Cheap RF Power meter vs Wifi - Page 1
Forum section: electronics / rf-microwave

author: muvideo
author_rank: Frequent Contributor
timestamp: January 03, 2021, 11:16:03 pm
content: Hello,lastly I had need to compare the rf power output of several wifi adapter boards I'm using for a little project.RF and microwaves ar not my field, by far, so I decided to try to spend the least possible money on this.Enter the cheapest rf power meter I found around, bougth for 22€ on aliexpress.The unit is advertised to be able to read up to 8GHz signals, the frequency range must bepreprogrammed by hand and it is possible also to preset the attenuation of an eventual externalattenuator, the value is simply added to the rf power.The useful range is something like -50 to -5dBm.Once connected to PC, the unit appears as a com port, and the readings appear as text strings on aserial connection of 9600bps 8N1.Somewhere in my head there was the approximative notion that wifi is not very nice to measure, but I tried anyway.Naturally the meter screen went crazy once the signal is connected.The next step is to open an check what is inside the RF Power meter, with the hope to better understand the situation.The device itself is made of two PC boards, the top one carries an mcu, display, and power connector, while the bottom oneis more interesting.It is a logaritmic detector, and seem to be based on an AD8318.

author: muvideo
author_rank: Frequent Contributor
timestamp: January 03, 2021, 11:24:12 pm
content: The next thing to do is check how is the log detector output behaving.The result is revealing: the board is fast enough to show that the RF is pulsing,but not fast enough to have meaningful results.The AD8318 output is at around 4V when no signal is connected, while it goesto lower voltages in proportion to signal intensity on it's input.The board has two 100nF capacitors that slow down the step response to about0.5ms per Volt.The capacitors are marked C8 and C6.So let's try to remove entirely these capacitors and see what happens.Whitout C8 and C6 the response time is very fast, but now the signal is wildly ringing.

author: muvideo
author_rank: Frequent Contributor
timestamp: January 03, 2021, 11:34:34 pm
content: Let's try to add a 1nF cap to C8 place, and the result is much better:The problem now is that the mcu board is sampling the signal at around 10Hz,while the numbers are available on the serial interface, the samplig rate is too slow for me.So next step is to understand what is the relationship between the output voltage of theboard and the power reading.To understand this I conncected a voltage source to mcu board and mapped the power readingvs input voltage and frequency preset.The result is attached here for curiosity, the map will be useful to relate the peak voltage readfrom the oscilloscope to the input power.Naturally the numbers are not checked agains a calibrated source, and I supose that the unit isnot that precise. But honestly I don't know how to check if they are correct.Will be useful for relative comparisons.Maybe I could build a peak detector that locks on the signal valleys of the output, and resetsafter 100mS or so, that could work for my purposes. I have not much will to reflash the mcuwith an alternative fw, but if anybody wants to or knows of some such project please let me know.Also any suggestion to any cheap way to check for calibration if possible, will be very appreciated.Thanks!

author: mawyatt
author_rank: Super Contributor
timestamp: January 04, 2021, 01:00:06 am
content: Most of the Log Amps are based upon a Successive Detection technique which requires some low pass filtering as you've discovered. For your intended application (WiFi power output measurements) I think a modest LP filtering (1nF) and a DSO is the best device for capturing and comparing results from different WiFi modules. I don't have the AD8318 but seems good enough for your purpose, but remember to use an attenuator on the AD8318 input, as a direct WiFi power amp will likely saturate or even destroy the AD8313 as it's specified with a maximum input of +12dBm (~16mw).Best,

author: muvideo
author_rank: Frequent Contributor
timestamp: January 06, 2021, 10:07:51 am
content: Seem that a peak detector between the log detector and mcu board is able tofix the problem of reading rf pulses. Now the reading of the mcu board is stable andprovides a number proportional to rf "peak" power during the bursts.Yellow trace is peak detector output:

author: bob91343
author_rank: Super Contributor
timestamp: January 06, 2021, 08:40:33 pm
content: What is the model number and manufacturer?

author: HB9EVI
author_rank: Frequent Contributor
timestamp: January 06, 2021, 08:59:21 pm
content: the AD8313 gets into saturation above -4dBm; that has to be taken into consideration not only for measuring but also for calibration

author: muvideo
author_rank: Frequent Contributor
timestamp: January 06, 2021, 09:04:41 pm
content: Quote from: bob91343 on January 06, 2021, 08:40:33 pmThis One:€ 21,45 25%OFF | 8GHz 1-8000Mhz OLED RF Power Meter -55to-5 dBm + Sofware RF Attenuation Valuehttps://a.aliexpress.com/_mqEql6d

author: jjoonathan
author_rank: Frequent Contributor
timestamp: January 06, 2021, 10:34:33 pm
content: I checked these out a few years back. Unfortunately, I found that in this case performance was consistent with price.https://photos.app.goo.gl/jRck3HwGcTZiP18dAThese modules aren't useless, but they aren't what the banner specs suggest, not by a long shot.

author: eb4fbz
author_rank: Regular Contributor
timestamp: January 06, 2021, 10:51:35 pm
content: Next problem you will face is measurement error due to OFDM high PAPR, also known as crest factor. Even detectors specified as "trueRMS" have a big error with high PAPR signals.

author: muvideo
author_rank: Frequent Contributor
timestamp: January 08, 2021, 09:58:19 am
content: Quote from: jjoonathan on January 06, 2021, 10:34:33 pmDoes that mean that the input impedance of the adapter is very poor over 500MHz?The use of an attenuator (for my use at least 30dB ones) will be able to reduce the effect of the poor behavior on the DUT?Will the cheap attenuators be decent or they will be as unreliable as the power meter, any experience with these?I'm referring to these kind of attenuators:https://a.aliexpress.com/_m0hlXjPQuote from: eb4fbz on January 06, 2021, 10:51:35 pmThat will prevent me from having any meaningful power reading, is this calibratable out, or at least will the reading be repeteable?My main achievement will be to compare the output level between some dongles at the same frequency, and on the same dongle with varying power out parameters set on the software driver.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: January 12, 2021, 10:31:03 pm
content: The attenuator will prevent reflections off the meter and back at the wifi device, but it won't prevent reflections internal to the meter between the SMA adapter, microstrip, and the Analog Devices chip. Since you have a practically unlimited noise figure budget you could try padding both ends with surface mount components, or you could just live with the restriction of only measuring relative dB levels at a given frequency.I don't know if the attenuators you linked are any good. The readily available attenuators back when I was still playing the cheap equipment game were a different design. They were OK up to a few GHz but deteriorated rapidly after that, and they were internally constructed with surface mount components directly soldered to the center connector, a design which all but guarantees eventual mechanical failure. Fortunately, it looks like brand-name attenuators are reasonably priced; there are VAT-30+ available for $13 shipped on ebay.