Use of the 9kHz to 100kHz range in spectrum analyzers

[FlexibleMammoth]

Hi all,
I am planning to get a Spectrum Analyzer/VNA for hobby purposes. For that, I have my eyes on a JDSU JD785 (8GHz SA, 6GHz VNA) or JD745 (4GHz SA+VNA) base station analyzer, because they are "cheap" and pack a lot of features.

I would prefer the 785 for looking at wifi signals, but they are harder to get in EU; additionally, the 745 SA only starts from 100kHz. It is my understanding that they are both not RTSAs, which is unfortunate, but e.g. a siglent R-X series starts at 4k€ for 3.2GHz range (feel free to point me to other RTSAs that include a VNA).

So my question is - what are practical applications of the 9kHz to 100kHz range in spectrum analyzers? Is it a dealbreaker to miss that range?

BR Andreas

[radiolistener]

this range is covered with LCR meters

[TopQuark]

I have used that range on a spectrum analyser to measure noise PSD of circuits before. I actually wished the lower end could go further down.

VNAs that can work down to 9kHz can be very useful for measuring power delivery networks (PDNs) on PCBs and what not. I wished my VNAs can go that low as well.

[FlexibleMammoth]

Thanks for the replies!

Unfortunately, the VNA runs only from 5MHz - a common limitation in these handheld devices. Is there anything I can do with just the SA in these frequency ranges?

Edit - just realized you were talking about SAs in the first paragraph. From my understanding for noise PSD a signal analyzer would be better, as they often run from 20Hz?

[TopQuark]

Edit - just realized you were talking about SAs in the first paragraph. From my understanding for noise PSD a signal analyzer would be better, as they often run from 20Hz?

yes, but sometimes you just have to use what you already have, and my Siglent SA will only go down to 9kHz. It still gives reasonably close to theoretical PSD values.

[pdenisowski]

So my question is - what are practical applications of the 9kHz to 100kHz range in spectrum analyzers? Is it a dealbreaker to miss that range?

Most people who need an analyzer with that low of a frequency range are using them for EMC: some compliance testing (for example, FCC part 15) requires that you go down to 9 kHz:

https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15#sp47.1.15.b

For hobby purposes, you're probably fine with an analyzer that starts at 100 kHz.   

It's worth mentioning that the frequency range of modern "professional" spectrum analyzers now often starts in the single-digit kHz range.  Even our entry-level FPC starts at 5 kHz.

https://www.rohde-schwarz.com/us/products/test-and-measurement/benchtop-analyzers/rs-fpc-spectrum-analyzer_63493-542324.html

And our high-end FSW spectrum analyzer bottoms out at 2 (yes, two)  Hz

[AndyC_772]

I'm not sure there's a well defined requirement here; "hobby purposes" isn't a technical spec.

We need to know what you're actually doing, not just the fact that you're doing it for fun; always buy the tool you need after you've determined what you need it for. Buying a tool 'just in case' is the best way to end up with expensive kit on the shelf that you never use... been there, done that.

For what it's worth, I use the low frequency range on my spectrum analyser because I'm involved in ultrasonics and acoustics. It's interesting and relevant to me to be able to measure from the audio spectrum up to single digit MHz.

[David Hess]

So my question is - what are practical applications of the 9kHz to 100kHz range in spectrum analyzers? Is it a dealbreaker to miss that range?

Low frequency performance is limited by transformer coupling, and then rising flicker noise of the local oscillator and first mixer.  Swapping ports on the input transformer allows operation down to DC but noise is still a problem at low frequencies.

If you need lower frequency operations, or lower noise, then there are other instruments like dynamic signal analyzers.

[FlexibleMammoth]

Very valid point.
I have a background in IoT wireless technologies (LPWA) and network protocol research, and would like to get some hands-on experience in modulation analysis of LPWA technologies, LTE and WiFi. Other than that, I want to build some antennas, sweep some PCBs for EMI and troubleshooting. Finally, I need the SA for troubleshooting RF gear - something I avoided until now, since my scope tops out <1GHz.
BR Andreas

[FlexibleMammoth]

I'm not sure there's a well defined requirement here; "hobby purposes" isn't a technical spec.

We need to know what you're actually doing, not just the fact that you're doing it for fun; always buy the tool you need after you've determined what you need it for. Buying a tool 'just in case' is the best way to end up with expensive kit on the shelf that you never use... been there, done that.

I should have probably pinged you in the reply above, sorry about that.