How much BW for the spectrum analyser? - Page 1
Forum section: electronics / rf-microwave

author: tszaboo
author_rank: Super Contributor
timestamp: July 24, 2022, 09:03:46 am
content: The company I work for is planning to buy a real time spectrum analyser, however I'm not sure how much bandwidth is actually needed for such a thing. I think it's the maximum bandwidth of the RF technology in question and some spare?In any case I'm mainly interested in BT, 804.15.4, GSM, LTE-M and NB-IOT all of which would fit into the usually cheapest 10 MHz bandwidth of these. Any reason I would need to get higher bandwidth?I know about EMC precompliance testing, and it is not planned to be done with this device, since that would mean getting something with 26 GHz and honestly, even with the tool, I don't think the company would be able to perform these tests properly, so I rather pay the lab to perform them.

author: radiolistener
author_rank: Super Contributor
timestamp: July 24, 2022, 10:07:49 am
content: Quote from: tszaboo on July 24, 2022, 09:03:46 amYou're needs more wide bandwidth to check out-of-band leakage or unwanted emission.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 24, 2022, 05:20:07 pm
content: Quote from: tszaboo on July 24, 2022, 09:03:46 amYou can do a lot of tricks not to have to have higher bandwidth than the signal but here are a couple of examples why you might be forced to:-You want to capture the frequency hopping of BT-You want to capture adjacent channel leakage in a time division multiplexed system, where there is no available separate trigger signal and the adjacent channel power is too low to trigger off of-Test time matters (high volume production test) thus you want to do all of your modulation measurements and leakage metrics in one go on a single packet-You figure out you want to do WiFi or any other spread spectrum signaling (If I'm not mistaken tho LTE-M can already use a 20MHz channel, at least per spec, I'm not familiar with practical use.)-You have to do time domain analysis of the nonlinear distortion of your constellation (PA tuning, digital pre-distortion etc.), thus need the adjacent channels in real time-You're doing coexistence testing where the timing relationship between the different standards is not deterministicDo you have a plan of what exactly are you going to measure?

author: tszaboo
author_rank: Super Contributor
timestamp: July 25, 2022, 03:01:57 pm
content: Quote from: Marsupilami on July 24, 2022, 05:20:07 pmUsually looking at the output of Microcontroller, that have the wireless technology built in.The 2G/LTE-M development usually doesn't happen in this office, it's something that would be nice to look at on occasion, but they are modules that are expected to work. Production testing is not really the plan either, nor WiFi.Release testing for FW to verify if it's using the RF interface properly.Me as a HW engineer, have little need for it, I have a VNA for the antenna matching, and otherwise it's just a black box that is relatively easy to use, and never had issue with the certification after following best practices and the documentation. It's surprising how easy it is really.Looking at collisions and coexistence would definitely be important.The best info I can find, LTE-M is 5MHz.Quote from: radiolistener on July 24, 2022, 10:07:49 amYes, that's a good point, that's why I'm looking at least 2x the BW of the analyzer than the BW of the signal.

author: LM21
author_rank: Regular Contributor
timestamp: July 25, 2022, 11:30:16 pm
content: Are you sure a spectrum fits best for your needs. Protocol analysers or some general test station with SA and other devices, would show what your HW is doing.

author: tszaboo
author_rank: Super Contributor
timestamp: July 26, 2022, 04:38:46 pm
content: Quote from: LM21 on July 25, 2022, 11:30:16 pmWe have our own custom RF sniffers for seeing packets for the 802.15 and it can be used for BT with some hammering on the FW. I don't think seeing CAT-M1 packages would be useful since it's undocumented, and even if something is wrong, we didn't write the FW for the modules. So far we rented the SA when it was necessary (you know, the usual 5 days before product release and "something in RF" doesn't work), but now it's time to invest in one.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 30, 2022, 10:47:39 pm
content: Quote from: tszaboo on July 25, 2022, 03:01:57 pmI'm not saying that an RTSA is not useful but based on this and without knowing the price-benefit ratio it's hard to tell if you'd have more success with a higher instantaneous bandwidth instrument.Here's a comparison of an RTSA snapshot of a 1ms NB-IoT packet vs the same thing on an ancient swept analyzer. Obviously the duty cycle has to be taken into consideration when calculating power levels but other than that it's totally usable for a lot of things.(My analog analyzer input is likely screwed up so ignore the absolute level.)Power vs time showing the 1ms packet on the 2nd pic.I don't know what's your budget and you can always find a use for a good instrument, but you don't necessarily have to overspend on something that you don't have a clear, well defined use case for.("We have a definite budget allocated this quarter and I have to spend it or we'll lose it" is a completely reasonable excuse though to buy something that you might not even use, but that's always harder to justify in terms of explicit utility.)Are you contemplating between any specific models?

author: tszaboo
author_rank: Super Contributor
timestamp: July 31, 2022, 09:29:54 am
content: RTSA because then you don't have to rewrite firmware to capture the RF continuously. I would say that a bunch of our devices have power limit that doesn't even allow the device to do that in normal operation. Meaning you would need to change the HW the FW, the output to coax to not jam everything, and what else to be able to do a test.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 31, 2022, 02:02:26 pm
content: Quote from: tszaboo on July 31, 2022, 09:29:54 amAh, ok. That makes sense.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: July 31, 2022, 04:17:53 pm
content: > rewrite firmware to capture the RF continuouslyHey, that's what gated sweep is for -- but yes, once you have to consider salary*time or time-to-market, it makes less and less sense to spend time making due with old equipment.