FEST3D

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-interface

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 🙂

InterdigitalWireframeWeb

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

More examples here.

 

My SDR – Nice band pass filter

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 :

LPF7M4P-web

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

LPF7M-Simul1

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

LPF7M-Simul2

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

S11_BPF7M_1-30M_Web2

S11 response, 1-30MHz (measurements here)

S11_BPF7M_5-10M_Web2

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

S21_BPF7M_1-30M_Web2

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

S21_BPF7M_5-10M_Web2

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

PA upgrade

J’ai bossé à améliorer mon petit PA avant le Field day de ce week-end. J’ai a présent un joli contrôle de puissance intégré avec la puissance transmise et réfléchie.

Les deux transfo des wattmeters visibles en gros plan

Le tout arrive sur deux galva pour un style tout smooth 🙂

Hey, le coté oldschool à du bon 🙂

En amont du Wattmeter, j’ai posé un filtre passe-bas 5 pôles. On voit clairement les 2 tores T130-2 sur la photo.

Filtre 5 poles avec les 2 tores Amidon

Au final, voilà la board du prototype. Le tout reste très simple, mais j’ai rencontré pas mal de problèmes intéressent et j’ai mis le doigt sur la réalité des spec’ constructeur. Prochainement, je ferais un CI plus propre en gardant le principe d’un PA par bande. En effet, le prix des Mosfet est ici ridiculement bas (mosfet d’alim à découpage) et parmi les éléments les plus chers, on trouve les tores et les relais. Alors tant qu’à faire, j’aime autant faire un PA dédié.

Concernant les perfs, la sortie est moins élevée que je que j’attendais, mais le bon coté est que l’IMD est meilleur que prévu 🙂 En PSK, je reçois des -27, -29 dB d’IMD et je n’en espérais pas autant. Update : Les reports sont soit vers -30, soit vers -20dB, et l’expérience m’a montré que l’IMD est vers -20. Ne pas croire tout ce que les gens peuvent dire 🙂 La polarisation est la même pour les deux Fets et les puristes auraient mis 2 polars ajustables, mais l’IMD me convient et j’aurais plus peur au final de dégrader la solution et d’avoir un système plus difficile à régler.

Nouvelle mouture de mon prototype, toujours en fonction et qui à déjà passé 1 contest et le Field day

Je mettrais le schéma du PA et le typon une fois le projet finalisé. Avant ceci, il me reste 2 gros morceaux à abattre, l’alimentation (actuellement, j’utilise une alim à découpage du commerce) et le frontend SDR sur IP. Le but est d’avoir une solution TRX amplifié autonome, directement utilisable avec un ordinateur connecté sur un réseau local. De cette façon, le TRX peut-être mis plus près de l’antenne, voire à l’extérieur, ce qui est un bon point pour les évolutions vers les hyperfréquences.

Je ne lâche pas mon idée et j’y bosse ^^