Watkins-Johnson SE222-50 Backward Wave Oscillator: the magic helix

Recently, I got a Marconi Instruments Model 6651 RF Plug-in (26.5-40 GHz), for the 6600 sweep generator. These were basically the 1st generation sweep generators extending into the millimeter wave region.

A number of plug-ins were available, mostly based on Watkins-Johnson BWO (backward wave oscillators), which are a particular type of electron tube.

These sweepers were first introduced in the late 1960s, early 1970s. New plug-ins became available as development of BWOs and similar sources advanced.

I have no price list, but surely these items came at a hefty price tag in these early days, even not quite cheap today.

My unit had a pretty “new” 1990 date-code BWO, I expect, it had been replaced by its former university owner at least once, also the cables and connectors of the BWO looks newer compared to the rest of the plug-in. Because of their nature, the BWOs only have a limited life-time, a few 1000 hours at best.

There are numerous warning labels, because to focus the electron beam, a strong magnetic field is needed.

The output is a gold-plated waveguide flange, for Ku-band waveguides.

The general layout of a BWO: there is an electron gun, producing, in our case, a hollow electron beam, with the necessary heater, kathode, anode. Then, the helix, and the collector (which is basically at helix potential or a little bit higher, say, 100 volts higher. The helix voltage is quite high, for exmaple, 1~1.5 kV.

While there is no information from Watkins-Johnson available online for any of these devices, a lot can be learned from related Hewlett-Packard microwave sweeper plug-ins (8697A), where (arguable) the same or very similar BWO has been used (some HP units may use Varian BWOs).

Unfortunately, after quite some effort with high voltages and pretty dangerous tests, it became clear that the tube barely gave some power. Maybe, because it had not been operated for a long time (some people and datasheets say that the BWO should be run at last once every 6 months, or similar, to preserve its function…).

But anyway, I have other, more stable and easy to use sources for this range, so I didn’t want to scrap the BWO without having a closer look at this. When do you normally get a chance to check out the secret workings of such marvelous devices!

First things first, hidden inside a heavy magnet, silicon rubber, and a waveguide coupler: the tube. A very mysterious piece of engineering, only a few companies ever mastered to produce these.

Also by the numbers and hand-writing, pretty clear that these units were all hand-made and assembled by engineers and highly skilled people.

The mount is all gold plated, a tapered wave guide eventually bent by 90°, so the microwave radian can be coupled from a small gap in the silver coating of the tube, to the outlet. This tube was meant to generate about 10 dBm.

The helix at the collector end.

You can see, the helix is kept centered by 3 glass (quarz?) rods, these rods are just held in place by the tension of the helix, but normally won’t move during operation and shipment. Surely, will all the glass-metal interfaces, mounts and connection points, it is no good idea to drop these units – some interfaces may break, or mis-align beyond repair. Furthermore, several Watts of powder will need to be dissipated, calling for a reasonably elaborate design of the mount not only to conduct the microwave radiation from the tube to the output, but also to dissipate the heat from the collector and helix.

All the high voltage socket part is embedded in two layers of silicon rubber. First they put some denser rubber around the socket to fix everything in place, probably run some tests, and then sealed it all by filling the further space with some pretty soft, highly filled silicon rubber. What they didn’t consider – the cable also has silicon isolation, and deteriorates in contact with other silicon materials. This may be one of the reasons for the unstable operation, because some wires were pretty close and the isolation seems very brittle.

Furthermore, I did some measurements of the critical parts, just in case I would even want to fabricate own BWOs, at least I have some working dimensions to start with.

The helix is normally made from flat molybdenum strip. At the bottom (gun end) it is mounted to a small ring. Scale is 210 px per 1 mm, in case you want to take dimensions…

A typical power supply – I am thinking about building a BWO power supply for some other old BWOs I have around (for other frequency regions), but it is fairly involved – the old design used high voltage 2 kV transformer with further high-voltage stable windings to provide the collector offset voltage, several expensive high voltage capacitors, and high voltage regulation vacuum tubes. I think it will be a good challenge for winter to build a 2 kV regulated and low-ripple power supply, solid state, probably with some series-connected MOSFETs.

Any of such future supplies will need a pretty special plug: heater voltage, anode and cathode voltages, helix voltage, collector voltage. At least I can go without any analog linearization voltage, if I manage to make the helix supply digitally controllable.

