Question about spectrum analyzer bandwidth - Page 1
Forum section: electronics / rf-microwave

author: spacehen
author_rank: Contributor
timestamp: December 11, 2023, 12:45:05 pm
content: Let's say I am a newbie designing a power amplifier for 450MHz and make measurements using an inexpensive 1.5 GHz spectrum analyzer.Now I need to check the basic harmonic performance of my PA. With a 1.5 GHz analyzer I can obviously view up to the 3rd harmonic.Is this generally enough bandwidth for successful PA design or do you usually need to characterize higher order harmonics?My thought was that harmonics generally decrease in power quite significantly so perhaps even the second harmonic would be enough.I found this quote from keysight:"A good rule of thumb is to choose an analyzer with a span that is at least twice the bandwidth of the signal you want to measure." but not sure if this applies to power amplifier design.Thanks in advance!

author: jonpaul
author_rank: Super Contributor
timestamp: December 11, 2023, 04:57:33 pm
content: check FCC/IEC/UK emission regs on harmonics applicable to the device eg for transmitters, medical, avaionics.Generally a calibrated analyzer is needed.With a lot of WiFi, 4G/5G and avaioics in or near 450, 3, 5, harm eg 2.4 GHz, you may have interference.j

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: December 14, 2023, 02:26:35 pm
content: “Push-pull” output stages tend to suppress the 2nd harmonic, so just having sufficient bandwidth to see the 2rd harmonic may not be a reliable indicator of additional harmonics.The low cost TinySA Ultra covers up to 6 GHz, so you can comfortably use one to check a 70cm TX.SJ

author: MichaelB
author_rank: Newbie
timestamp: December 14, 2023, 03:37:30 pm
content: The 1.5GHz "spec" is the 3dB bandwidth of the scope. So, in theory (i.e., all other factors being equal), you'd see the initial harmonic of 900MHz at close to the right level, but after that (even at 1350), there will be some added attention from the scope alone. Since harmonics are (typically) at much lower levels, this would likely make it difficult to get an accurate reading of that third harmonic. Since this is an amplifier you are testing - you'll also need to add an external attenuation (or coupling) so as not to damage the scope. This added attenuation will also attenuate the harmonics - making it even more challenging to get an accurate reading. I've read a lot of "rules of thumb" regarding BW vs. highest frequency. "Back in the day" with older analog scopes - the rule of thumb was to use a scope with 3X the highest frequency. I've seen manufacturers of the newer digital scopes (including KeySight) recommend 5x the highest frequency. The 'best' way to measure those higher frequencies (on a budget) is the TinySA or NanoVNA. But - make sure you put an appropriate attenuator on the input if connecting to a PA.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: December 14, 2023, 04:52:28 pm
content: “The 1.5GHz "spec" is the 3dB bandwidth of the scope”Bandwidth isn’t quite the correct way to describe the frequency span of a spectrum analyser. Think of it like a swept frequency receiver with a panoramic display. There is resolution and video bandwidth for a spectrum analyser, which relate to the amount of detail you can resolve on the screen.SJ

author: fourfathom
author_rank: Super Contributor
timestamp: December 14, 2023, 05:57:16 pm
content: A 1.5GHz spectrum analyzer has a tuning range from 0 to 1.5GHz. Bandwidth is important but probably appropriate for a 1.5 GHz unit. Unless you use external oscillators, mixers, and filters to shift those harmonics down in frequency you won't be able to see them. And even then accurately measuring them is a challenge.It's not like an oscilloscope where you can cheat and look at signals higher than the-3dB spec.

author: kj7e
author_rank: Frequent Contributor
timestamp: December 15, 2023, 04:13:47 pm
content: A 1.5GHz SA would do what your looking for. However, I would not focus about directly measuring the 3rd harmonic of your UHF amplifier as your first priority. Instead, I would build a good low pass filter and sweep it for performance. You can optimize it for a null on your 2nd and 3rd harmonics. Then you should easily meet the spurious emissions requirements.

author: G0HZU
author_rank: Super Contributor
timestamp: December 15, 2023, 10:14:15 pm
content: Usually, the formal requirements spec for the device under test should dictate the harmonic suppression levels and the range of frequencies to measure over. If there are no formal requirements then I guess you can make up your own. Usually, for ham stuff there will be a spec for harmonic distortion and it's best to measure over a range of frequencies that include most modern domestic appliances. 30 years ago this meant testing up to about 700MHz or so for UHF televisions etc. Nowadays, with WiFi and Bluetooth, it makes sense to measure up to 2.5GHz or even 6GHz if possible.If you do end up with a well filtered amplifier and you still want to verify the harmonic suppression, you will need to be careful how you set up the spectrum analyser. It's generally a good idea to make sure the internally generated harmonic distortion in the spectrum analyser is at least 15-20dB lower than the level of distortion you want to prove or measure.Ideally the internal distortion in the analyser should be 20dB lower than the distortion of the amplifier under test. This will keep the measurement uncertainty of the spectrum analyser (a kind of error) below about +/- 1dB in terms of the uncertainty caused by its own internal distortion. This usually means the operator has to set the analyser to have a fairly high internal attenuation level at the front end. The better the analyser, the less extra attenuation will be needed to meet this >20dB requirement.However, if you just want to make informal or casual measurements, then this can be relaxed a bit.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 17, 2023, 11:36:44 am
content: Depending on the application, I would imagine you rather want to use a VNA that can do power sweeps to also check your compression curves. Depends, of course, on what you mean by 'design a PA'; I would usually assume that the compression behavior is more important, as the harmonics can simply be suppressed through filter/matching network/antenna design.