appropriate equipment for 2.4GHz radio module calibration - Page 1
Forum section: electronics / rf-microwave

author: brumbarchris
author_rank: Regular Contributor
timestamp: July 03, 2019, 08:00:54 am
content: Hi All,we are looking at implemented an automatic RF output power adjustment for our RF radio modules (calibration) , directly on the manufacturing line. This is not so uncommon, it seems, as module to module variation in output power is quite significant, simply due to the stacking of component tolerances.I am personally, however, rather inexperienced in the RF field. I have been indicated we will need to us a power meter to measure the radiated output power of the modules, and depending on the result we can adjust a non-volatile parameter of the module to either increase the power or decrease it. I have difficulties in selecting the necessary equipment for this job.Our modules operate in the 2.4GHz band (several channels in 5MHz steps between 2405...2480MHz) and put out 6...20dBm (conducted power), depending on setting . When measured radiated, using a small RF chamber + spectrum analyzer, we get obviously much lower power levels, somewhere between -7...7.5dBm. We have the ability to configure the modules to continuous CW mode, this is the mode we generally use to measure output power.I am currently looking at Keysight equipment, but most of the ranges they have seem an overkill for this job. The EPM, EPM-P series and P-series seem to be a type of power meter that are capable of a very broad range of measurements, using different externally connected power sensors. However, we need to use such an equipment in a very limited role, basic power measurement, with a reasonable degree of accuracy (0.5dB accuracy will be sufficient, I suppose), the only other requirement being to have the possibility of retrieving the measurement data automatically through some sort of interface (serial, USB, GPIB, etc).The Keysight V3500A looks appealing to me, but it looks like a toy, compared to the others (price is also just around 2.5k USD, compared to minimum 4.5k for something in the other ranges). However, despite being labeled as "handheld" it is also advertised in the datasheet as being suitable for manufacturing floor use. Some limitations I am aware of are the 10MHz minimum resolution for the frequency (but this could potentially be acceptable, we do not need to calibrate on every channel) and the fact that it can only do "average" power measurement (so not peak); I am not sure if this is suitable for me, with the module set to continuous CW operation; as I said, I am lacking some fundamentals. It seems it has an "integrated" power sensor and that no other additional element is needed (well, apart from some sort of an antenna + RF cable, I suppose).Has anyone experience with the V3500A? Are there other considerations I should have in mind? Do you have alternative suggestions?Thank you and looking forward to your reply.Best regards,Cristian

author: tautech
author_rank: Super Contributor
timestamp: July 03, 2019, 09:00:51 am
content: Channel power measurement using the AMK-SSA3000X option with a SSA3032X.https://www.siglenteu.com/accessory/amk-ssa3000x/https://www.siglenteu.com/spectrum-analyzers/ssa3000x-series-spectrum-analyzers/

author: ogden
author_rank: Super Contributor
timestamp: July 03, 2019, 09:49:41 am
content: I know only hi-end power meters, yet have opinion- As long as specs of V3500A are fine for you and software works as well, it seems like good choice especially considering that your product can output CW. As you noticed with SA, power measurement of modulated signals can be trickyDon't forget to get decent attenuators. +20dBm source shall be attenuated (by 10dB or 20dB) to be calibrated using +20dBm meter.

author: brumbarchris
author_rank: Regular Contributor
timestamp: July 03, 2019, 11:59:50 am
content: Thank you for your suggestions!A low cost spectrum analyzer looks like a good option too, so we might consider it.We do have some good quality passive attenuators around, I just need to check if they have the right connectors. Although I am pretty certain the power at the input of the power meter will never reach 20dBm, if we will measure it radiated, through an antenna. I am actually quite sure it will not exceed 10dB; I suppose in such a case the attenuators are not required anymore, right?Cristian

author: ogden
author_rank: Super Contributor
timestamp: July 03, 2019, 12:16:52 pm
content: Sure - you may use it w/o attenuators indeed if you do not exceed absolute max power +23dBm. FYI this sensor have best accuracy in the middle of the range.

author: SpencerTC
author_rank: Contributor
timestamp: July 03, 2019, 01:02:06 pm
content: I use a E9322A power sensor with a 5348A power meter (although that meter is quite a bit more than you require and any meter that supports the E9322A sensor should suffice) to do just what you are asking about. I use a NI GPIB-USB-HS gpib to usb adapter and a simple program I wrote with VBA.What connector do you need to connect to? This sensor has an N-m- by default and depending on the volume of your production, you *might* look into a push-on variant of the connector and a fixture to apply consistent pressure to that connector to aide in quick and easy swapping of units. If you choose not to use quick connect varient of the connector beware that Keysight advises that you connect the sensor (and all connections for that matter) with a torque wrench. For power only measurements at this low of a frequency, I can't see it making much of a difference, but it is the standard procedure none-the-less.Good luck, and I hope you find a solution that works well for you.Best regards,Trent Spencer - KM4YTU

author: brumbarchris
author_rank: Regular Contributor
timestamp: July 04, 2019, 11:29:07 am
content: Hi Trent, thank you for the suggestion.Not sure why the connector would be a concern. I envisage a setup which would not require fiddling with the cable to the power sensor, something like in the attached picture. I wold only have to replace the DUT when needed, but the measuring antenna, power sensor and power meter would be permanently fixed, together with their connections. I would use a mechanical jig that would neable us to alway place the DUT in the same location relative to the measuring antenna, in order to get repeatable measurements.Is this not feasable?Regards,Cristian

author: Gribo
author_rank: Frequent Contributor
timestamp: July 04, 2019, 06:43:13 pm
content: In production, you usually want to calibrate the LO too, as it affects your channel's center frequency. For that, you need an SA, to measure the CW's frequency.

author: brumbarchris
author_rank: Regular Contributor
timestamp: July 05, 2019, 07:55:49 am
content: What does LO stand for?Cristian

author: tautech
author_rank: Super Contributor
timestamp: July 05, 2019, 08:17:57 am
content: Quote from: brumbarchris on July 05, 2019, 07:55:49 amLocal Oscillator.As Gribo points out this might affect the transmission frequency and hence apparent range performance where the frequency might be outside optimum for the antennas used.Transmitter power means nothing if it's not at the desired frequencyandmatches the resonant frequency of antennas used.

author: ogden
author_rank: Super Contributor
timestamp: July 05, 2019, 01:26:15 pm
content: Quote from: tautech on July 05, 2019, 08:17:57 amGetting out of antenna range is not an issue for 2.4GHz transceiver ASIC which supposedly is used here. Even using cheap crystal oscillator with 100ppm freq tolerance means 240 KHz drift off 2.4GHz. It could miss channel/frequency (lock range) of the receiver, but frequency range of antennas are much wider than that.Yes, frequency tolerance is problem when comparably narrow channels are used and it is good idea to test it as well. So I agree that it could be good idea to use SA to measure everything - TX power, frequency tolerance and radiated EMI. In case of failed/missing/wrong component of matching filter, it could be so that power is ok, yet transmitter do not meet EMC specs.