Inside the 6651 plug-in the BWO is connected to the board with some screw terminals. Seems to work well even at the high voltages. Surely there is no touch protection for these devices, just a few warnings not to get too close!

Wood and Steel, covered in Paint: a new garden bench

Recently, some heavy metal work in the precision workshop. Amongst other projects, I decide to build a new bench, to set it up behind the workshop building, to enjoy warm evenings during summer. Surely you could buy a bench, but I wanted certain dimensions, in particular, flat and somewhat wider seat area. A rough sketch shows the main dimensions. A cushion can be added, it matches 200 cm wide, 50 cm deep size.

All is made from solid flat hot rolled steel, one piece bent (after heating it with an oxygen torch, and using a small bending fixture), the other pieces welded (TIG).

All the angles were nicely hit.

I made a full size template drawn on stiff cardboard, so the welding and alignment could be easily done, then, fine adjustment by using the electronic level.

To get the right angles I had a look at some benches in public parks, etc., better to compare new design and existing know-how!

The seat, a little inclination to make sure water will drain.

The wood, douglas fir, it has a fair amount of resin and is quite resistant against weathering. It may darken, but it is not prone to rotting. I painted it with some sun-protecting varnish, two layers.
My uncle kindly provided the wood, from my ancestors village, ready to use. Saved a lot of time, compared to cutting and preparing the wood with my humble small woodworking tools.

The metal parts, carefully removed the rolling scale, added rust-proofing primer, and then two layers of solvent based lacquer. You could also consider hot zinc treatment, but such solid steel, painted, should last a hundred years, or longer.

Finally, after some hours of work, and drying the varnish for 1 week during a business trip: finished and at its final place. Very comfortable!

HP 11517A aka 08747-60022 Harmonic Mixer: a little study of a very intriguing device

As part of a HP R8747A 26.5-40 GHz reflection/transmission test unit (for the 8411A network analyzer; 6300 USD in 1973 — about 40 kEUR today), I got two HP 08747-60022 harmonic mixers, one didn’t seem to work right, the diode has just 0.2 V voltage drop. These were fairly fragile devices, only designed for 1 mW of power, and very static sensitive, point contact devices.
In addition to the regular 11517A, the 08747-60022 has a bias connection (needs about 1.5 V DC bias, center positive).

The main unit can work from 12 to about 40 GHz, with a set of adaptor waveguides.

The unit can be taken apart, all precision machined.

The diode is pressed in, on some holder (haven’t tried to remove it from the case).

There are several other precision parts, a coaxial resistor, held on to the diode with a spring.

There is also a spacer, with a very flat capacitor, a DC block. The spacer is modified to connect the center conductor to a surface at the perimeter (used for DC bias).

Further up, there is a low-pass, machined from a single piece and gold plated.

The N-type connector, stainless, is screwed on.

The DC bias uses a small 1.5 kOhms resistor, and a custom connector, so that the resistor is pushed onto the spacers’s connection.

Here, a quick schematic. Seems a lot of engineering went into this device…

Finally, we need a microscope to study further here, the diode (the square), about 0.25 mm side length. It is isolated from the case by an air gap.

The diode is made by a contact junction, a small tungsten(?) whisker.

Probably, this whisker needed some adjustment during manufacturing. I tried to adjust it a bit, but this didn’t change the diode characteristics of my broken unit, unfortunately (anyway, these devices are more for study than for use; currently using only cartridge-based diode mixers).

Also, looking into the waveguide of the assembled mixer, with good long-range optics, I could get a shot of the actual point contact in action. Very interesting historic technology.

Spring repairs: Fixing and frost-proofing ball valve

Recently, I have been setting up the watering system for my vegetable garden again. In my area, the soil is a little sandy, not bad for growing vegetables because slugs and snails don’t like such soil, but surely it needs proper watering.
All in all it is about 200 meters of 16 mm drip irrigation pipe, 16 mm size, with drippers every ~33 cm, nominal 2.1 L/h each. The system is run at about 1.5 bar. Some specific plants like pumpkins and certain berry shrubs in other area of the garden have their own supply by 4 mm tubing and individual adjustable drippers.

In the past years, the system has severed me well, and just last year, I have upgraded the water supply by setting up a new well. So there is plenty of water, and every day, at least in summer, the system supplies about 400 L a day, in two portions (like, at 6:30 am and 6:30 pm). Glad I don’t have to carry all that water.

