All posts by Simon

Regulated High Voltage Power Supply: Switchmode control, 30 kV, about 1 mA

2014/12/08 Simon 12 Comments

Disclaimer: This circuit description is for your information only. Do not attempt to duplicate! Danger! High voltages can kill you!

For many purposes, a regulated high voltage supply can be very handy. Purchasing one is not easy because such supplies are in high demand by all hobbyists that love sparks, and used units are quite expensive.
Fortunately, high voltage transformers are much more commonly available, from TV sets, so-called “Flyback” transformers. These provide, depending on type, up to 30 kV of DC voltage, at a pretty decent power of about 20-30 Watts.

Quite a few circuits are around, to provide drive signals for flybacks. These work, but often only for a short time – after a few sparks, the driver stage transistor blows. And, most of them have no means of adequately controlling the voltage.

Control of the high voltage typically requires measurement of the high voltage, a task that is not easily implemented – requiring expensive and bulky divider resistors.

The circuit described here is time-proven, and eliminates many of these shortcomings.

(1) Snubber networks around the coil and switching MOS-FET eliminate spikes in case of a shorted output (sparks act like shorts on the output).

(2) The primary winding – it is of very low inductance, just a few turns. This makes it easier to control the voltage spikes on the primary, and allows low voltage drive circuits (much reduced risk of electric shock, much better for amateur use). Still this circuit is dangerous! Do not attempt to replicate!

(3) Switchmode regulation with current limit – also this provides added protection against overload. Dead-time control limits the maximum duty cycle (power).

The design, all build around a real classic, a TL494 switchmode regulator.

Power is supplied via mains filter, and two transformers. Second transformer is only a needed for the LCD panel meter (Voltage display in kV). Note that the regulator and primary coil is on a floating ground. Earth ground is used for the high voltage coil, with a 1 mA full scale amp meter. Actual output current can be above 1 mA, depite the caption “0.5 mA”. Make sure to adjust the maximum current conservatively, and also the dead time, to keep the output power limited – otherwise, the flyback transformer will suffer and eventually stop working.

The most interesting part, the flyback driver and snubber circuits. The snubber circuits were designed with a lot of effort, using a scope to probe the overshoot voltages, etc. – if you change the flyback or primary inductance, make sure the check proper dampening! The BY329-1200 is a fast diode, with rather slow (soft) recovery. This will lead so some extra power losses, but this can be tolerated here. The VDRs add some more protection, but actually, they were added more for peace-of-mind than for any real purpose.
One thing to be improved in further design updates is the gate drive: the gate to source voltage is currently the full voltage of (about 24 Volts), it works, but it is running close to the limit of the IRFP450 device, or even above the limit. Furture circuits will include an independent linear regulator, to run the TL494 from a 15 V supply, derived from the main supply, and maybe a Zener diode added to the gate drive signal path, for added protection agains gate voltage excursions.

The IFRP450 is driven rather hard, via a 47 Ohm gate resistor, and has very small switching losses. No heatsink required, mounting it to the rear panel will provide ample heat dissipation.

The flyback itself, a 0100170 equivalent to HR 8409 type. The only coils used are the secondaries. The high voltage coils, for the output, and the 11-12 coil, for the voltage feedback. The voltage derived from this coil by half-wave rectification is a very accurate representation of the high voltage output. This has been checked at multiple voltages and load conditions!
The primary winding is 8-1/2 turns of rather heavy copper wire. You can also twist multiple thinner wires, if no thick wire is handy. I used double-isolated type; the device below is just a lab demonstrator for personal use, for professional cirucits, it is suggested to use PTFE (Teflon) or silicon tubing to provide additional isolation of the primary winding from the ferrite core.

Keep your fingers (and other wires) off the other primary windings!! These carry dangerously high voltages, at significant power, and might be more dangerous than the acutal high voltage secondary!

The actual unit:

The rear panel has the high voltage output: a big isolator, machined from HDPE plastics, with a 4 mm receptacle hidden inside.

