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.

22 March 2022

Again problems with LiquidCrystal_I2C Arduino library

I am having issues again with third party Arduino code that uses the LiquidCrystal_I2C library. The code was written in 2015-2017 and since then the said library has been hijacked, abandoned and is not backward compatible. In other words: it doesn't work. But I mostly do not like the "hijack and abandon" part.

The second part of the problem is using a mjkdz I2C adapter that reacts on address 0x20 but has a different wiring to the display, let alone the IC part number scratched off. This adapter needs a flexible library that allows remapping the PCF8574x outputs, which is not the case for most common LiquidCrystal_I2C. Or custom code and direct I2C drive. No thanks.

Conclusions of few hours spent debugging this stuff are:

  • throw away mjkdz modules so I don't fall again in the same mistake few months from now;
  • use LiquidCrystal_PCF8574 to avoid ambiguities and LiquidCrystal_I2C hijacked libraries: they are 95% compatible (just chance the instantiation and call to begin method)

05 March 2022

February 2022 news

I haven't been inactive in February 2022, I've just not updated the blog.

I have been playing extensively [time permitting] with WE 439A/6167 dekatrons, trying to get them spin reliably, which I haven't succeeded yet. In return I found out that they did contain a bit of radioactive material which is not indicated on the box nor on the glass. I am still unsure which isotope they used, since it is still pretty active after 72 years.

After receiving a small batch of IV-22 VFD displays from Ukraine and planning a clock, today I re-checked the Bad IV-27M bought in 2016 and, with the correct datasheet, I found the filament on pins 4+5 and it does light up properly. Hooray! By the way, the filament runs at 3V 160mA and segments light up with 12V.

22 January 2022

Two triode valve-LED multivibrator

 Just a couple of pictures to show the two valve astable multivibrator built as a freeform electronic "art piece" that blinks two (green) LEDs.

Two triode astable multivibrator and freeform electronics exercise.
Two triode astable multivibrator.

I used (old, dirty) sockets because I had no idea if the tubes would work, so they allow an easy exchange. And that was needed indeed to balance the blink duty-cycle. I think that at these low voltages these tubes have huge differences in all their parameters.

The tubes are PCF80 or PCF802, filaments are in parallel at 9V, and HT is 9V as well. If you want to give it a try use whatever triodes you have or can get

Since the capacitors are quite small at 1 uF or less, touching one of their legs changes the astable frequency and it feels like a "do not touch" feature.

Next attempt will be to build the blinker with just one triode-pentode valve, that is just one PCF80 or PCF802 in my case with the pentode connected as a triode (g3 with cathode to ground, g2 to anode or +HT).

Two triode astable multivibrator and freeform electronics exercise.
Freeform two triode astable multivibrator (PCF80).




17 January 2022

Abraham-Bloch valve-LED multivibrator

Mr. Abraham and Mr. Bloch invented the astable multivibrator during World War I. It used two triodes to produce a square wave, actually two waves, with 180° phase shift.

LEDs have been mass-produced since 1962 according to wikipedia. I am not sure when the high-brightness LEDs hit the market.

Regardless, I have built a circuit that spans a century of electronics: a LED blinker whose active elements are vacuum tubes. And it was built with an improvised freeform architecture.

The blinker is based on the Abraham-Bloch configuration and lights up two high brightness green LEDs

I challenged myself to use whatever tube I could find in my box, it just had to contain a triode. Since I have few PCF80 (and pin-compatible PCF802) triode-pentode, I built the first circuit around their triode. 9V to the filaments (in parallel) and 18V to the anodes, and it worked at the second attempt, as I forgot to ground cathodes at the first smoke test.

Abraham-Bloch astable multivibrator.
Abraham-Bloch astable multivibrator.

Component values for an HT of about 20V are: R=10 or 15 kohm, C=0.47uF/50V , R1=R2=1 Mohm. Filaments in series (18V, 300mA) or parallel (9V, 600mA) as you prefer.

First discovery. I decreased plate voltages down to 9V and it kept blinking. Now I can power the whole circuit with a single 9Vdc source (capable of at least 700mA). Component values untouched. Yes, filament and anode are powered from the same source. Vacuum tube and safe (not lethal) voltages, what a mix!