All the water system, pump and distribution piping had been empty over winter, to avoid frost damage to the pipes. I keep the ball valves at about 45° angle, but this year this didn’t help to prevent damage to one of the valves (all the other valves are OK).

It is a combined, low cost China-made ball valve with strainer, I bought two of these some years back.

It is tight when closed, but when opened, water comes out at the crack indicated in the picture.

How can this have frost damage? Well, with the pipe empty, it should be safe? There is considerable dead space in this valve, the ball also has an indentation (cut-out) at the side opposite to the handle. It is brass, chromium plated.

As luck would have it, I had the second valve still in a box, it had been in service for some years until someone broke-off the handle (never throw away parts that may be suitable as spare part donor…). Surely it would be quick to just replace it, potentially, even with a better quality ball valve, etc., but why now study it and fix it?

Breaking open the valve, the two parts are screwed together fairly hard, and glued with some kind of epoxy or similar glue.

But nothing that can withstand a large spanner and a pipe wrench! So, the front part with the PTFE seal looks good. Also the spare valve (with the handle missing/broken off) was disassembled in no time.

Now, how to frost-proof ball valves? It is quite easy, and a well-known trick. Just dill a hole, about 3-4 mm in the far side (low pressure side) of the ball, then it will drain the dead space, when left open (or at an angle).

It is essential to remove any burrs, so I countersunk the hole, and polished the edge with some find sandpaper.

Everything put back together, I used some acrylic compound to seal and tighten the screw threads of the two-part valve. Good as new!

Fixing a really-high-frequency phase locked oscillator: a noisy opamp and a little bit of sticky tape

Recently I got hold of a really marvelous bit of kit, an assembly of millimeter wave phase locked oscillator, all the way from 60 to 89 GHz. It is already 35 years old, but all based on solid state Gunn/Impatt oscillators, so there is little aging. For each frequency (there are 5 discrete frequencies), there is a full assembly of reference upconverter (100 MHz reference to some intermediate frequency in the 6-12 GHz range), a voltage controlled fundamental oscillator (e.g., bias-tuned Gunn diode), the necessary isolators and splitters and occasional attenuator, along with a harmonic mixer. The IF is 100 MHz or 200 MHz, depending on the unit.
Only one of the oscillators has been more than 20 kEUR in 1988 currency, and there are five such assemblies on this plate, along with control equipment and power supply…

Only one of the sources is playing up, not achieving phase lock, it is the 79 GHz oscillator. Notably, some of the small screws are missing and the lid of the oscillator cavity has been removed, which points to prior repair attempts. The Gunn has a protection network, a 2.2 µF capacitor in parallel with a ~10 Ohms, ~470n series network — this is to protect the Gunn diode from the inductance of the bias current supply (when the diode snaps-off according to its characteristics, it will induce a very fast current spike that could increase the voltage at the diode above its damage threshold).

Upon close inspection, the 2.2 µF capacitor had been tampered with, bad soldering, only one end connected, and in reverse polarity (it is a high-reliability tantalum cap).

When checking the down-coverted output, there is no stability at all, it has a very strange FM modulation. What could be the root cause?

Checked a few items:
(1) Upconverted reference, 11.3 GHz (7th harmonic will be 79.1 GHz): it is a very clean and stable signal, well locked without any visible noise.
(2) Checked all the cables and connectors by pushing gently, no sign of any trouble, all unchanged.
(3) The supply voltages are all practically noise free.
(4) Also fixed the 2.2 µF capacitor at the gunn diode cavity. No particular effect.

(5) Disconnected all the phase lock, bias driver, and driving the oscillator from an external supply.

Finally, with just an ordinary power supply connected and the voltage ramping up (don’t ramp it up too slowly or let the oscillator sit in an unstable region or at the Gunn peak current for any length of time!), there is a stable (surely, non phase-locked) output, but none of the strange modulation. So it seems, the oscillator is good. But wait, with some wobbling and touching on the part, it is shorting out the supply. Hmm. Time to go a little deeper, and I decided to remove the biasing rod.

Just for explanation, the biasing rod is isolated from the cavity (the metal block holding the diode, with the waveguide cavity in the middle), and conducts the current to the diode, which is at the tip of a metal screw holder.