Stepper/Multiphase Motor Control

Stepper and Multi-phase Motor Control: LMD18245 based driver

2014/12/06 Simon Leave a comment

For quite a few projects, I need to control DC, stepper or similar motors, with moderate power, anywhere from 0.5 to 2 Amps. For smaller motors, I have a well-established circuit using ULN drivers (to be described elsewhere), and for powerful motors, I generally use the reasonably inexpensive Leadshine or Leadshine-compatible controls – but for the intermediate range, below circuit has provided great service in many applications over the years.
Mostly, it is used together with bipolar stepper motors, like, in a big engraving machine build about 10 years ago. Recently, I re-used the design to control a rather uncommon 3-phase stepper motor.

The original prototype:

The key part is a LMD18245 from National, now, Texas Instruments, about USD 10 per piece. This is a full H bridge, with 4-bit DAC current control, integrated diodes, and utilizing DMOS technology. It is working up to about 3 Amps, 50 Volts; and has overcurrent/overtemperature protection. Not bad, and it allows for very small designs, without going to the trouble of thermal engineering of power SMD components used in more recent designs (and reliability issues, if such design is not properly done).

The LMD18245 uses a remarkable current sensing technique – the main DMOS switches are made up of about 4000 elements, and only one of these is used, along with a current sense amplifier, to provide a 4000:1 scaled version (250 µA per Amp) of the coil current. This eliminates the need of heavy/expensive low-ohm low-inductance resistors.

To protect the circuit, two capacitors are used – a 1 µF film capacitor, close to the VCC input of the LMD18245, and a 470 µF electrolytic (1 for each pair of phases).

The digital interface is very simple, and has been used in assemblies of multiple motors/multiple phases with success. The data bus input is buffered by 74LS374 edge-triggered D-flip-flops. Many units can be connected to a common bus, using a ribbon cable, and solder bridges for the address (LS374 clock) lines.

Typically, these are set by a micro-processor, using a look-up table (if MCU pin number is limited, a shift register, 74LS164 or similar, can be used instead). This allows full control of magnitude of current (4-bit DAC), and direction (via H bridge).

Judging from experience, the LMD18245 is a very robust device that can be employed of all kinds of motor control, in particular, if you need a easy to implement, but still fully customizable, reliable solution.

Miscellaneous

FKM349VL Benchtop Mill: control, EMC2 LinuxCNC interface

2014/12/06 Simon 2 Comments

The FKM349VL is one of many similar benchtop mills, Made in China. It’s size and power requirements make it quite suitable if you need a small machine that is still capable of machining aluminum alloy, and to some degree, even steel.

General characteristics – X travel = 490 mm, Y travel = 160 mm, Z travel = about 330 mm.
Table size is 700×180 mm

Spindle is MK3 with M12 draw-bar (this is the most significant limitation – only manual tool change!).

The linear stages use 16 mm, 4 mm pitch ball screws. Motors are 4.5 Nm, 6 Amp nominal. These are quite powerful, plenty of torque for this machine. This allows velocities of about 1800-2200 mm/min with no steps lost.

The control electronics are all housed in a cabinet attached to the machine. All pretty nicely made (motors powered by roughly 50 VDC, from the toroidal transformer; the blue transformer provides 12 VDC for the control circuits):

The stepper drivers – Leadshine units, up to about 5.5 Amps, configured for 4.3 Amp peak, 3.1 Amp RMS. Type MD556, V2.5. The units are similar to the Leadshine M542 and M752 units. Aka, KL-5056, aka, Rhino RMCS-1102 – many similar units exist.
The stepper motors have 200 steps/rev; the drivers are configured for 8 microsteps per full step – this results in 1600 steps per rev.

