21 April 2017

HB100 10 GHz module frequency agility

Before I can get my hands on two Sat-TV LNB's and work on a 10 GHz WFM transceiver, I wanted to measure the frequency agility of a stock HB100 radar module.

The plan was to use one module as a fixed receiver on the factory preset (presumably 10.525 GHz) and tune a second module. Remember that HB100 is both a TX and a RX at the same time, and it emits a continous carrier. If the two TX frequencies differ, their difference will be visible at the ultra-broadband (unfiltered) I.F. output.

At first I connected the IF output of a receiver module to the frequency counter (100 MHz), and I could read up to 10 MHz delta.

Then I routed the IF output to the oscilloscope (100 MHz bandwidth, 10 MHz probe) and, as long as the signal was above 1 mVpk the sine wave period indicated a signal of about 20 MHz.

Time to turn on the spectrum analyzer, that starts at 45 MHz. Bingo! The maximum delta I could get between an untouched HB100 (let's assume 10.525 GHz) and a fully unscrewed tuning harness was 232 MHz, so it was operating roughly on 10.293 GHz.

Also, the closer the tuning screw is to the DRO, the higher the frequency. Screw in to go up in frequency, unscrew to decrease frequency.

Last but not least, I confirm that the receiving mixer works better if it is terminated on some medium impedance. I will try to visualize the signal on the oscilloscope in parallel to the S.A.

14 April 2017

Measuring frequencies around 10 GHz

While changing the frequency of HB-100 10 GHz radar modules is pretty simple, it is not so easy to understand where exactly it has been set. Unless you have got access to an expensive frequency counter, of course.

A common workaround seems to be using a satellite TV LNB so that the whole 10 GHz band is downconverted below 2 GHz, which is more affordable to be measured even with an RTLSDR dongle! As long as everything sits in the same room, there should be enough signal to do meaningful measurements.

I need to dig out one of those LNB's and do some cut and solder.

10 April 2017

Changing DRO frequency: add dielectric (hypothesis)

In order to bring HB-100 radar modules into the 10 GHz HAM allocation, their frequency has to be reduced. Reports say that the stock screw is enough, better if replaced with one with finer thread.

Then I investigated how Dielectric Resonator Oscillators work (wikipedia, no more no less) and tried few simulatons with the provided formula: the more dielectric material, the lower the frequency.

So, assuming that we have some "extra" dielectric laying around, like from a similar (dead) module, it could be worth trying to add it on top and see the resulting frequency. Besides providing a sort-of fixed frequency, it would reduce the number of factors that are influended by temperature and cause frequency instability.

For first experiments I will stick to the screw method. Reports of success/failure are welcome.

02 April 2017

Just ordered my first PCBs

After fiddling with KiCad for few month, reviewing the same circuit over and over again, I decided to stick with a relatively general purpose PCB for four ZM1332/NL5870S nixies. It is a multiplexed design that includes the decoder IC on-board (7441 or equivalent). Anodes must be multiplexed on the logic board since everyone has his preferred way of doing it (pnp, optoisolator, pmos).

This is the 3D render (by KiCad) of the boards I have ordered on firstpcb.com:
Since I was not satisfied with the result of the embedded autorouter I did it myself. I have never designed a 2-layer PCB, so I used this extra degree of freedom only when I was stuck. I ended up with only two vias, and other transitions were handled at pads when needed.
ZM1332 cold cathode displays are small and not too tall, so I put all components on the back side of the board, leaving only tubes on top. There are no overlapping components, so it won't matter which side I start soldering.
In order to simplify routing I have remapped the outputs of driver IC to actual digits, so this will have to be taken into account in the firmware. The two headers mantain a 0.1" spacing even if they are far apart.

I am really curious to see the resulting boards, and to build them of course! By the way, the size is 10x5 cm.

If all goes as planned, I will publish what is needed to reproduce this project. Fabricator emailed me they should ship on April 7th, 2017. No idea how long it will take to get here!

30 March 2017

Challenges of the HB100 10 GHz module

As listed in the previous post, the HB100 10 GHz sensor module poses some challenges when repurposed as an RTX. Let's list them, and possible workarounds.

1) The RF power is in the order of 10 mW (10-12 dBm).
On 10 GHz it is easy to assemble and use a high gain antenna. Some people report that an IKEA lamp has the perfect shape. Just remember that higher gain means narrower beamwidth.

2) Frequency stability was not a design goal for the original destination of use.
Frequency instability can be tamed with proper thermal insulation of the module. The more the merrier. In any case all narrow band modes are out of question. WFM is the way to go.

