€26 Chinese Powermeter - Page 1
Forum section: electronics / testgear

author: McBryce
author_rank: Super Contributor
timestamp: May 12, 2017, 12:31:48 pm
content: Hi all,I've searched the forum but found nothing....Has anyone tried one of these:http://www.ebay.de/itm/331681024984Does it work at all? Is it even close to accurate?I should mention. I don't normally have much use for a powermeter and they tend to be relatively expensive, but on occassions I've wanted to do a quick check to roughly know what the power is, so I thought this might be a handy toy to own.Any reviews available? Any and all feedback (and mild ridicule) welcome.McBryce.

author: CJay
author_rank: Super Contributor
timestamp: May 12, 2017, 01:03:06 pm
content: I reckon, from the specs, it'll be using an LT or AD log detector IC so it should be capable of reasonable accuracy.Have just placed an order for one so a couple of weeks before I find out.

author: kripton2035
author_rank: Super Contributor
timestamp: May 12, 2017, 01:08:15 pm
content: I would like one of these, but where you can specify the frequency you want to measure. not the whole spectrum the antenna is capable of.

author: macboy
author_rank: Super Contributor
timestamp: May 12, 2017, 02:17:17 pm
content: Quote from: kripton2035 on May 12, 2017, 01:08:15 pmIf you need to know signal power at a specific frequency or range of frequencies, you need to use a spectrum analyzer. That is a very very different piece of equipment than a simple RF power meter.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 12, 2017, 02:54:31 pm
content: looks funother flavors:http://m.ebay.com/itm/1-500Mhz-digital-OLED-RF-Power-Meter-Power-Set-RF-Attenuation-Value-software-/152441268017?hash=item237e352731%3Ag%3A9esAAOSwOgdYqCO7&_trkparms=pageci%253A23805c6c-3722-11e7-aec1-74dbd180115b%257Cparentrq%253Afd2267d815b0a7f8f8cbbc13ffff6d82%257Ciid%253A7http://m.ebay.com/itm/8GHz-1-8000Mhz-OLED-RF-Power-Meter-55-5-dBm-Sofware-RF-Attenuation-Value-/182265220856?hash=item2a6fda8ef8%3Ag%3AC4IAAOSwMgdXyVHj&_trkparms=pageci%253A23805c6c-3722-11e7-aec1-74dbd180115b%257Cparentrq%253Afd2267d815b0a7f8f8cbbc13ffff6d82%257Ciid%253A6- read the reviews for this second unit (contained within the ebay ad); lots more info- looks like a relatively inexpensive useful learning tool

author: kripton2035
author_rank: Super Contributor
timestamp: May 12, 2017, 03:10:21 pm
content: Quote from: macboy on May 12, 2017, 02:17:17 pmit's ok for me that for a range of frequencies, this is called a spectrum analyzer.but for one single frequency the user can enter, how do you call this ? also I've never seen such instrument.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 12, 2017, 03:17:23 pm
content: in a portable mode might be handy for checking on some wifi or other signal strength

author: edavid
author_rank: Super Contributor
timestamp: May 12, 2017, 03:20:26 pm
content: Quote from: kripton2035 on May 12, 2017, 03:10:21 pmIt's still called a spectrum analyzer... you use "zero span mode".

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 12, 2017, 06:29:33 pm
content: Quote from: kripton2035 on May 12, 2017, 03:10:21 pmSelective level meter or Selective power meter, although the last one is normally reserved for optical power meters.

author: technogeeky
author_rank: Frequent Contributor
timestamp: May 12, 2017, 06:33:48 pm
content: Quote from: edavid on May 12, 2017, 03:20:26 pmWouldn't it be difficult to accurately characterize power using zero span? Your DUT and spectrum analyzer would have to be perfectly frequency locked, and the 1 Hz window would have to be perfectly representative of the power level of the entire signal, no matter the signal width (e.g. you might be selecting a 1Hz span of a 6Mhz window)The "real" way to measure power is a power sensor (diode or thermal). Yes, you're going to measure power over the whole spectrum. But you'll get a measurement that's accurate to 0.1 dBm or so.As for this device, the main thing that would annoy me is the upper limitation on power. I always want to know power of things in the 0 dBm to +40dBm range.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 12, 2017, 06:37:18 pm
content: Why would you use zero span mode?

author: kripton2035
author_rank: Super Contributor
timestamp: May 12, 2017, 06:54:18 pm
content: Quote from: PA0PBZ on May 12, 2017, 06:29:33 pmyep ... not very portable instrument ...

author: sanman
author_rank: Contributor
timestamp: May 12, 2017, 07:04:50 pm
content: I bought one. Seems to be okay for $30. I bought the unit that goes out to 800M.There is a pretty good YouTube video here:

author: edavid
author_rank: Super Contributor
timestamp: May 12, 2017, 07:06:52 pm
content: Quote from: technogeeky on May 12, 2017, 06:33:48 pmZero span does not mean "1 Hz window". You have to set the RBW appropriately.Quote from: PA0PBZ on May 12, 2017, 06:37:18 pmMostly if you want to look at power vs. time, but it's still the best answer to the question asked by kripton2035.