By default, this machine came with a “CNC-Workbench” CNC controller, offered by W+W Automatisierung (www.ib-weigelt.de). I gave it a try but soon found out that it is not up to my requirements; it’s a nice little controller, for what it is, no complaints, but really only for very basic uses, and difficult to interface with other CAD/CAM software. Most of my other machinery either uses industrial control, or EMC2 (LinuxCNC), so the decision was soon made to adapt the control to EMC2.

EMC2 has a powerful hardware layer, using the parallel port for control input and output. To allow proper speed and noise immunity (very important if you don’t want to run into issues!), a little interface circuit was fabricated, on a piece of perf board:

Nothing too fancy – low pass filter, Schmitt trigger, LED driver (the stepper driver use optocoupler inputs). The limit switches are combined by diode OR connections, switches are normally closed – to prevent machine damage in case of a broken wire.

The internal interface of the machine, originally used by the “CNC-workbench” controller uses a pretty uncommon high density D-SUB connector – 44 pins!

First time I have seen this type of connector, but it offers a fair number of contacts, for a pretty reasonable price, and quite a bit of soldering effort!

The software implementation – let me know in case you need the EMC2 HAL files for reference. Also attached a little incremental encoder as a “handwheel”, using a second parallel port. Quite amazing what you can do with a second hand computer, a few parts, and free software!

Micro-Tel MSR-904A Microwave Receiver

Anodizing 7075 Alloy: Micro-Tel handles

2014/12/02 Simon Leave a comment

Quite a few good tutorials exist for anodizing of aluminum, and pretty decent results can be achieved in any home shop equipped with a a sink and a few chemicals. For good results, with all the basic items mastered, the most critical item is the aluminum alloy. Generally speaking, any type of pure aluminum, and Al-Mg alloys are very much suitable for the anodization process. Zn, Si, Mn (and to some extend, Cu containing) alloys don’t work well.

The handles for the Micro-Tel MSR-904A receiver were machined from 7075 alloy, because of its strenght, and availability. 7075 has about 6% Zn, and 1.5% Cu, both of these elements are known to cause trouble when anodized. However, one can still try.

A quick, step-wise description of the process:

Step (1) – throughly clean/degrease the workpiece: first, I use methylated spirits, then, hot water and detergent, then rinse with water. Wear clean gloves.

Step (2) – etch with about 10% caustic soda. Room temperature.

As you can see, the part will turn black. This is typical for certain alloys.

Step (3) – use about 5% nitric acid to remove the black layer. Dip for a few minutes only. There will be some faint grey residue which needs to be brushed of mechanically (use a very clean brush – otherwise, it will contaminate the surface). Had to repeat the caustic etch twice to get a uniform and shiny surface.

Step (4) – anodize. Mount the piece with heavy aluminum wire. For 7075 alloy, pure aluminum wire works. Alternatively, use thick titanium wire. Current needed is about 1.5-2.0 Amps per 100 cm2. I used 2 A, for the handle. As cathode, use a sheet of aluminum, lead, or titanium. I use just plain aluminum and it is working just fine. For the liquid, about 15-20% sulfuric acid (dilute 37% battery acid with destilled water 1:1 ratio). Keep at room temperature, cool with some ice (applied to the outside) if it heats up too much. Typical time needed is 30-60 minutes, depending on the temperature and layer thickness. Don’t let the acid heat up too much – the layer will stop growing.

Step (5) – densify by boiling in distilled water. Needs to be really boiling, not just hot!

After the first attempt – everything looked fine after step 4, but the handles turned pretty dark after densification.

Pretty much, a full failure.

So, etched off the oxide layer with 10% caustic soda, and repeated the process, with two modifications:

(1) Keeping the acid rather warm, about 30°C, and reduced anodizing time to 20 minutes. This will give a thinner layer.

(2) Added a bit of ammonium acetate to the water used for densification. You may also add a very small amount of acetic acid. Keeping the bath slightly acidic prevents darkening during the densification process for Zn/Cu containing alloys.

The final result:

It’s not perfectly silvery color, but a slight yellow-orange color (like lightly colored wood). And the layer is certainly not very thick. But good enough to protect the 7075 alloy from forming corrosion spots over time.