Second discovery. Since the circuit is symmetrical, if LEDs blink uneven the difference must be in the tube. Blinking frequency is dependent on the plate voltage and at 9V I am pushing the tube limit. I managed to balance the ON times by replacing one tube. I wonder if this visual effect can help to identify tube efficiency.

First question. PCF80/ECF80/PCF802/ECF802 also contain a pentode that can be configured to work like a triode: can I halve the power consumption and use just one of these tubes?

"Poor tubes!" "Don't misuse them!" I think that this kind of misuse is better than "no use to the eternity". Unless someone comes up with a use-case where vacuum tubes of unknown source and efficiency must be used.

16 January 2022

Notes from InnoSent Application Notes on 24 GHz RADAR

In the download area of the innosent.de website there are four application notes. They contain 24 GHz wisdom, but you need to read through them to notice it. 

Since I am still convinced that some HAM radio result can be achieved using those radar modules, I collected below the interesting information I picked from those A.N.'s.

For the IPM-165 and the CDM-324 clone we should have faith and believe that the maximum EIRP is +20 dBm, AKA 100 mW. Typically 16 dBm.

Frequency decreases with temperature -1 MHz/°C (IPM-165). This is one of the largest challenges of these radar modules we already faced on 10 GHz with HB-100. I insulate from fast variations (wind) and keep the module in the shade when operating outside (sun/cloud transitions).

Frequency increases with voltage +25 MHz/V (IPM-165). They work from 4.5V to 5.5V, then they blow up. Vcc can be used to frequency modulate the carrier, while it must be as clean as possible: I chose linear regulators.

The A.N. also mention that the I.F. output impedance is middle-ohmic. I'd put that in the 5k-10k range. They suggest to use a HI-Z amplifier. I will consider this improvement once I am sure some sort of "DX" is possible. Remember: there is no cheap way to receive 24 GHz!

The I.F. output bandwidth reaches 100 MHz. Good! This confirms that there is not a LPF in the I.F. output. I would stop at 85 MHz to stay away from the FM broadcast band.

Regarding the installation, the A.N. lists how much common materials adsorb. For my imaginary setup I need to know that "microwave penetrate plastic very well – 0.5 to 3 dB loss with optimized thickness and correct spacing." I would rather not loose even 0.5 dB, but that gives some form of thermal stability. More from the A.N.:

The following suites very well:
•  coverage with plastic materials (ABS, PVC etc.) as long as they aren’t in direct contact with the antenna patch structures and the correct thickness and spacing has been evaluated
•  foams like Styropor and similar materials, whose relative dielectric constant is close to 1, they can even be mounted in direct contact with the antenna surface.
For 24 GHz the following rule of thumb can be used:
•  thickness of the plastic material around 3mm or 0.12 inches
•  air spacing to antenna surface similar, around 6mm or 0.24 inches

I will try surrounding the module with styropor and put everything inside a thin plastic container.

The application notes also describe the theory behind a microwave radar, with some light maths too. I think the formula ignores free-space-loss, that even at 20 metres return (obstacle at 10m) accounts for 86 dB! Using that data and the observed SNR of my experiments, I think I can establish a module-to-module contact at 200 to 300 metres. Unless the IF circuitry is saturating/clipping therefore cutting the SNR visible on the SDR.

10 January 2022

Giants' Star by James P. Hogan

While I loved watching science fiction series/movies like Star Trek, in my teenage years I was not a keen reader (nor my parents) and I read just a couple of Asimov books. Then I was introduced to the mindblowing HHGTTG by Douglas Adams (SK), and that was it.

Following a renewed interest in reading (real) books and leaning towards science fiction novels, a friend lent me part of his collection of Urania books from 1970's and 1980's.

Urania series by Mondadori publishes a sci-fi book once a month (in Italy). Today drawings on their cover receive comments, likes and dislikes, as an art form on its own. And they keep up with the monthly publishing frequency!

I picked "Giants' Star" (1982) by James P Hogan (SK) from the pile of old books and I loved it. He has an interesting theory on the evolution of humanity, which is not totally impossible. Also his sci-fi artifacts like space travel are described in a technically possible way. No spoilers here.

In the end I discovered this is the third book in a series, so you might want to start from "Inherit the Stars" (1977), continue with "The Gentle Giants of Ganymede" (1978) before reading the book depicted in the picture.

These blog posts on books are mainly for my future reference, but who knows some reader might enjoy a reading suggestion?!