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?!


27 December 2021

Signal source for 24 GHz

UV-82 400 to 520
UV-82 400 to 520 MHz
While waiting to go outside with the 24 GHz experimental setup I tried to measure the module frequency. The simplest way to generate a 24 GHz signal of known frequency is to multiply a UHF source through a diode.

That's nothing new, it's cheap and worth trying. So I rigged up the multiplier shown in the picture with an unknown Schottky diode.

The Baofeng UV-82 RTX transmits between 400 and 521 MHz; that's plenty of choice to get an harmonic show up in the GHz range.

In order to check if it works I first looked for the harmonic on 10 GHz through an LNB, and sure it was where expected.

Unfortunately I did not see (receive) anything on 24 GHz, even at the higher power setting of the UV-82 source.

A better view of the diode and adapters.

Then I tried with a source that covers 2000 to 2990 MHz and there was no sign on 24 GHz while it was very strong on 10 GHz. I think the LNB amplification chain makes a lot of difference.

Still a 24 GHz module can receive another module at 12 metres distance with S9 and probably more once the antennas are properly aligned. I postpone the frequency measurement and spend time on checking which distance is possible in a radar-to-radar setup.

19 December 2021

Observations on CDM-324 (IPM-165)

After adding the sponge under the lid of two CDM-324 radar modules (probably clones on InnoSent IPM-165) I repeated the transmission-reception test over a range of 4 and 10 metres.

While the SNR does not deteriorate of a visible amount with the increased distance, I noticed few things about these radar modules.

They are very sensitive to power supply voltage variations. Any noise on the power supply line is reflected on the output frequency. I opted to use a 78L05 linear regulator and a battery power source.

The IF output(/input) has low impedance. I tried modulating via a 1+1 Mohm divider but no signal was available at the IF pin of the radar module and no modulation could be detected on the receiving end. I will try to lower the impedance of my source, or will move to Vcc modulation.

Sensitivity to surrounding objects is less than 50 cm. When moving closer than 50 cm to the CDM-324 there is noticeable frequency drift. When the obstacle is removed the frequency returns more-or-less where it was before. This effect could be used to generate a physical FSK modulation for slow visual CW, for example.

They are sensitive to temperature. This is not a surprise, as HB100 on 10 GHz feels temperature variations too. That's intrinsic to DROs. But on 24 GHz the effect is 2.4 times higher. As long as receiving and transmitting end are subject to the same delta_T, their frequency difference remains constant.

Given all the causes of interference to frequency output are minimised, the CW radar-to-radar reception looks like this:

The waterfall took about 60 seconds to build. Both modules where in the same small room in free-air, not enclosed in a box to add temperature hysteresis.

I think the current SNR should allow the reception over a 300 metres distance, which I will try once I manage to modulate them. That will tell some more information about the antenna pattern.


15 December 2021

Trying again on 24 GHz

Two years ago I briefly tried to use 24 GHz radar modules (CDM-324) for HAM operations. Back then I considered them not suitable, also considering that I discovered they lacked the RF sponge inside the lid.

Last week at a flea market I could buy the RF sponge, so it is worth trying again.

Now there is a sponge in the lid.

I built two circuits with a 78L05 and proper DC filtering. I could get two modules at about 70 MHz apart and the frequency is quite stable (in a constant temperature room). The good news is that the hand-effect is negligible and there is no modulation.

Next step is to add modulation.

BTW, I have lots of sponge in small rectangles in case someone needs it.

BTW2, would the sponge have some effect on HB100 too?!

10 December 2021

tinySA Windows software

A friend had a problem with tinySA Windows software, the one you can download here http://athome.kaashoek.com/tinySA/Windows/ . Everything looked fine, the COM port was recognised, just the software would not load. He got an error that a resource was not reachable, usable, accessible.

When I visited him I noticed that the Panda Antivirus from Panda Dome suite kept deleting the .exe every time I ran it. Even if the tinySA program was run as Administrator. While I am against whitelisting programs in antivirus software, it was the only way to get it running.

So if you are stuck in a similar situation with that software, find the way to convince your antivirus that it is bening.