Miscellaneous

German Bakery: sourdough bread, ‘quark’ variety

2014/12/01 Simon Leave a comment

Living in the US, I don’t want to miss good home-style bread. Sure, all kinds of bread are available here, but at a price, and you never really know what is inside.

This bread is about 60% rye, 40% wheet. Rye flour always requires the use of sourdough, and we have to consider two cases:

(1) You are the proud owner of some sourdough, or have a friend that has some.

(2) No sourdough at hand. Don’t worry. Sourdoug can be prepared, without any starters, at home.

(a) mix 50 g of dark or semi-dark rye flour with 50 g of water; stirr; leave at room temperature for 24 hours.
(b) add 100 g of dark or semi-dark rye flour and 100 g of water; stirr; leave at room temperature for 24 hours.
(c) add 100 g of dark or semi-dark rye flour and 100 g of water; stirr; leave at room temperature for 24 hours.
(d) add 100 g of dark or semi-dark rye flour and 100 g of water; stirr; leave at room temperature for 24 hours.
At this point, you should have quite a bit of sourdough ready, with small bubbles, and with no mold.

Some key items:
(1) Use boiled or otherwise chlorine-free water; tap water is perfectly fine but boil and let cool before use to remove any chlorination
(2) Use very clean utensils; store at a clean place – away from any sources of mold.
(3) Use a plain plastic or porcellain dish; don’t cover it; don’t use a metal dish.
(4) Temperature is fairly important – not too hot, not too cold.

For later use, best put some sourdough (~100 g) into the freezer.

Now, as you have sourdough now, let’s get started.

Step (1) – Take 100 g of sourdough, add 200 g of rye flour, and 200 g of water; leave at room temperature for 24 hours.

Step (2) – Add 400 g of rye flour, 500 g of water. Leave at room temperature for 24 hours.

Step (3) – Add 750 g wheet flour, 200 g of (warm) water, 30 g of salt (measure accurately!, mix in with the flour), 1 cube of yeast (available mainly in Europe – 42 g each; can be substituted by one package of active dry yeast), 500 g of ‘quark’ = curd cheese – low fat type.

Step (4) Knead properly, using your hands or strong machine; cover the dough with a towel, let the dough sit for about 1 to 1-1/2 hour to rise.

Step (5) Form any shape and size of bread. I prefer multiple small pieces. Use some wheet flour – the dough is fairly sticky.

Step (6) Let rise for about 30 minutes; preheat oven to 220-230°C.

Step (7) Bake. After 10 minutes. Reduce temperature to 170-180°C (depending on oven; I use a forced convection type: 170°C). Bake for another 45-60 minutes, depending on size of loaf. If you prefer a shiny crust brush with water (with a bit of corn starch added) immediately after removing from oven.

Step (8) Let cool. Don’t cut off pieces for at least 1 hours – other wise, steam will escape.

Enjoy!

Note – rye flour refers to semi-dark rye flour (not whole grain). In Germany, known as Type 1150. Wheat flour – best use any ‘bread flour’, in Germany, known as Type 1050. I use Type 405 because it is commonly available and it is amazingly cheap (currently EUR 0.32 per kg!!).

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Microwave Receiver: a set of new handles

2014/11/25 Simon Leave a comment

For a while I have been looking for a set of of spare handles for the MSR-904A, but to no avail; the unit was missing the handles – had some rough rack-mounting fixtures. Fortunately, most of the Micro-Tel equipment uses the same kind of handle, so at least I know how they should look like.

These handles are actually of a very nice design, and don’t look too complicated to fabricate. So I thought I would give it a try and machine a set of spares.

For the material I selected a piece of 7075 alloy T651 temper (fully hardened, stress relieved). This is a quite strong alloy, and it needs to be because the MSR-904A is heavy, and the full weight rests on handles if it is set down on the floor/carried around, etc.

