22 April 2022

Milliwatt meter with AD8318 inconsistent readings

First of all: I do not own a calibrated generator in the frequency range of the AD8318 chip. The closest approximation is a Kenwood handheld that emits about 50 mW at 450 MHz. That is my source.

With 30 dB attenuation I could bring that signal down to about -13 dBm that the AD8318 reads as 0,904V, which is in the ballpark of -16 dBm according to figure 4 of the datasheet.

What I could confirm is the -0.025 V/dB slope by adding and removing attenuators. This let me compute the 0V intercept, but the Arduino firmware does something weird and tells me I am feeding a much higher power.

There also is a dependency on impedance matching at the AD8318 board input, since swapping my attenuation chain changes the detected value, appreciably. 

The NanoVNA did help to confirm my attenuation chain value and impedance mismatch.

For the time being, since I lack too much instrumentation to turn the circuit into something precise, I will use its output voltage to peak an RF signal and I will not care of its actual Wattage (trimming 10 GHz filters on PLL modules).

18 April 2022

Milliwatt meter with AD8318 Arduino code

I have published the Arduino+AD8318 milliwatt meter code on github. I have changed the slope (can be re-computed with calibration), intercept points and the table with correction factors.

Note that I use LiquidCrystal_PCF8574 library for driving the I2C 16x2 display to avoid incompatibilities between various LiquidCrystal_I2C versions (same name, different API!).

The hardware circuit around Arduino is the same of PA0RWE's design.

I still get too high power readings with about 5 mW UHF at AD8318 input and I suspect it's because the error is very high, at least according to the datasheet graph (900 MHz reference). 

Typ. AD8318 response at 900 MHz.
Typ. AD8318 response at 900 MHz.

14 April 2022

75 metres on 24 GHz

Finally! A proper Spring day and 90 minutes free to spare! I took my 24 GHz experimental setup to a park and checked how far I can receive my own beacon.

The previous successful test was on about 12 metres distance.

The experiment uses only radar modules, both on the TX and the RX end. This means that the receive mixer is the one inside the radar module (CDM-324 in this case).

The frequency difference between the chosen modules was about 54 MHz and the modulation was WBFM through Vcc on the TX side.

The transmitter was arranged on a tripod and I walked away keeping the receiver both oriented and tuned to the right frequency. Since both modules were not thermally isolated, they followed the breeze. Fortunately the sky was cloudy and I was close enough that the "wind" affected both modules so they changed their frequency in the same direction, keeping the delta_F quite constant (+/- 1 MHz).

The Kenwood scanner could pick up the signal at about 75 metres. If someone had been at the transmitter changing its direction, probably I could have achieved few tens of metres more.

Now, if I add 30 dB gain ... can the distance increase of the same amount, which is 1000x?

First things first: add thermal insulation. Then get a second experimenter to help estimate the right radiation patterns of those patch antennas.


06 April 2022

Three hundred thirty three mA

In-person HAM flea markets are back in business after the pandemic years, at least here in Italy in April 2022. I was looking for some wall-wart power supply for (my future) circuits that use Vacuum Fluorescent Displays (VFD).

Most VFD's do light up with an anode/grid voltage as low as 12V, but higher brightness require more, like 20-40V.

From a box of wall-warts I dug out a 30V power adapter from an HP printer curiously rated at 333 mA. Why "three hundred thirty three"? Did they really design a circuit that stops working at 334 mA? Or the design requirement was 330 mA and the designer responded with 333 mA?

Since it's the time, sound like an April's Fool or an Easter Egg to me.

I will need 100 mA for the filaments in series, 10 mA for grids and the rest for digital circuits: I should fit within the 333 mA limit.

02 April 2022

Milliwatt meter with AD8318

Pushed by a friend who needed help with the firmware of a AD831x milliwatt meter, I (think I) decided to build a real circuit around the AD8318 module I have.

I used it for making differential power measurements of a 10 GHz generator to get the maximum juice out of it, so a voltmeter was enough to spot the lowest output voltage (the output is -25mV/dB, so the lower the better).

Nevermind. I have everything at home to build the whole power meter, so why not? I'm using PA0RWE Arduino firmware that is freely available and quite comprehensive. But his firmware was written for the AD8317, which has a different output slope (-22mV/dB) and different non-linearities.

I think that the slope value could be changed with a recalibration of the meter, but not the correction factors table. That's why I produced a different firmware with the right values for AD8318 (and I2C 16x2 display only) that will be up on my github repositories soon.