It's raining elephants – VA2GKA

†2013-03-07

No more post here, the blog is converted into static pages

Recently, I had the opportunity to try FEST3D (Aurorasat), a professional software used to design microwave filters and passive components based on waveguide technology. It can analyze and optimize almost every kind of topology found in this field.

I especially appreciated the very large number of examples available out of the box, because as a not-expert in microwave applications, it allowed me to see all the work made by generations of RF engineers. And there is a diversity that I had not suspected!

About the speed of calculations, the software is simply amazing! The approach is to put the components into equations and calculate the solution, unlike EM full 3D simulation programs which are very time consuming. The result is usually obtained in a few seconds, while an EM simulation software take several minutes to get the result, and up to a some hours to complete a full frequency range! Moreover, the results are very close to those obtained using full EM simulation.

Fest3D main interface, reentrant cavity filter example

So, I played with this nice piece of engineering to calculate some band-pass filter, low pass filter and duplexer for our hyper-frequency amateur bands. But, my constraints for Ham bands are not the same that the professional ones. In fact, the pros often need a high bandwidth, and the band pass filters are designed to drop any inter-modulation distortion on the adjacent band. For me, with only some kHz on voice and and less one hundred Hertz using CW, I don’t have these kinds of problems (BTW I can swim :) My constraints are more focused on the construction easiness and the rejection of even and odd harmonics.

Another impressive feature, but totally out of scope for me, is the capability to estimate the maximum permissible power. I can see that my filter can supports up to 2.8kWatts, fun for me, but for sure a precious tool for a doppler radar application or a satellite broadcast design.

I will write a new post soon about these designs but I want to present my new article on slotted antennas first. Hey, first the antenna, and after, the filtering bank :)

Interdigital filter blocking a frequency (calculated with Fest3D, ParaView for the render)

More examples here.

“Coupling Designer” is a new software allows to synthesize and optimize RF and microwave filters. OK, another you will say, but this one runs on iPhone/iPad !! Ôo

Again, on iPhone/iPad… Funny, and for 30$ only. Hihi, why not ! :)

Check the website here : Coupling Designer

Braahh… I made a mistake in my last order and I got a 12.1 ohms resistor instead of a 12.1k ohms. By misfortune, the value of this resistor is critical and I can’t replace it by a 10k or something close. It’s used by the internal voltage regulator of the Ethernet chipset (LAN8720A). So, I made an extra order, payed my 11 cents for the component and the rest for the shipping cost :/

Beside, I assembled and tested the first filter. The result is excellent and I can’t expect have a better response. This band pass filter (4 poles, Butterworth) was originally designed to be used on my contest transceiver, with some close antennas in the same area. For this board, I scale it down with a maximum permissible power is 10 Watts (air cooled) on a side band (ex: parasitic TX on the 14 MHz, while the RX is centered on 7MHz).

I use a miniVNA for my measurements and I cannot read below -40dB, but the expected results looks good. The attenuation on 14 MHz should be around 70/75 db. The calculated insertion loss is around 0.5dB, and I measured in practice 0.66 dB, simply great! Some screenshots follows :

The LPF, using Amidon toroids (T37-2), and SMC capacitors

Schematic of the filter, and response (in red, a Monte-Carlo analysis)

Zoom on the top flat, around 0.5dB of insertion loss

S11 response, 1-30MHz (measurements here)

S11, close view on the 40m band (measurements here)

S21 response, 1-30MHz (miniVNA can’t read below -30dB with the directional coupler in place. Measurements here)

S21, close view, -0.66 dB on the flat top (measurements here)

I received my new PCB this Friday and started to solder it with a new approach. Instead of working by stages, I soldered every series of components. It’s very fast because the components identification is visual and easy on the computer. It’s a bit boring for the large series, but I the counterpart is I soldered more than 350 surface mount components in less 4.5 hours. I used the solder mask (steel foils cut by laser) to put the right quantity of solder paste and the result is simply excellent. Now, I have to winding the toroids and finalize the board. Some pictures of my new prototype :

Fresh new PCB, top face

Bottom face

Close view on the solder paste pads

The final board soldered, without the inductors and transformers

Bottom face of the final board

I just finished to design a new version of my remote SDR, with some improvements and corrections.

The final Gerber render. Outch my eyes ! :)

Update :

Component face

Solder face

Cool, another free EM simulation software : emGine. The interface is close to CST Studio, visuals are nice and it seems to be fast. I made some test for fun :) Try it here : http://www.petr-lorenz.com/emgine/

Unwanted solder joint between pin 1 and 2 (flux teardrop at right)

Audio Codec

Clean pads for the Pic32

Band pass filter, Amidon T25-2

Now, I have to put extra effort into the code to finalize my transceiver.

My board with the programming/debugging Microchip interface

Yearrrr! I received my PCB Friday, just in time. The quality is here, and it’s another good point for PCB Layout. The day after, I started to solder a large part of the SDR transceiver, but unfortunately, I missed the CODEC in my components order… Braaaa, components are delivered in J+1 ^^

TinySDR borad, my 4 layers PCB

Colors are fine for Christmas :)

For my next 2m band SDR transceiver, I plan to build two different architectures, because testing them is probably the best way to decide which is the most valuable for me.

The first will use a classical topology with an FI on 30MHz. The front-end LNA sends the signal to a mixer, the FI was demodulated by my HF Tyloe mixer, converted by a 24 bits ADC and sent true Ehernet on HDSDR. The mixer chosen for this first version is a ADL5350. Nothing exceptional on the performance side, but I’m sure it will do the the job, and it’s an easy way.

The second one is a zero IF transceiver. Regarding the huge amount of cell phone sold by year, we can say now that it’s a classical topology too :) Usually, the mod & demod chips integrates a phase shifter (90°) and I can reuse my PLL design based on a ADF4157. After this stage, the I/Q signal is sent to a OpAmp (LPF + Gain) and after to the ADC. I selected the duo LTC5584/LTC5598 for this design, but at higher frequencies, SKY73012 & SKY73010 should be  fine too.

I cannot prognostic the winner, but I’m sure it will be very interesting to test and compare :)

In a few words, I completed my 4 layers board, the gerber files were sent and I’m now waiting for it. This prototype is design to work on the 40m band, because it’s easier for me to debug it at these low frequencies. The next versions will operate at 145MHz and 1.3GHz, with an IQ and a classic mixer. The different parts could be clearly identified on the board (power, microcontroller, ref clock, Ethernet, RX mixer, TX mixer, PA, RX filterbank, TX filterbank) and I tried to keep it as simple as possible.

The next step is to solder SMC components (small 0603 for res & cap) and test the microcode. All the data will send over Ethernet and HDSDR will process it, if everything works fine :)

TinySDR, my SDR board, for receiving/transmitting over Ethernet

Update : Production process @ Beta Layout