Update to Yesterdays update - Double sided PCB and Giga Hertz frequencies.

Well I was getting very weird results yesterday.......

........ but today I now understand them better!


I am used to using high frequency that is in this case  > 1 Ghz kit! I am not however used to building it! Well I have learned a new lesson for me!  Here am I trying to build a low pass filer for around 150 Mhz, I didn't think I needed to learn construction for >1 GHz, Wrong!

The filters I was building were exactly as planned and I was using double sided board with the lower side being a pure ground plane. However there behavior above 3-400 Mhz was wrong, and there behaviour at 1.2Ghz was just silly (2db attenuation when I expeced over 40db down!).

What I had not thought was that my Top layer ground fill was contiguous but greatly narrowed in places by the other components. I had made a series of resonators working in the Gigahertz ranges. I had built tuned slots and all sorts of things that were just not as they should be.

These boards are self designed and self fabricated. I had forgotten to put any vias between the top and bottom layers except at the ends where the SMA sockets connected the sides to each other.

Pick up drill, drill a number of via holes in sensible places. Insert copper wire in vias solder both ends trim off excess wire and re test.

Now my board is behaving with all below -40db above the 300Mhz level right up to 2.7Ghz. Most below 65db down and I could probably get the hump at 1.4G down if I added some more vias.


Lesson learned I may think I'm playing with 2M radio, that doesn't stop my boards playing up above 1Ghz. Grounding is critical watch how well things are grounded, and use vias!

--
Peter

Update on Crystal measurement

I am making progress.  I have been playing with a resistive fitting to do the impedance matching and following that by a small two transistor post amp to bring the signal back into a measurable range.

It's looking promising.

I am also experimenting with producing PCB's by toner transfer and that is looking promising as well using SMT components. I have started on building filters with SMT components and that is also promising but I have a lot to learn. I have built a 148Mhz low pass filer and it isn't quite right yet. It's come down to 140 Mhz. I think that's due to the components I have being only 5% and almost all the capacitors on the high side (or is that just my SMD device measuring tools?) Of course stray capacitance is present in all PCB's (and stray inductance as well!) I'm not yet good at compensating for that.

The worst part is I have a lot to learn about using SMD Inductors! The filer I built looked usable up to about 240Mhz then the attenuation rises and above 500Mhz I would rather not comment about but at some frequencies between 500Mhz and 2.7G (As high as my measuring gear go's) the attenuation was under 10db! As I say I have a lot to learn.

--
Peter

Making a headache by musing on electronics measurement

I have been experimenting in a field well outside my comfort area! I have started to play with RF construction and measurement. I have a reasonable set of test gear from my Radio Ham days but very little of it is suitable for serious "Research". Well I will never have the best gear but I either have or can build some measurement tools that let me start to get a better feeling for the fundamentals of RF.

Currently I have an RF Explorer Spectrum Analyzer (which is very good for it's price) and an RF Explorer Signal Generator (also a bargain). These together cover about 25Mhz to 2600Mhz (actually a bit more as the generator go's up to 6Ghz and the analyzer go's down to 10Mhz)  but I have started building some home brew kit to cover from about a Megahertz up to about 50 Mhz.

Signal generator / recording voltmeter

Front Crystal test jig / Back 3db splitter

Overview of setup

The first picture shows a PSOC5LP development board and an AD9851 module. This board takes the output from the AD9851 and converts from the 200ohm filter output to a 50ohm SMA socket using a bifiliar wound impedance matching transformer.  The PSOC5LP also supports a high resolution ADC which is used to measure the output voltage from the Log Amp that is in the diecast box on the left.
The code on the PSOC5 is partially mine and partially stolen from other open source projects. This code lets the PSOC configure and drive the AD9851. It also reads the high res ADC fed from the Analog Devices logamp.

The second picture shows a crystal test jig that is basically two SMA sockets with a pair of 4:1 impedance transformer to better max the crystal characteristic impedance which is around 111ohms.

You can also see the Mini-Circuits splitter (ZFSC-2-2500-S+) that I used in my tests.

I firstly ran the test with a wire jumper in place of the crystal, With nothing between the generator and the Crystal test rig, I then inserted a 32Mhz crystal and took another set of readings. I then inserted a Mini circuits splitter setup with a 50ohm dummy load on the second output. Next was a home brew resistive splitter also terminated on the unused output .



The blue line is the reference run with the short circuit instead of the Crystal.
The orange line is the setup with just the crystal.
The yellow line is with the 6db attenuation of the resistive splitter.
The grey line is replacing the resistive splitter with a high quality (transformer) splitter rated for 10-2500Mhz usage.


As you can see the grey line is the closest to the expected classical shape. at the extreme right the lines are spaced as expected in 3db intervals (The sensor has a slope of 25mv per db of power detected).

The left hand side is not as expected. The splitter insertion loss showing as near zero!
Also at the anti-resonant point the shape is much closer to what I would expect.

However as you see I have a lot to learn and understand. My head hurts!

By the way all of this is because I want to build a few crystal filters and to do that I have to characterize the crystals (I have a bag of 100 32Mhz crystal purchased very cheaply on ebay).

G4DCP - Peter D Hull

Musings on the Analog Devices ADALM1000

In this post I take a look at the Analog devices ADALM1000. This is an interesting piece of electronic kit that has not received very good reviews as far as I can see, this is a pity as this is a very useful piece of kit and Analog is working hard to improve the support software for this tool.

This piece of hardware is basically a voltage and current source. It is part of the requirements for a basic analogue electronics laboratory. On its own it is useless to anyone who does not understand the value of such a tool. As part of a collection of equipment for analogue experimentation it is an essential part.

The device has two identical channels channel A and channel B. Each channel can provide either a fixed voltage, a fixed current, a function generator offering sine wave, triangular wave, sawtooth wave, square wave, staircase wave with all of these available in either voltage or current sources.

The range of voltage available is from 0 to 5 V, and the range of current is approximately plus or minus 200mA.

At any one time each channel can be set either to source voltage and measure current or to source current and measure voltage or to high impedance and measure voltage.

The supporting software Pixelpulse2 is being developed as an open source product on GITHUB with Analog Devices employing one of the developers. The software is available for Linux, Mac or Windows. It is perfectly correct that the current documentation is extremely lacking and the number of people have failed to get this product working on Windows. This is sad because a working Windows version exists and is relatively easy to get hold of. The project needs to update the README file on  GITHUB so that people can easily find the released versions.

The current stable version can be found at:-
 https://github.com/analogdevicesinc/pixelpulse2/releases with a Windows installation package.

This version is basically functioning but has a few stability issues. These issues have been fixed in the latest version, which is available at:-
https://ci.appveyor.com/project/analogdevicesinc/pixelpulse2/build/artifacts 
what should be understood is this is a development version and changes as they develop the program. At any particular time this could be fully working or broken. If you have difficulties it is easy to raise an issue with the project and issues seem to be looked at quickly.

If you know what this tool can do and what it's limitations are it is a very value for money purchase.

If you buy this thinking it is going that you are going to get a complete course at this moment and that this tool is all you need then you will be very disappointed. Sadly the device is being sold  and the information on the websites and box is very optimistic, offering far more than is easily available today.

Analog on their site does have a full course of  instruction on analogue design however as yet this is more aimed at users of Diligents student tools than at this particular piece of hardware. Hopefully Analog will start rewriting this to use the ADALM1000 in the near future.

In summary if you know what this tool can do and what it's limitations are it is a very good value for money if you don't then wait till the documentation and other aids are available as of yet this is not a beginners tool.