VNA versus SA - Page 1
Forum section: electronics / testgear

author: RikV
author_rank: Regular Contributor
timestamp: September 17, 2023, 07:58:01 pm
content: To what extent is a vector network analyzer usable as a spectrum analyzer with tracking generator? They of course are meant for different purposes but their application domain is greatly overlapping I gues???

author: nctnico
author_rank: Super Contributor
timestamp: September 17, 2023, 07:59:32 pm
content: A spectrum analyser typically has a greater dynamic range.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: September 17, 2023, 08:54:07 pm
content: In addition to dynamic range, SA receivers typically have better harmonic/spurious/image performance than VNA receivers. This is nontrivial and usually involves tunable filters / filter banks and multiple conversion steps, but it is worth the trouble for a SA: when you see a signal on a well-designed SA you can generally trust (maybe to 60dB) that the signal is really there and not an alias of something off-screen. Attenuators and preamplifiers are also common to deal with large/small signals and improve matching. These things are less important in a VNA: if you control the stimulus amplitude, you can probably live without attenuators and preamplifiers. If you control the stimulus frequency, you don't need 60dB of preselector attenuation at the image frequency. Therefore, VNA receivers are typically much simpler than SA receivers. Which is good, because you typically need 4/8 of them, and multiplying the filter banks of a SA by 4 or 8 would get very expensive indeed.I believe there is an option to buy attenuators and noise receivers on the high-end Keysight VNAs, but you pay a pretty penny for itI've also seen a "Spectrum Analyzer" mode that uses Software Image Suppression on the VNA receivers, but I would warn that SIS usually comes with gnarly fine print (like "no signals within such-and-such bandwidth of each other"). It's convenient if you know what you're doing and a real foot-gun if you don't.

author: TimFox
author_rank: Super Contributor
timestamp: September 17, 2023, 09:29:41 pm
content: I naively tried to use a high-end -hp- 8753A VNA as a SA in my younger days, and it did not work at all well.The SA is needed when your input signal is "free-standing", but can also be used with its tracking generator (if installed) to get frequency response, etc. (without phase information).

author: tautech
author_rank: Super Contributor
timestamp: September 17, 2023, 10:06:44 pm
content: Quote from: RikV on September 17, 2023, 07:58:01 pmLimited.Needs ? Budget ?There are instruments that are both SA's and VNA's that operate in different modes for each type of measurement.

author: Bud
author_rank: Super Contributor
timestamp: September 17, 2023, 10:46:55 pm
content: 200% usable because VNA will measure frequency response AND phase response. SA + tracking generator will only measure frequency response.However for freewheeling input signal spectrum measurement VNA in general is not usable unless you know what you are doing, or the VNA specifically has SA feature.

author: TimFox
author_rank: Super Contributor
timestamp: September 17, 2023, 11:05:34 pm
content: Yes, the VNA is great for measuring the response of a network or amplifier to the VNA’s generator.With a reflection bridge, it allows measuring impedance and S parameters.It is not useful for analyzing an existing signal, nor non-linear distortion products of an amplifier, where you use an SA.Both devices have their uses.