Cheap USB power meters?

[W4PJB]

Does anyone have experience with the cheap $30-$90 USB power meters available from the usual websites that carry such items? I've watched a few videos, and they actually seem to perform pretty well on CW signals in their "sweet spot", which is about -30dBm to 0dBm.

What I haven't seen is how well do they work on swept signals, such as from an SA or sweep generator? In the videos I've seen the software wants to know what frequency is being measured, presumably to apply some type of correction factor. This of course would not be possible on a swept signal, or even for picking up a harmonic higher up the band.

Pics attached for reference.

[Shock]

OP is looking for a USB powered and interfaceable? RF power meter (analyzer).

[W4PJB]

Yes, I'd love a "proper" power meter, but as we all know these can be insanely expensive. I've noticed these guys (I'm sure they all run on the same chip) and curious about their performance. Obviously I don't expect them to compare to a multi-thousand dollar Giga-tronics or such, but I don't have those needs, either.

I guess for $30 I'll bite the bullet and give one a test drive! 

[4thDoctorWhoFan]

Have you seen this video about the one with the LCD screen.

[W4PJB]

Thank you for your reply. Yes, I did see the video from The Signal Path, and as always, it's very educational. But what I am curious about is how well they work on swept signals, and that is not something I have seen anyone try yet.

[radiolistener]

Does anyone have experience with the cheap $30-$90 USB power meters available from the usual websites that carry such items? I've watched a few videos, and they actually seem to perform pretty well on CW signals in their "sweet spot", which is about -30dBm to 0dBm.

What I haven't seen is how well do they work on swept signals, such as from an SA or sweep generator?

They using logarithmic detector inside. You can buy logarithmic detector modules on aliexpress they selling it with different chips depends on your needs for frequency range and dynamic range.

For example:
https://www.aliexpress.com/item/1005006837297453.html
https://www.aliexpress.com/item/1005007089277354.html

Your device is just uses one of these logarithmic detectors and has some microcontroller to measure it's output DC voltage and convert it to dBm units with simple math and show it on display.

You can do the same manually, using one of these Chinese modules and DMM.

[radiolistener]

Thank you for your reply. Yes, I did see the video from The Signal Path, and as always, it's very educational. But what I am curious about is how well they work on swept signals, and that is not something I have seen anyone try yet.

You can read logarithmic detectors datasheets (for example AD8310) to see how it works, technically it has many amplifiers in a row with detector after every amplifier.

They often used in a sweep devices. For example NWT uses these logarithmic detectors and they work pretty good from several kHz to 1-10 GHz sweep.

But be careful, it measure power within full bandwidth. So if your sweep generator has odd and even harmonics, you can't distinguish it from main carrier, logarithmic detector measure all frequency components without frequency selectivity

[Dave_g8]

Hi,
For general information, I have one of the cheap RF power detector modules which uses the AD8362. The cost was around £8 in the UK.


On the board that I received, the balun on the front-end was incorrect, so I ended up modifying it to single ended mode, which reduces the performance, but adequate for basic testing.

The meter shown in the video appears to use the AD8317 detector and a programmable front-end attenuator.
I expect there is some form of adjustment made in the firmware based on frequency, using stored calibration data.
It would be interesting to see the response of the meter with a fixed frequency entry.

As stated by “radiolistener”, in the simplest case, one of the cheap RF detector modules with a DMM may be adequate.

With any of these low-cost options, the absolute accuracy and variation is going to be questionable, but with calibration it is probably adequate for amateur radio users.
73, Dave

[G0HZU]

If you want to sweep fairly fast, then the other option is to use a low barrier Schottky diode based detector. Commercial versions work up to >20GHz with low VSWR and +/- 0.3dB flatness up to about 10GHz.

The main disadvantage is that you have to calibrate them for absolute power level, but this can be done at 50MHz and then you rely on the flatness of the detector up to 10GHz.

They only operate over a limited amplitude range and they are also probe to errors caused by harmonic distortion, so it's best to keep the harmonics below about -30dBc if you want to use it for levelling purposes, i.e. you wanted to level a sweep generator output. This becomes less of an issue if you operate the detector down in the square law region (below about -20dBm) , but you would need to minimise any temperature changes after calibrating the detector as it will be prone to thermal drift.

For several decades, this was the classic way to externally level a sweep generator. I've done this many times over the years with good results.

[DaneLaw]

Got a few of these "budget" RF power meters.
Some are quite old, but with the limited tools I have to validate, they seem to play along, as long as you stay in their sweet spot.. sub 6G and 0to-55dBm

There are quite a few models... some will do 40Ghz, some will visualize a spectrum on the tiny display, some have a databank of 10 log-entries with different dBm/hz offsets for each log and some have inbuilt attenuator..(0-31.75dBm)
I haven't spent much time with them... briefly to get an idea of the RF power in various RF products.

I have 3 different RF power meters... two of them will work with PC incl that unit Shahriar's video, - one of them uses a USB3-interface, and the last is mainly optimized for walkie-talkies, etc with RF wattage up in the 3 digits.

Signal Path's video is quite informative on how it stacks up to reference-class R&S RF-gear.
I haven't tested yet how much harmonics, etc skew the readings, but I would guess a bit on the Walkie-optimized 100W unit.

 

two older walkies, RF-powerlevel tests  (Quansheng L,M,H & Jianpai L,H both at 146.200)

 

[Dave_g8]

Hi,
For experimentation I purchased one of the low cost ARW227 based RF power meters for around £7 delivered to the UK.

The description from the seller.
  24-bit ADC
  USB Type-C power supply
  -55dBm ~ -5dBm 50dB dynamic range
  Frequency range : 100MHZ ~ 2.7GHz
  Input impedance 50Ω

The specification seems a little optimistic, since the information for the ARW227 quotes a dynamic range of 45dB for frequencies 0.1GHz to 2.7GHz.

The “Main” PCB is connected to the “Display” PCB via a 16-way fixed connector.
For curiosity, I separated the two PCB’s, which was a little difficult and I ended up having to repair a few of the tracks.
An image of the unit after modifying it to have a male and female connector is shown attached.


The basic architecture inferred from the PCB is shown attached.


Interestingly there is a separate 24-bit ADC which has a SPI serial interface to the MCU and an AZ431 (2V5) voltage reference.
The four buttons select the frequency bands, 100MHz, 700MHz, 1.8GHz and 2.6GHz.
I assume there are separate slope and intercept parameters for each of these bands.
I am not sure if there is a method to modify the parameters without reprogramming the MCU.

Using the limited RF generation sources that I have (NanoVNA and TinySA) and comparing the results with the AD8362, the ARW227 power meter reads about 2~3 dBm higher than expected.

As previously pointed out the meter will measure the power over its full bandwidth, so when using the NanoVNA for example, which has a nominal square wave output, the power reading includes the fundamental and the harmonics.

73, Dave