Why Sweeping Spectrum Analyzer Circuit Needs More Than One LO?

[Mechatrommer]

i'm learning SA topology... my curiosity is, why need 2 (or even more) Local Oscillator in the design? why cant just use one LO? in example diagram below, at the end after 3 LOs, the 10.7MHz signal is processed. why cant just use one 10.7 - 1010.7MHz VCO or if its too odd, use one 10-1010MHz VCO and process the 10MHz signal after RBW filter?



and while i'm at it. if you have more room to answer, this is the 2nd but less important question... i heard about phase noise (LO's?) business. whats about it? i mean how it can affect the SA accuracy? or noise level? or whatever the fuzz is? thanks for your time.

https://en.wikipedia.org/wiki/Phase_noise

[Tomorokoshi]

A lot of it comes down to how wide the frequency range is of the device. A spectrum analyzer can be thought of as a very wide-band tunable radio.

Here is a nice overview of a dual local oscillator approach to radios:
http://www.radio-electronics.com/info/rf-technology-design/superheterodyne-radio-receiver/double-superheterodyne-receiver.php

[w2aew]

The answer can get complicated pretty quick.  But, the main reasons have to do with achieving high dynamic range, good image rejection, good dynamic range, good frequency accuracy and low phase noise.

For example, to get good image rejection, you want the conversion images to be far away from the IF frequency to make them easy to filter at the front end.  Since the images are 2x the IF frequency away, the higher the IF the better.  But, in order to get good selectivity (low RBW), you want the IF to be low frequency.  You also have to consider the difficulty of getting low-noise, high-accuracy/stability LOs - much more difficult to do for very wide frequency ranges.

These are a few of many reasons that multiple conversion stages are used in Spectrum Analyzers (and receivers, etc.)

[BrianHG]

I remember one of my first home made spectrum analyzers.  I ended up using an old trashed TV tuner which had DC control in which swept the tuner from 30MHz at 0v to 1.2GHz at 22v.  The output was a 45Mhz IF stage.  I had a 45MHz 6MHz bandwidth saw filter there & an AM TV IF tuner there, and off of that, 2 different 4.5Mhz filters for really narrow band visualizations.  The first-LO coming out of the TV tuner was already pre-scaled down /16 if I remembered correctly.

I know that the tuner's internals were complicated to be able to tune and go through a 1Ghz span with a single voltage sweep, without any band switching like most more modern compact TV tuners.  The number of coils and copper strips & posts inside, all bent and tuned by mechanical position, doing most of the work, is a nightmare & I didn't touch anything inside there.