author: edavid
author_rank: Super Contributor
timestamp: May 12, 2017, 07:10:10 pm
content: I think it's misleading to call this device a power meter. It's more of a logarithmic RF voltmeter. The usual problem with this type of device is that the SWR is too high, varies wildly over the frequency range, and is not characterized. You can see that the listing doesn't say anything at all about SWR (AKA return loss).

author: kripton2035
author_rank: Super Contributor
timestamp: May 12, 2017, 07:18:31 pm
content: also all the ebay $30 devices tell all you want and don't want about their characteristics...8GHz in the title, 50MHz in the description, ... so who cares about the SWR ?

author: technogeeky
author_rank: Frequent Contributor
timestamp: May 12, 2017, 07:29:45 pm
content: Quote from: kripton2035 on May 12, 2017, 07:18:31 pmCorrect me if I'm wrong, since I'm thinking aloud here. But if the SWR is 1.0:1, then you can be sure that all of the power you put into the device is being absorbed by the meter. But if the SWR is e.g. 5.0:1, then a bunch of the energy will be reflected and therefore not be counted by the power meter? Or it will be counted out of phase? Bad SWR doesn't change the frequency of the tones, right; so it's not like it will be shifted to higher harmonics.I was dealing with a similar thing last night.Thisdevice shows a graph for directivity, but doesn't actually state the return loss for any (or each) port. But more importantly it doesn't actually show the coupling. It lists return loss as "-35dBm" but obviously this isat bestan average, and clearly it should be shown over the whole frequency range. Also, the graph is pretty heavily skewed because of the semilog presentation. The vast majority of the graph is concerned with the first few MHz.In any case, as usual, it's a problem of $10. So like a chump I gave $10. We'll see.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 12, 2017, 11:36:49 pm
content: Quote from: kripton2035 on May 12, 2017, 01:08:15 pmLooks like some models have a PC interface

author: eb4fbz
author_rank: Regular Contributor
timestamp: May 14, 2017, 11:24:55 am
content: Are you serious? A terminal block connector for RF? And you are afraid of the detector IC input impedance?

author: ebclr
author_rank: Super Contributor
timestamp: May 14, 2017, 11:30:01 am
content: "It's still called a spectrum analyzer... you use "zero span mode"."No it's not , it's a called a receiver with power indicator

author: technogeeky
author_rank: Frequent Contributor
timestamp: May 15, 2017, 02:52:36 am
content: Quote from: ebclr on May 14, 2017, 11:30:01 amOr a bona-fide power meter? Isn't this exactly what the HP/Agilent/Keysight and Anristu power meter devices are designed to do? They measure RF power, calibrated, down to 0.01 dBm. What more can you ask for?

author: McBryce
author_rank: Super Contributor
timestamp: May 15, 2017, 07:29:07 am
content: Thanks for all the comments. I just ordered this one:http://www.ebay.de/itm/331958053726For the price it's worth it just to see if it works and if it does it could be useful. If not I have an OLED display and a few other parts for other projectsMcBryce.

author: tronde
author_rank: Frequent Contributor
timestamp: May 15, 2017, 06:27:51 pm
content: Found some comments on the 8000MHz here: ( Oct 04, 2016, 07:16 PM )https://www.rcgroups.com/forums/showthread.php?2744880-Cheap-RF-Power-Meter

author: McBryce
author_rank: Super Contributor
timestamp: May 23, 2017, 06:51:12 pm
content: So it arrived today. Unfortunately I don't have time to properly test it yet, but a 2 minute test gave these results:Test was done with a sinewave (500mVpp, no offset into 50ohm) coming from a Siglent SDG-2042X and the result was being compared to the measurement readout on a Rigol DSA-815TG.At 2MHz it is way off.At 25Mhz it is pretty close.I'll do proper tests later when I have time.McBryce.

author: G0MJW
author_rank: Regular Contributor
timestamp: July 31, 2017, 12:48:34 pm
content: I bought a 500MHz unit with an OLED display. Looks neat and the display is clear. Its not a colour display but is filtered so the top line is yellow and the next 3 are blue. Works quite well. I would rather have watts than volts but never mind.Calibration was not so good. I tested this with a 3GHz HP ESG signal generator and assuming this is accurate there is up to 3dB error. I think some of this is poor match. Fitting a 6dB attenuator on the input helped a bit. I have not tried it on my VNA yet. Normally you would want an input attenuator to extend the range anyway, unless you want to measure very low power. Typical application is either an absorption mode with power dumped in a load or by sampling a transmission line, e.g. with a directional coupler.Testing.At 435MHz it was 1.2dB out. I can make it read correctly by setting the offset. I did this at -30dBm. The power then tracked to within 0.3dB from -50 to 0 dBm. Rather better within -40 to 0dBm range. That's an excellent result and I can't really say if its the unit or my generator that's at fault. I suspect this is based on an AD8307. I have not tested temperature stability.Several frequencies were tried with varying calibration error but the linearity was always good.The CPU outputs 9600 baud data in plan text via a CH340 USB serial port. Not sure what the CPU chip is but I expect its going to be easy to hack, if necessary.MikePS - here is an image of the measurement head - chip marking filed off but certainly looks like an AD8307 - nice to see the input transformer and the matching resistor.