SDG2042X output to calibrate a cheap spectrum analyzer - Page 1
Forum section: electronics / testgear

author: black6host
author_rank: Contributor
timestamp: January 21, 2022, 03:25:34 am
content: Greetings all,I've got an RF-Explorer 6G spectrum analyzer, and I want to calibrate it using my Siglent SDG2042X signal generator. I know little of the uses of either of these pieces of gear. I am learning though and have read quite a bit about both and I've looked at many youtube vids.I know enough to know that I need to be very careful that I don't overload the front end of the spectrum analyzer and it seems the sweet spot for input is about -27dBm. What I don't know is how to figure out how much attenuation I need to add from the signal generator to the analyzer so as not to blow it up. How does vpp on output relate to all this, if at all? I don't seem to find any answer in the manual, that I can understand so I'm asking for help. Thank you in advance for any insight you might offer!Edit: I've since figured out I need a different bit of kit, to do what I want. Still, any input you might think helpful? I'm all ears!

author: DaJMasta
author_rank: Super Contributor
timestamp: January 21, 2022, 03:48:16 am
content: I'm not totally sure why you should need to calibrate it or why the SDG2042X would be the right gear for the job, but I think getting a dBm output should be simple:Either set the output impedance of the generator to 50 ohm and then change the display unit to dBmorSet the output impedance of the generator to 50 ohm and then convert Vpp to Vrms (with a sine wave, which is presumably what you're using, you multiply by 0.7071), then convert Vrms to dBm using: Power in dBm = 20 * Log (Vrms/(0.001 * System Impedance)^0.5)orJust use one of many online calculators:https://www.analog.com/en/design-center/interactive-design-tools/dbconvert.htmlIt's important to emphasize that your power calculations will be wrong unless you're using the right system impedance, though. Generally, for RF test gear, this will be 50 ohms, and the generator also needs to be set to that for your measurements to be correct. -27dBm is fairly low power, so it may be easier to do with an inline attenuator than just turning down your function generator (if it will even give you a signal that small).

author: tautech
author_rank: Super Contributor
timestamp: January 21, 2022, 03:59:54 am
content: DaJMasta is quite correct, the 40 MHz SDG2042X is not suitable for spectrum analyzer calibration however it would be just fine to use as a sanity check to see the SA is somewhere near accurate but to do it with any precision both the SA and SDG need be connected to a 10 MHz reference source to align their clocks.Then up to 40 MHz the SA marker accuracy can be checked and as DaJMasta points out in 50 Ohm mode the SDG can be set to display dBm so to do SA amplitude checks.

author: black6host
author_rank: Contributor
timestamp: January 21, 2022, 04:02:45 am
content: Thank you. I'll use the formula you provided. The function generator doesn't let me pick output in dBm so that will help. I plan on using external attenuators, I've got two 30dB and a 10dB on the way. As far as for why I have to say that I've completely bungled this whole concept as I need to calibrate it at a frequency higher than my signal generator will do. Sorry for the interruption!

author: black6host
author_rank: Contributor
timestamp: January 21, 2022, 04:06:10 am
content: Quote from: tautech on January 21, 2022, 03:59:54 amThank you! Yes, I figured out my SDG won't put out the signal I need. But, sanity checks are good. I need lots of them, dailyI'm slowly learning about this gear. I just got my ham radio license and it's an exciting time. So much to learn. I aim to build an HF transceiver sometime in the near future and I'm learning how my gear works to help in the process.

author: DaJMasta
author_rank: Super Contributor
timestamp: January 21, 2022, 05:04:35 am
content: Well if you're working in HF, the signal generator may be a good source - and on the radio side of things, there are plenty of good uses for a dual channel sig gen. There's some built in modulation capability to do direct synthesis of stuff in the HF bands or to generate a test signal to feed into a modulator, and it's a reasonably clean signal given the bit depth of the DACs for use as an LO, at least in prototyping, though the power level is going to be too high receiver testing and generally too low for transmission levels - so it's not a turnkey solution, more of a building block and diagnostic tool.Especially as a ham, have a look around for calculators online. There are a lot of people and amateur radio organizations who have calculators available for all sorts of things, and especially if you're not 100% on the math, it can be convenient to use or just a good check of what you're trying to evaluate.