3) Receiver is direct conversion.
This one, combined with #2, is a bit harder to tackle. You can't do CW or SSB. You can't do FSK. The solution proposed by a U.S. HAM is to work full-duplex. Transmitters are on different frequencies. The received signal is then at an Intermediate Frequency equal to TXQRG difference. If the difference is about 88-108 MHz, you know which WideFM receiver can be used! Actually an RTLSDR dongle receiver allows more frequency agility and flexibility, thus allowing to operate I.F. outside the crowded FMBC band.

There are other reports that HB100 is sensitive to microphonics (mechanical vibrations are picked up and turned into electrical/RF signals). Not hard to keep under control, either.

29 March 2017

Easy way to 10 GHz

I read it on hackaday, then again on G3XBM's blog: there is an easy way to play with 10 GHz. And (very) cheap too!! The idea is to repurpose something originally meant to be used as something else. Like the RTLSDR TV dongles, the 74HC240 buffer and many more in this wonderful hobby.

The HB100 is a microwave sensor module designed to be used as motion and speed (doppler) detector. It operates on 10.525 GHz and can be retuned below 10.500 GHz into the 3 cm HAM band (Italian bandplan) with a screwdriver. It can be frequency modulated through the power supply (I guess you get some AM too). It features both the transmitter and the receiver, (patch) antennas included.

How much? Less than 3 EUROs including shipping. That's three espresso coffees standing in an Italian bar. Or three of the cheapest burgers in the "M" restaurant (their own definition, not mine).

Drawbacks (A.K.A. "challenges"):
1) The RF power is in the order of 10 mW (10-12 dBm).
2) Frequency stability was not a design goal for the original destination of use
3) Receiver is direct conversion

I have ordered 2 pairs and a spare one. I am curious how far the unmodified version will go.

By the way, I have spotted a similar radar device operating on 5.8 GHz and others on 24 GHz (InnoSent IPM165). Maybe ... ?

27 March 2017

UDN6118A VFD driver IC

I haven't tried it myself yet, but the (obsolete, discontinued) UDN6118A IC is an 8-line driver specifically designed for driving vacuum fluorescent displays. With two chips of these, up to 8x "7-segment + decimal-point" displays can be controlled through multiplexing.

Apparently UDN6128A and XO-951 are suitable replacements.

These IC's don't seem to be cheap either, but they do simplify wiring.

18 February 2017

Raytheon 2051 Thyratron vacuum test

The WikiPedia page about Thyratrons mentions that they contain some gas. Either neon, argon, mercury or xenon. This means that the tube can be tested in the same way I do with Nixies, using the high-voltage AC generator.

Now that I built a HV-AC tester out of a CCFL driver crcuit the test is fast. And the picture shows the result:

My Thyratron lights up, so it contains some gas. Which one, according to the color?!

Only the lower half of the glass lights up this way, which confirms something I've read: the gas tends to leave deposits on the glass. I am now holding the tube upside down to see if something changes.

15 February 2017

Tektronix 7A13 plugin ... fixed!

Contary with what I stated in the previous post, I tried to fix the 7A13 plugin. I couldn't leave a burned capacitor in there.

Messages on the TekScopes list suggested a simple test: measure with an ohmmeter the impedance across the tantalum capacitor: if it is zero, then there is a problem. And it measured zero.

So I cut off the original C158 capacitor and promptly replaced it with a modern 100 uF 25V electrolytic. Why cutting instead of desoldering? Because pads might not sustain several re-heating and, in any case, that component will not be reused. Even if the board is densely populated, I could operate without disassembling it, that could have been a nightmare.

With the new capacitor in place, the meter reading was still zero. So I located another couple of tantalums and one of them was dead short even if it looked brand new. Off it came, and a shiny 10uF 16V electrolytic got in the scene.

No more 0 readings across those capacitors. Time to a test in the 7603 chassis ... GO! The baby lives, again.

Left: C158. Right: C165.
This 7A13 has at least one more tantalum that I should replace before I forget where it is located. Then I should do the same replacement in my other plugins, just in case!

Out of curiosity, the burned capacitor measures open circuit and does not smell: I suspect it went up in flames long ago. The smaller one measures short circuit.

11 February 2017

Small change in HP 5082-7300 clock firmware

The picture doesn't show "3" is weaker.
I always feel awkward when I flash a new firmware in a clock. That was the case again with my 4x HP 5082-7300 bedside clock.

After more than six months of uninterrupted operation, the tens of minutes display started loosing brightness and the decimal point burned out. In order to prolong its life I changed the code to switch off the display between 9.00 and 21.00 unless a large change in the incoming light is detected. In that case the time is shown for few seconds, and then off again.

Moreover during night hours the display is PWM'ed to reduce brightness, and heat dissipation is decreased as well.

The updated code is on github.

