Power Meter Recommendations - Page 1
Forum section: electronics / rf-microwave

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 12, 2019, 03:59:20 pm
content: I'm going down the RF rabbit hole and find I need an RF power meter to verify my equipment (sig gen, vna, etc). I'm workingin the under 1.4GHz range.I see some cheap RF power meters on ebay out of China, presumably based on the LTC5596 or AD8318, which might be suitable. Anyone try one of these?More traditional analog or digital HP and others are fine, but once the sensor gets included it rapidly goes beyond my budget.I'm not adverse to building one, but I have the chicken and egg problem. I'd need to calibrate the meter and I need the meter to calibrate my instruments.Suggestions?

author: CJay
author_rank: Super Contributor
timestamp: February 12, 2019, 05:20:02 pm
content: The AD8318 modules worked for me, really good value for money.Standy by for the 'muh bird ain't fayke Charnesium crap' posters but for the most you can ignore them, the Chinese modules are generally pretty good IME

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 12, 2019, 07:23:13 pm
content: From what I can see, if you get a module you still need to calibrate it for best accuracy.Getting a module + arduino + LCD + a couple buttons is no problem; I have everything except the module in my stock and those are amazingly cheap. It would certainly cover my requirements. Writing or grabbing code is also not an issue, but the software calibration is.It would probably be asking too much if the modules came with calibration information?

author: CJay
author_rank: Super Contributor
timestamp: February 13, 2019, 06:53:28 am
content: Definitely no cal sheet with them but they were surprisingly good and consistent, I could swap them over to get the same measurement from both and the output was the expected 25mV/dB from the datasheet for a given power input from the CMU200 sig gen so it seemed accurate out of the box, good enough for me and it worked as a drop in replacement for the AD8307 sensor I homebrewed.I guess the question is how accurate do you need it to be?

author: JohnPen
author_rank: Regular Contributor
timestamp: February 13, 2019, 10:22:06 am
content: I bought one of the Chinese 8318 power modules, linked it to a 3 digit voltmeter module and installed them into a small diecast box. I powered it up with an external 9v power block. The result is shown in the attached pictures and a calibration curve. The calibration was achieved by using my home built Scotties Spectrum Analyser. The latter had been calibrated using a Sam Wetterlin 'Leveler' module and switched attenuators. The Power meter certainly seems to be very consistent although I am unsure of its accuracy above 1 Ghz as I have no equipment capable of confirming it's accuracy. Also I have no signal sources above 2Ghz to check whether it really does go up to ~4Ghz. Still it is a very useful quick check Power meter and takes very little space up.

author: mcafzap
author_rank: Newbie
timestamp: February 13, 2019, 05:37:26 pm
content: I too bought an AD8318 module and coupled it to small processor (MSP430G2552) to remove the offset, invert and provide gain to the signal digitally, then feed out the result via a PWM output to a conventional 1mA fsd meter. Works very well with pretty accurate results (long time since I checked it).

author: CJay
author_rank: Super Contributor
timestamp: February 13, 2019, 07:50:59 pm
content: Further to that, there's an article in EPE Mag this month which has been published previously by Silicon Chip, there's also an article in Radcom, the RSGB member's magazine.

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 14, 2019, 02:53:17 pm
content: SV1AFN did an implementation of the reference design in an AD app note, I think I will duplicate that.There are some nice implementations that couple that with a color touchscreen, but a 2.4 inch touchscreen seems a bit cramped. I will look into the UI. Soft buttons may be better than a touchscreen.There's also a 31db programmable attenuator chip available using the HCM470 chip. Might be nice to integrate that into the front end.Just what I need, another project. It will be a while, but I'll post what I end up with.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 14, 2019, 02:57:52 pm
content: Quote from: CJay on February 12, 2019, 05:20:02 pmNot my experience, unfortunately. They're bad. They might still have a good value proposition for your particular use case, but you should familiarize yourself with the tradeoffs and make sure that they continue to make sense as you proceed down the rabbit hole.Power Meter:https://photos.app.goo.gl/jRck3HwGcTZiP18dAVariable Attenuator:https://photos.app.goo.gl/fdysAuMYQ8JX3yY86The tool you have will always beat the tool you don't have. Hopefully those S11 and amplitude measurements give you some indication of the difficulties you will need to face in order to use these cheap tools effectively. Godspeed.

author: dmills
author_rank: Super Contributor
timestamp: February 15, 2019, 11:20:26 pm
content: S11 is fairly easily cured providing you have power to spare, a 10dB pad will get you a guaranteed 20dB return loss providing the pad is up to snuff, I bought a nice set of them (and paid far too much) but a collection of calibrated attenuators of known goodness really are the RF guys secret weapon.Welcome to the really expensive end of gear acquisition syndrome, you will know you have arrived when you have more then one 'cal kit' in a walnut box AND a full set of connector gauges (The VNA crew will know the horror to which I refer).73 Dan.

author: CJay
author_rank: Super Contributor
timestamp: February 18, 2019, 10:22:34 am
content: Quote from: jjoonathan on February 14, 2019, 02:57:52 pmAbsolutely, it pays to understand the tools you have.I have far better power measurement than these cheapie boards but at a far higher price, the cheap boards work, they do have shortcomings but are excellent value for money IMHO.

author: radiolistener
author_rank: Super Contributor
timestamp: February 18, 2019, 11:48:37 am
content: I played with this one:https://www.aliexpress.com/item/1-8000MHz-AD8318-RF-Logarithmic-Detector-70dB-RSSI-Measurement-Power-Meter/32829231631.htmlI don't have generator and analyzer for gigahertz frequency, I'm using it for 500 MHz max.Here is my measurements of it's input impedance and SWR.My analyzer has a slightly worse precision above 200 MHz, because it's generator is 200 MHz max and it works with harmonics above 200 MHz, so there is a noisy line.As you can see it has to high SWR on frequencies above 100-200 MHz and this is because of reactance component in it's input impedance. This is because it has some small capacitance and there is no compensation circuit. So, I think it will useless for gigahertz frequency. There is need to implement compensation circuit. But I thik it will be too hard for gigahertz frequency