Under the microscope, the very thin isolating tape (looks like some PET/Mylar transformer tape) is quite damaged and some metal of the bias rod exposed. Also the spring holding down the rod on the diode (which is fixed by a nylon screw) can contact the case easily. So all was newly isolated, and the screw and spring positioned carefully. Surely with all the soldering at the capacitor, things may have shifted a little. At least now no isolation problems any more. Sometimes when switching the oscillators on the assembly, the 79 GHz signal is not coming up, the Gunn drawing much less current. But it can be fixed by just power cycling the assembly, so it seems to be some rise time issue of the power supply.

This is the driver circuit, it has a hybrid high-speed Kennedy electronics 722 amplifier, but the low frequency path is a common NE5532 low noise opamp.

After checking around the low frequency path, the opamp seems to introduce some noise, note that for this level of noise at GHz levels, a few millivolts are more than enough to cause a lot of disturbance.
Fortunately, the opamp was socketed, so I replaced it with a OPA2277, which has about the same noise compared to the (low-noise) NE5532, and much lower offset voltage drift, and 40 dB better CMRR, excellent low frequency characteristics, and lower gain at >1 MHz.

Now, with stable current drive, we can measure the output power (downconverted by an harmonic mixer driven by a 11.45 GHz source, 7th harmonic: 80.15 GHz — shown is the minus mixing product, i.e., 79.0 GHz correspond to 80.15-79=1.15 GHz).

You can see clearly, the oscillator has good power from 79.1~79.25 GHz, but falling down just around 79.0 GHz. Note that at such high frequency, every mW counts…

After some thinking, I decided to try to lock at 79.2 GHz, and as it turns out, it is working fine at that frequency. With the 11.3×6=79.1 GHz, and 100 MHz IF, it works out, and the PLL doesn’t seem to be affected, regardless if you use the minus or plus side IF.

The IF signal can be probed conveniently at a test port, it is pretty clean.

Also the down-converted signals look good (this is 10 MHz span at 79.2 GHz center, corresponding to 80.15-79.2=0.95 GHz downconverted).

Also the close-in side bands are good and a very clean signal.

Outdoor movie theater: a digital projector ceiling mount

In preparation for warm spring weather and long summer nights I am already gearing up my outdoors installations, in particular, a video projector will be handy to screen some movies.
It will be working together with a Miracast Wifi display, so it can show streams from practically any device.

The projector is a fairly lightweight model, because I only desire an about 150 cm wide screen.

There are various commercial ceiling mounts, but they all seem about flimsy and inaccurate to adjust. So I quickly fabricated a precision-adjustable mount.

First, a metal plate was affixed the the projector, with a standard UNC 1/4″-20 screw. It is about 10 mm thick hard aluminum, just a piece of leftover scrap (that’s why there are various holes in it).

On the projector side, a 1 mm NBR rubber plate is used to avoid movement and scratches.

The adjustment uses three M6 screws, 1 mm pitch will allow very precision adjustment by Allen key.

For the ceiling side, we use an 18 mm thick piece of plywood, and a M10 bolt. Surely, a longer bolt, rod or pipe could be used depending on the needed distance and rigidity. For the current setup, the M10 bolt is plenty rigid enough.

There are three springs (stainless) to keep the holding plate – mounting plate system tensioned so that the adjustment screws can work precisely.

Still, we need to get a suitable projection screen, and some speaker.

But already now looking forward to various summer movie screenings.

Laser cutter setup: air supply, off-gas and various cutting tests

Finally I find some time to document all the remaining parts of the laser cutter, Workshop Upgrade: Laser cutter and engraver SCULPFUN S9. The cutter itself is just the common off-the-shelf kit, but the air nozzle and enclosure has been custom made. If you want to cut wood, paper, plastics, there will be a lot of bad-smelling and potentially toxic fumes, so better you enclose the machine and provide adequate ventilation. Also, for wood, paper and such, you will need a strong air flow to ensure clean cutting without burn marks.

The setup is now arranged in the basement, so that it can be used quickly and without setup time. There is a metal plate inside, zinc plated steel, so thin materials can be fixed by magnets.

The enclosure, made from 15×15 mm square steel tubing, painted, and the openings closed with white PVC sheet, and yellow (laser-blocking) Plexiglas.

The exhaust is a fan I had handy, a Dalap AP series 125 size, it is quiet and powerful, but surely any similar fan could be used.

There are some openings around the cover (upper) part of the enclosure, accordingly, air can enter and flush out the fumes. The off-gas is connected to an old, disused chimney.

Next, we need a reliable air source. In the main workshop, I already operate a larger air compressor, but it is noisy, and there is no pipeline to the house. Rather than building such pipe system, I decided to setup a second compressor, a quiet compressor, to make the work with the laser cutter more comfortable (hard to focus on any work close to a running compressor…).