06 February 2017

Tektronix 7A13 module on fire (almost)

I spotted it in a drawer, so I tried to plug a 7A13 input module into my Tek oscilloscope chassis. In less than 10 seconds on the CRT a bright spot appeared and then all lights went faint. I shouldn't admit that I replugged the unit and tried a second time, and the experiment lasted just two seconds since all lights were faint since I pulled the switch (on this device you pull to power up and push to switch off). No smoke/smell was released.

Next I did two things:
  1. visual inspection of the 7A13 but nothing was obviously burnt (but I met many nice looking components)
  2. searched on TekScopes Yahoo group for similar failures
The search revealed that tantalum capacitors do fail abruptly, and they were used to decouple power lines inside the instrument.

Back to the book then! The user and service manual is a work of art itself, so it was a pleasant experience to go through it. Looking at parts list and their position I finally spotted a polarised capacitor (C518) hidden under wires and behind the module structure:

C518 is the burnt blob in the centre.

I really doubt it originally looked "burnt brown", with a shade of orange towards its pins. So, this is the starting point to fix this instrument.

I will not go much further, since I don't really need this module and it already had a broken relay when I acquired it (debugging and broken relay posts from 10 years ago). Needless to say, this piece of history is the result of 1960's and 1970's engineering and I am glad to have had the chance to see and touch it. Oh, is has a digital display too!

28 January 2017

Arduino Nixie voltage booster: 12 V to 200 V

Nixie PSU kits are cheap and robust, but sometimes they can be "too much" for a project: too big, too costly, too powerful . It is the case of single tube circuits, where the current requirement is pretty low and board/case space too.

The Net is full of projects that use the 555 in place of specialised IC's: a PWM signal controlling a MOSFET it's (almost) all that you need. Since my circuits are usually based on Arduino, why not use its embedded PWM generator? Again, there is at least one fully working Arduino code and diagram on the Net, and that's where I started (thanks to Ian of nixieclock.biz).

While I could get clean HVDC, it was too high: even more than 350 V! But but but it would drop to about 200V when current was drawn. It behaved like a far-from-ideal voltage generator with a high internal resistance.

All of this could be fixed with careful run-time trimming of PWM parameters in software after a thorough (software) calibration, but still the firmware would lack the real-time response provided by specialised circuits. So I went a different way.

I used a Zener diode to keep the voltage at 180 V. When the Nixie lights up it "pulls" the current its way, effectively cutting out the Zener. The trick here is to calculate the drop resistor Rz before the Zener in a way that it will load the power supply slightly less then the Nixie. When displaying a digit this Rz becomes part of the anode current limiting resistor you need to add anyway. Since the Nixie sustain voltage is less than the striking/Zener voltage, you need Ranode > Rz with Ranode = Rx + Rz. Just do the math in such a way that the anode current is a few hundreds of microA higher than the "stand-by" current.

Yes, I know I am wasting power into the Zener, but this way I keep control over the maximum voltage if something fails upwards in the PWM chain. Moreover, since I fully control the firmware, I can save power by turning totally off the voltage booster when nothing is to be displayed. Last but not least, as in the original circuit, a resistive voltage divider lets me read the output value though one of Arduino ADCs.

I will share the diagram in a second post on this topic.

12 January 2017

From Eagle to KiCad

For Xmas 2016 I gave myself a little time to test KiCad as a replacement of Eagle.

Even if I used it for very few hours, I could not get the right feeling with Eagle interface and most of the boards in my wishlist would not fit the free version limit.

So I jumped in the Hack-A-Day tutorial series on "Creating a PCB in Everything" and tried KiCad. I love their old-school keyboard shortcuts. I love the way the interface responds when drawing a schematic diagram. I could easily (cough... I had to) design my own symbols for Nixie and VFD and their respective footprint.

So far it has been a positive and productive experience with ZM1332/5870S and IV-6 drawn in KiCad, both symbol and footprint.

I need to define a couple more tube display components and then I'll work on my first PCB to send to a fab house.

Meanwhile I keep up-to-date my own KiCad libraries on github.

09 January 2017

Nixie Tube Ciapapuer - 3 - final firmware

Lately I have pushed to github a new version of the Nixie Tube Ciapapuer firmware.

The main change lies in the way the random number generator is initialised, that now relies on an external library called Entropy. For simplicity I added the library in the github area too. It relies on small speed differences of oscillators within the Arduino microcontroller.

A second change is that the depoison routine (or slot-machine effect) can go either backwards or forward, and it's all chosen randomly.

That's it.

03 January 2017

Ham calls in a stationery store

I was walking in the neighborhood this afternoon and at the stationer's shop window I saw these bags:

They show supposedly U.S. vanity car plates with HAM callsigns. W6HQF seems to be expired, but NCA is not.

Looking up other callsigns is left as an exercise to the reader.