The Mystery Box

The Mystery Box – A multiband software controlled receiver-transmitter for 50MHz…6GHz frequency range.

This is my latest project. Main feature will be simplicity. Only one USB connector for computer and one SMA connector for antenna. Other key features are small size 120*54 mm and wide operational temperature range. It will be also cheaper and more hackable than it’s competitors because there is no any FPGA’s.

https://www.xilinx.com/products/silicon-devices/fpga/what-is-an-fpga.html

I’ll probably program my own controlling software to that radio. Other alternatives are for example popular GNURadio, but it’s design is very complicated and there is not enough information on how to implement IIO environment to embedded system without before mentioned FPGA. I think IIO forms some kind of ring buffer to device, but this is where information ends.

There is however a plenty of information how to render audio stream on Linux operating system. For example on this page.

https://github.com/scottstensland/render-audio-openal

Then I only need to parse received audio from quadrature and in-phase signals from ADC. Unfortunately Wikipedia is very cryptic about these signals and so are most web search results. However there is I/Q data for dummies.

http://whiteboard.ping.se/SDR/IQ

This is one of rare pages that clearly states that I and Q signals are 2D representations of 3D world from side and from above. Very simple to understand. Worth reading!

Butterworth filter design

One new thing that I learned during development was calculating correct component values for Butterworth lowpass filter. I found almost perfect calculator from RF-Tools.com website.

https://rf-tools.com/lc-filter/

That was good starting point for filter design. Input values for that calculator that I use was following. Response: “lowpass”, type: “Butterworth”, topology: “Series First”, order: “8”, cutoff frequency: “57MHz”, input and output impedances: “50 ohm” and component values: “exact”.

My circuit uses 50MHz local oscillator. That’s why I selected 57MHz cutoff frequency so that only at most 50MHz signal goes through it without any attenuation. All received signals is downmixed to that frequency before filter. After filter there is quadrature demodulator circuit.

Back to filter design. From Wikipedia I found important information how to convert that filter circuit to it’s balanced version.

https://en.wikipedia.org/wiki/Balanced_circuit

Capacitors are untouched but inductors are doubled for both sides of circuitry and their values are divided by two. This topology is yet verified with CircuitLab.com circuit simulator and it works as expected. Frequency response is identical with original results.

Balanced-T attenuator design

That was easy part. All component values and circuit design was ready on that web page.

https://www.electronics-tutorials.ws/attenuators/t-pad-attenuator.html

I needed only to select component values from table with desired attenuation and then choose correct circuit version to have balanced design. Attenuation is needed because MCU’s ADC uses 1.8 volts analog voltage and quadrature demodulator outputs 2V p-p signal levels.

Arbitrary Transmission Line Calculator

http://atlc.sourceforge.net/

Atlc is a program which can be used to calculate differential PCB traces impedance among many other things. It seems to be very accurate when compared to other calculators on the Internet and I think it’s result is most accurate of them all. That’s why I selected it to be my main tool when calculating PCB traces.

First thing when using Atlc from command line is to produce data file which is then used to calculate result. Data file is actually a BMP bitmap and Atlc comes with several helper programs to make them.

I needed to design 50 ohm coupled lines so I used my favorite method that is trough trial and error and come to these values below.

create_bmp_for_microstrip_coupler -b 8 1.4 0.2 0.2 0.36 0.035 1 4.3 out.bmp

BMP size is selected with b option to be 8. It’s default value is 4 so now result accuracy is doubled. Trace width is 1.4mm and spacing between them is 0.2mm and spacing between traces and surrounding ground plane is 0.2mm. There is also ground plane below traces on the first inner PCB layer and that thickness between these layers is 0.36mm. Copper thicknesses are 0.035mm and air permittivity is 1 and PCB’s is 4.3.

Result is calculated by Atlc with this way.

atlc -d 0xac82ac=4.3 out.bmp<br>
out.bmp 3 Er_odd=   2.912 Er_even=   3.355 Zodd=  24.866 Zeven=  31.882 Zo=  28.156 Zdiff=  49.732 Zcomm=  15.941 Ohms VERSION=4.6.1

We can see from that result that Zdiff is 49.732 ohm which is almost perfect for our target use.