How to machine such handles – well, it is best done using a CNC mill, and luckily, I have one in the basement, even if it is just a small machine.

It is a FKM349VL table-top mill, not a very heavy machine, but pretty capable if used correctly. This is not for heavy loads and throughput-optimized toolpaths, but it can yield very usable accuracy, and the surface finish is very nice, provided that good tools are used. Typically, I get parts that are reproducible to within 0.01 mm, and absolute accuracy typically better than 0.03 mm, depending on how the machining goes.

Starting from a plate, first, the opening of the handle was machines, and chamfered. The original handle uses a radius chamfer, but well, I only have tooling for 45° chamfers around, fair enough.

Then, the holes were drilled, and countersunk, followed by machining of the left and right (short) edge.

A bit more tricky, the piece where the handle is mounted to the case. This is fairly thin, and some excess metal was left to allow easy clamping of the piece. Sure, there are other ways of achieving the same result, but it saves a lot of time if the vice doesn’t need to be re-adjusted, and if everything can be done in two or three clamp positions.

As one of the last steps, the excess metal is taken off.

All in all, considerably more effort than I thought:

(1) Cut the plate to approximate size; machine one long side flat

(2) Machine handle cut-out

(3) Machine chamfer (upper side)

(4) Machine left and right edge (short sides)

(5) Spot drill, drill, and countersink holes

(6) Turn around – align

(7) Machine other chamfer

(8) Machine mounting piece

(9) Mount set of 4 pcs upside in vice

(10) Cut-off excess

(11) Cut recess of mount piece, left and right

(12) Machine front chamfer, 2x

(13) Deburr, finish

These were the tools used –

And, not to forget, EMC2 with the Axis interface. Thank You EMC2 (LinuxCNC) team for providing such great software, free of charge!

I might still anodize the handles – but first need to do some tests with 7075 alloy (never anodized this alloy before, and don’t want to waste the handles!).

Miscellaneous

Xmas Bakery: Nougat Plätzchen(=small cookies)

2014/11/23 Simon Leave a comment

These are one of my favorite Christmas “Plätzchen”(=small cookies). A kind of nougat shortbread, covered with chocolate.
First, prepare a dough by thoroughy mixing/kneading (no machine needed, best use your hands):

250 g nougat (soften in microwave)

80 g butter (soft)

60 g sugar

300 g plain flour, German Type 405

1/2 teespoon baking poweder (throughly mix with flour)

a little bit of salt

1 egg

some vanillin powder (or vanilla essence)

Once properly mixed, make two rolls, and wrap in some plastic foil:

Let rest for several hours in the fridge, or overnight.

Then, form nice cookies, either manually, or by rolling it to about 3/16″-1/4″ thickness. Don’t roll too thin!

Bake in oven at medium temperature. For best result, use 165°C circulating air. Baking time is 10-12 minutes.

Let the cookies cool to room temperature. Eventually, your efforts will result in a pile of cookies ready to be chocolate-coated.

Then, melt

200 g baking chocolate

150 g of really dark chocolate

using a water bath (also to keep the chocolate warm during the coating process).

For guests that are worried about too much chocolate, it always is a good idea to leave some of the cookies partially covered.

Use non-stick paper, and let the cholocate solidify for at least 18 hours before packaging.
Store in odor-free, dry place. For best taste, let them rest for at least a few days.

Miscellaneous

Xmas Bakery: Christstollen

2014/11/23 Simon Leave a comment

The time is right to get started with some Christmas bakery. Stollen, a long standing German Christmas tradition. It comes in various kinds and modifications, but there is only one truely original recipe.
Don’t use any self-rising flour, or baking powder – really Stollen always uses yeast to rise.

1 kg plain wheat flour, Type 405 according to German industrial norm (DIN) no. 10355

60 g of baking yeast (about 1.5 regular yeast cubes), can be subsituted by active dry yeast, but the real thing is always baked with fresh yeast, commonly available at all German supermarkets.

