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Cheap USB power meters?

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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

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