Because just the first LO is sweeping this can be a free running VCO, but the second LO need to be a PLL, or at least a multiplied X-tal LO, otherwise the frequency drift of LO2 will create a mismatch between first and second IF (3GHz and 50MHz)

For second question, yes you can do what you mentioned.