250 mL milk (warm up; not too hot!)

First, to get the yeast started, put all flour in a suitable bowl (plastic bowl most prefered), make a hole in the center, put in the yeast and mix with some milk to form a semi-liquid dough. Cover with some of the flour, and let it sit for 30 minutes.

This is how it should look like.

Add:
1 egg, 1 egg yolk (some traditional recipes don’t use eggs, but I highly recommend it, otherwise, it will be a pretty ‘flat’ Stollen, unless you have a suitable mould)

180 g sugar (extra pure saccharose – just kidding; refilled with ordinary, plain sugar)

200 g butter (room temperature!)

and, the remaining part of the milk.

My traditional recipe doesn’t need any almonds – but these are common additions to other Stollens. Therefore, you might add 125 g of ground sweet almonds, and 50 g of ground bitter almonds, along with some more milk.
Also, you can add mace spice, but not too much!

Knead to a firm dough. If too dry, add some more milk. Don’t add too much, dough should not be sticky or too soft! Properly knead with your hands. No machines are required for this recipe.
Let this dough raise for at least 60 minutes at a warm place.

In the meantime, we prepare the additions:
200 g citronat (rather finely cut!) – candied peal of a special type of lemon
100 g orangeat (finely cut!) – candied orange peal
500 g raisins (wash throughly with at least 1 liter of boiling water, and pick out any that don’t look 100% perfect)

Key thing is that the citronat and orangeat are finely cut, nothing is worse than big chunks of this in the final Stollen.

Once the dough has rested long enough, the citronat, orangeat and raisins are added – best by flattening out the dough to about 3/4″ thickness, additing the ingredients, and then folding the dough – this will reduce the effort to form a uniform dough.

Finally, form a Stollen:

Make sure to remove any raisins from the surface (these will scorch and give bitter taste) – just push them in, or remove and eat.

Let the Stollen raise for about 30-45 minutes (avoid air drafts, keep in well-heated room).

Then, bake in a pre-heated oven. Temperature: For best result, an electric baking oven is much prefered, circulating air type. Pre-heat to 200°C.
Insert the Stollen. This will help with retaining the shape.
After 10 minutes, set temperature to 165°C, and bake for another 60 minutes (cover with a piece of aluminum foil after 30 minutes).
Test with a wooden stick – if no dough adheres, everything went fine. Otherwise, leave for another 10-15 minutes.

While still hot, apply about 200 g of molten butter with a soft brush.
Cover with a layer of powdered sugar (prefereably, use pure powdered sugar without any other ingredients that are commonly use in ‘icing sugar’).
Carefully wrap in aluminum foil to provide a good seal.

Now, the most difficult part, let it rest for about 4 weeks at a reasonably cool and dry place (guest bedroom, corridor, or similar place that is free of strong odors and free of damp).

Note: this recipe is all in metric units, rather than cups, ounces, etc, because this is about exact replication of a traditional Stollen. This will only work if done accurately, with precise weights, and well-controlled temperatures all throughout the process.

Wiltron 6659A Programmable Sweep Generator

Wiltron 6659A Programmable Sweep Generator: EPROM images captured, some rubber items

2014/11/17 Simon Leave a comment

The 6659A is running on no more than 10 kilobytes of code, stored in 5 pcs 2716 EPROMs. These were programmed in 1984 – just about 30 years ago. Just in time to capture a copy. Well, not just of their exteriors.

Their contents –
wiltron6659a
-also contained are the data of the 4 frequency correction EPROMs.

Looking through the board, a very intriguing part: a DAC72C-CSB-I

It is a 16 bit DAC, 30 ppm INL, 7 ppm/K gain drift. Merely, a resistance ladder, with a build-in 6.3 V reference. Still going for about 40-50 USD each! Very impressive case, for 1984 vintage, and still working just fine, to the current day.

