€26 Chinese Powermeter

[McBryce]

Hi all,
     I've searched the forum but found nothing....

Has anyone tried one of these: http://www.ebay.de/itm/331681024984
Does 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.

[CJay]

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.

[kripton2035]

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.

[macboy]

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.

If 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.

[Electro Fan]

looks fun

other 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%253A7

http://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

[kripton2035]

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.

If 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.

it'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.

[Electro Fan]

in a portable mode might be handy for checking on some wifi or other signal strength

[edavid]

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.

If 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.

it'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.

It's still called a spectrum analyzer... you use "zero span mode".

[PA0PBZ]

but for one single frequency the user can enter, how do you call this ? also I've never seen such instrument.

Selective level meter or Selective power meter, although the last one is normally reserved for optical power meters.

[technogeeky]

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.

If 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.

it'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.

It's still called a spectrum analyzer... you use "zero span mode".

Wouldn'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.

[PA0PBZ]

Why would you use zero span mode? 

[kripton2035]

but for one single frequency the user can enter, how do you call this ? also I've never seen such instrument.

Selective level meter or Selective power meter, although the last one is normally reserved for optical power meters.

yep ... not very portable instrument ...

[sanman]

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:

[edavid]

It's still called a spectrum analyzer... you use "zero span mode".

Wouldn'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)

Zero span does not mean "1 Hz window".  You have to set the RBW appropriately.

Why would you use zero span mode? 

Mostly if you want to look at power vs. time, but it's still the best answer to the question asked by kripton2035.

[edavid]

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).

[kripton2035]

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 ?

[technogeeky]

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 ?

Correct 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. This device 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 is at best an 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.

[Electro Fan]

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.

Looks like some models have a PC interface

[eb4fbz]

Are you serious? A terminal block connector for RF? And you are afraid of the detector IC input impedance? 

[ebclr]

"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

[technogeeky]

"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

Or 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?

[McBryce]

Thanks for all the comments. I just ordered this one: http://www.ebay.de/itm/331958053726
For 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 projects

McBryce.

[tronde]

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

[McBryce]

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.

[G0MJW]

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.

Mike

PS - 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.