It is a Hyundai brand silent compressor, quite decent built quality, and inexpensive for what it is.

With these data, it is running about 30% of the time, when the cutter is taking the full amount of air.

The tank is running with 6-8 bars pressure, by on-off regulation. The line pressure is set to 5 bars, so the pressure to the laser cutter system is stable.

There is already a moisture (water droplet) filter at the compressor, but I added another air filter, a simple model, EIF 4000-04, which is a centrifugal filter including a 5 micron particle filter. This is prevent particles from getting into the needle valve (potentially affecting or blocking the air flow), and removing any water droplets (condensate) in the line.

For easy use, there is a cut-off valve, and precision needle valve (Festo GR-QS-8) to set the air flow at the desired value.

The GR-QS-8 was cheaply available, but sure any similar precision needed valve will do.

The flow meter has a built-in needle valve, but strangely, when using this valve (partially closing it at inlet pressure of 5 bar, outlet pressure basically atmospheric), it causes the metering sphere to rotate quickly and with noise, showing completely incorrect readings. So I believe the design of this built-in needle valve is somewhat flawed.
Be sure to install any valve BEFORE the flow meter, because if you operate the flow meter under pressure, it will show completely incorrect readings. 16-18 L/min is plenty enough for the cutter to work without any burn marks. I have not optimized this much, but maybe you could also work at 12 L/min for most situations.

Cutting plywood works just great, with maybe 0.2 mm cut width.

All the contours are nicely defined.

Even stars or pointed objects can be cut without any trouble. These are just about 3-5 mm size!

With some materials, like, rubber and aramid enhanced seal papers, these don’t cut well, or not at all. And even the vendor (Klinger of brand Klingersil) doesn’t recommend or even support laser cutting of these materials, such seals still need to be cut or punched.

Other seal materials, like, reinforced paper (cellulose) materials including Elring Abil brand materials, these could perfectly fine.

Hewlett Packard HP-85B: a marvelous desktop computer revived

At its time, the HP-80 series of computers were really desirable and expensive computers, mainly intended for control of test equipment and associated calculations. There is a screen, a printer, a tape storage device, all in one case.

For its age, still looking great!

Some cleaning of the CRT, and some other parts of the electronics to remove dust. But no other repairs were needed to the electronics.

All the shielding, all the parts, it seems to have been hand-assembled in small series, with each part individually checked and hand-labeled…

Only trouble is with some stuck keys. These can be operated, but don’t spring back easily.

Unfortunately, a common problem that can’t be fixed easily. Reason is the age, fatigue of the plastic. So I just switched the few broken keys with less-used characters. This is easily done by pulling on the white actuators firmly.

Surely, the HP-85B had quite severally limited memory, and there were many programs available that would need to be purchased piecewise, but thanks to great enthusiasts (google: EBTKS), there is a solution: the EBTKS extension board will provide huge additional memory, mass storage, and a replacement of the tape drive.

A little test program, to set the time, date (no year 2000 problem…), and even the printer still works (just needed a little cleaning).

The paper is thermal paper, but easy to read and can be even used to print simple matrix graphics.

Baking 1&1: delicious cocos cookies

These cookies are not only delicious at xmas time:

250 g soft butter
250 g sugar – beat thoroughly
add 1 egg – beat again thoroughly
add a dry mixture of 1/2 baking powder, 200 g cocos (ground), 250 g flour.

Make rolls of about 2.5 cm diameter (about 30 cm long so that you can easily handle). Let these harden in the fridge for a few hours or overnight. Cut slices about 1 cm thick. These slices will change shape while baking. Surely you can also make other shapes.

Bake at 185°C upper-lower heat for 12 minutes (until the edge is just a little brown).

Baking 1&1: hazelnut cake

A very quickly made cake, if guests announce their coming unexpectedly:

100 g soft butter
80 g of full far margarine (80% fat)
150 g sugar
3 eggs
beat thoroughly.
add 90 g milk, mix thoroughly.

add 180 g flour mixed with 1/2 baking powder (or self rising flour) and 200 g of ground hazelnuts.

Bake at 180°C upper-lower heat for 40 minutes.

Let it cool down in the mould for 15 minutes, then remove.

SimonsDialogs – A wild collection of random thoughts, observations and learnings. Presented by Simon.