While silicon chips last, rubber is subject to aging – in particular, the 4 rubber stand-offs that were holding the fan. Removal was no easy task – they were glued in with some Loctite, but eventually, I could remove them, and they were replaced by regular screws – not too much vibration anyway.

R820T/RTL2832U USB SDR: Various hacks and measurements

USB RTL SDR 28.8 MHz Reference: VCXO

2014/11/16 Simon 1 Comment

One of the shortcomings of the USB RTL SDR devices is the build-in oscillator. It is actually very stable and sufficient for all kinds of everyday uses, but I am using these SDR devices for narrowband applications, with down-converted microwave signals. So utmost frequency stability is a must.

Not only needs to frequency be stable, it is also a good idea lock all oscillators to a common reference, which typically is derived from a 10 MHz rubidium source (like in my lab), or a GPS-controlled VCXO.

How to get from 10 MHz to 28.8 MHz – well, not all that difficult, but needs quite a few parts. First, we need a circuit that can receive 10 Mhz signals, and clean them up and prepare them to be used by a PLL. Then, we need a VCXO (voltage controlled quarz oscillator) that can be tuned by the loop filter of the PLL to keep it at 28.8 MHz. The loop BW will be very very narrow, a few Hz at maximum. Comparator frequency can be up to 400 kHz, the largest common divider of 10000 and 28800; but I might select a value more like 100 kHz which can be readily derived from a 10 Mhz reference. There are plenty of programmable PLLs around, but I might just use a hardware solution here (only need to put together :288 and :100 dividers using some TTL logic).

The circuit-

– nothing too fancy, and still needs some fine tuning. The xtal, it’s the original part de-soldered from the RTL SDR stick. These are actually pretty stable and well-behaved, at least for the devices I sourced from China.

The circuit employs a Pierce oscillator, build around a J310 FET. This is coupled into a common-bias amplifier, another J310, which provides the low output impedance. A matching network is added the make the circuit rather insensitive to changes in the load impedance. The circuits draws about 20 mA at 12 V. Not quite a power safer, hey, but this is not the objective here.

The items circled are just temporary parts, will need further optimization.

The big question – tuning range (pullability) of the xtal. Ideally, it should be a few 10s of ppm, to give the PLL some room to operate, and to account for aging effects over the years. Temperature-induced changes are on the order of a few ppm (see earlier post); but there is also drift, and other factors.

A quick test with some capacitors, and, stable oscillation can be found in a range of -1.8 to about 1.8 kHz around the 28.8 center frequency, this is quite satisfactory.

At the moment, still run with fixed capacitors, but I will add a varactor network to provide about 8 to 40 pF tuning capacity, by voltage input.
In an effort to keep phase noise down, I might employ a circuit used a lot for earlier projects, with anti-parallel varactor diodes.

The spectra look pretty clean, and the power is as expected, about -10..-6 dBm. I will use this output to drive the PLL, and add another amplifier to drive the RTL SDR R820T reference input – well shielded from everything else to avoid spurs from the divider and PLL circuits.

A quick test of the phase noise – hooked it up to a 3585A Spectrum Analyzer – there are some mains spurs, which will be reduced by adequate filtering once the circuit is fitted to a shielded box. Other than that, nothing really suspicious. All very close or at the noise floor of the 3585A.

SimonsDialogs

All posts by Simon

Regulated High Voltage Power Supply: Switchmode control, 30 kV, about 1 mA

Stepper and Multi-phase Motor Control: LMD18245 based driver

FKM349VL Benchtop Mill: control, EMC2 LinuxCNC interface

Anodizing 7075 Alloy: Micro-Tel handles

German Bakery: sourdough bread, ‘quark’ variety

Micro-Tel MSR-904A Microwave Receiver: a set of new handles

Xmas Bakery: Nougat Plätzchen(=small cookies)

Xmas Bakery: Christstollen

Wiltron 6659A Programmable Sweep Generator: EPROM images captured, some rubber items

USB RTL SDR 28.8 MHz Reference: VCXO

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