author: Performa01
author_rank: Super Contributor
timestamp: January 21, 2022, 06:35:13 am
content: Quote from: black6host on January 21, 2022, 04:02:45 amYour claim that you cannot use dBm simply isn't true. The Siglent SDG lets you pick Vpp, Vrms and dBm for the amplitude.Whenever you enter a new output level via the keypad, you have to confirm it by the appropriate key to select the intended unit.You can change the display even without changing the value: just press the amplitude key for longer than two seconds in order to toggle between Vpp, Vrms and dBm.These generators are perfectly usable for calibrating (especially low end) SAs - as long as they cover the required frequency range, that is. So, there will be the main problem.Frequency accuracy is not that bad from the outset and the SDG can use an external 10 MHz reference, so you can get any accuracy you want.Amplitude accuracy is quite good as well and exceeds the specifications of most SAs quite easily.I would worry much more about the external inline attenuators. The SDG can go down to -56 dBm, but an accurate external 10 or 20 dB attenuator would be handy to protect the SA from high signal levels that the SDG can output at low frequencies and help to get down to even lower signal levels. As long as you don't have a precision step-attenuator, an assortment of precision inline attenuators will usually be fine in my experience, even though the specified tolerances could be as bad as 0.5 dB over the entire frequency range. So if you cascade more than one attenuator, the tolerances might add up - but in practice good attenuators from reputable brands like Mini-Circuits or narda have tight tolerances.

author: bicycleguy
author_rank: Frequent Contributor
timestamp: January 21, 2022, 09:56:28 pm
content: Quote from: black6host on January 21, 2022, 04:06:10 amTo display dBm on the SDG2042 you must first touch 'Load' on the screen and then press the 50 ohm button. In HiZ mode dBm doesn't make any sense and is not selectable. Wikipedia dBm for why.

author: black6host
author_rank: Contributor
timestamp: January 22, 2022, 01:28:25 am
content: Ok, it's time to go back and play with the SDG more. Thanks all, you've been very helpful and I think I've got a good start on how to proceed next. I've been interested in radio for a long time but never tried to learn much about it. And now I am. Thanks again!

author: rf-loop
author_rank: Super Contributor
timestamp: January 22, 2022, 11:10:27 am
content: Quote from: bicycleguy on January 21, 2022, 09:56:28 pmSmall addendum to this.SDG2kXAfter select "Load" and press "50ohm" button. 50 ohm is displayed because it is just default value (but it is user adjustable).There can be also other impedances and naturally still we can use dBm (power) but we need know impedance, in SDG case LOAD impedance. If load is not 50 ohm then user must tell this right load impedance to SDG so it can display right values.User can change this "50ohm".Range is 50ohm - 100kohm.Internally calculated and displayed level value is correct for user selected load value.In all cases without exceptions, Hi-Z or 50ohm to 100kohm output Load selected,generator real internal source impedanceis allways 50ohm, even when HiZ and DC out is selected.If external load is 50ohm and user have selected 50ohm "Load" then system can calculate displayed output level values right.If user have example 120ohm load but he have selected "Load" 50ohm then SDG displayed level is of course wrong. But when user have selected "Load" value 120ohm then it can calculate right displayed levels (Vpp, Vrms, dBm). Naturally mostly in usual RF systems 50ohm is used. But, for other load values in SDG this is also user adjustable.

author: black6host
author_rank: Contributor
timestamp: January 22, 2022, 07:42:54 pm
content: You know, I searched the manual for dBm, thinking that would lead me to some info on how to select that as an output criteria. I can find nothing in the manual about how to do that. I suppose Siglent assumes you know the tool you've purchased. Anyway, it seems I have it sorted out. Thanks again!