CitRad - Elektronik-Entwicklung

Ziele

  • Verstärkung der Radarsignale
  • Filtern des Rauschen auf den Signalen und Spannungsversorgung
  • Pegelanpassung an das Messsystem

Spezifikationen

  • Spannungsversorgung 3,3V … 5V
  • Verstärkungsfaktor im Bereich 1000x bis 50000x
  • Ausgänge: Line-Out (±1V), Logikpegel (0 … 3,3V)

System overview


The system is aimed to be felxible and ready to be used in different applications and setups. In genereal, to work with analog data from a radar module, the analog signal needs to be amplified. The gain depends on the actual application and distance one wants to measure in. The higher the gain, the more noise is in the system. Radar applications can range from simple setups of a radar module + amplifier board with a LM358 OpAmp + Arduino for data sampling to complex arrays of several radar modules with highly specialized amplifiers and ASICs for data processing. In this part we want to introduce possible solutions, so you can decide how much effort you want to put into your project.

Power supply

Pretty much all power supplies in commercial consumer products are switching mode power supplies. This means that they switch the voltage over a coil on and off. This causes a lot of noise, or ripple, on the voltage. Every small voltage variation will be amplified and will intefere with the radar measurements. This is why the power supply choise is crucial for radar applications.
The simplest solution is to use a 9V batterie and use a linear regulator to generate 5V from the batterie. There are no switching parts and the ripple is very low. However, a 9V batterie may not be sufficient for long measurements. A 12V car batterie might be better, but also more expensive and bigger.
To keep the system small we aim to use a USB connector for power supply and digital data transfer. This means we have to deal with the noise on the 5V USB line. For this reason, a power supply filter is implemented in the electronics. This is realized with a common mode choke and capacitors to filter the high frequency noise on the voltage lines. Additionally, a linear regulator is used to smoothen the voltage even further. The voltage feedback in the linear regulator works faster than in the switching regulator, which means there is less ripple. However, a linear regulator needs to have a higher input voltage than output voltage. It is not possible to go from 5V input to 5V output. There are regulators which require a very low drop-out voltage of 100mV to 200mV. These type of linear regulators are reffered to as Low Drop-Out linear regulators (LDO). Often the 5V USB line is in fact 5.2V to 5.5V, meaning it could be possible to use a LDO to smoothen the 5V USB voltage. To be on the safe side, a 3.3V LDO can be used.
For this reason the circuit is designed to work with 3.3V or 5V depending on the choice of power supply, radar module and amplifiers. The LDO can be short-circuited when the 5V USB voltage should be used directly.

Radar

Several radar modules are available. We are mostly focusing on models from Innosent and B+B Sensors due to the availability in our country, but all modules are very similar. We only use radars based on the Doppler measurement approach.
At the time of working on this project, most ready-to-use modules require a 5V voltage supply. The modules come on a PCB, which can be attached whever you need it, and are connected via jumper wires. The 3.3V modules are only available as bare components and require more work to fixate and contact the wires.
The modules can have different types of output. Since we are doing the data evaluation by ourself, the module needs analog outputs. The modules can have 1 or 2 outputs. The first output is I, or Inphase signal. Its output is the frequency shift generated by the Doppler effect. One output is enought to detect movement and the velocity of the movement. The second output is Q, or Quadrature output. It is similar to I, but has a phase shift of 90°. By comparing I and Q it is also possbile to determine the direction of the movement.
For example, a 3.3V compatible bare module would be InnoSenT SMR-333 and a 5V module on a PCB would be B+B RSM2650. Both modules have two outputs.

LNA

A Low Noise Amplifier (LNA) is a circuit with a OpAmp, which adds nearly no noise to the signal. Every electric component generates noise due to the movement of the electrons and heat generation. These specialized OpAmps are build in a way to reduce these electric and thermal effects. However, since the are specialized, they are also expensive. They are not required when the measurement distance is low, let’s say <2m, but become mandatory when the distance is larger. It is up to you to deceide whether you want to use expensive low-noise OpAmps or cheaper generic OpAmps.

LPF

To reduce the noise on the signal even further, a Low Pass Filter (LPF) is added to the LNA amplification stage. It is designed with a corner frequency of 10kHz. This means that the signal is attenuated by -3dB at 10kHz. The used radar modules operate at a frequency of around 24GHz. When the Doppler shift is calculated, this leads to a shift of 44Hz per 1km/h velocity of the measured object. With 10kHz we can roughly measure speeds of up to 220km/h, which leaves plenty of room for applications in and outside the city.

VGA

The Variable Gain Amplifier (VGA) is the last part in the amplifier chain. Depending on the working distance there might be a need to increase the gain. It is implemented as simple non-inverting amplifier. The gain can be set by potentiometers.
This stage could be accompanied by an Automatic Gain Control (AGC). In this case the gain is not set by manually potentiometer, but by digital potentiometers, which are controlled by a microcontroller. The microcontroller samples the analog data on the output and determines whether it is in a good voltage range or not. When it is not, the microcontroller sets the resistance of the digital potentiometers and can adapt the gain. While this is very handy, it is also complex to integrate and limits the versatility of the system.

Output

The amplified signal needs to be processed furhter by a measurement system. This can either be an Arduino or similar microcontroller or a PC. For this reason the output is limited to the voltage range of the input voltage, e.g. 3.3V or 5V, and can be connected directly via jumper wires to the controller. In addition, there is a line-out output, which can be connected to any line-in or microphone input on a PC. To use two channels a line-in input is required, as the microphone input only has one input line. Using an external USB soundcard it is also possible to attach it to a Raspberry PI or similar embedded PC.

Simulationen

LTspice Dateien
Input_Stage_10kHz_v2.asc (6,7 KB)
VGA_Potentiometer.asc (1,9 KB)
LC-Filter.asc (816 Bytes)

LNA

VGA

RC Filter Versorgung

Schematic + PCB

Tests for supply filtering

Two designs to generate and filter the 3.3V supply voltage for the radar module were tested. The noise on the output of the radar module was measured.

First the noise is measured while the radar module is supplied directly from the laboratory power supply with 3.3V. The resulting noise is about 45mV.

In the first design a 5V input voltage from the power supply was first filtered by a choke and then regulated to 3.3V by a LDO. The resulting noise is about 15mV.

In the second design a 5V input voltage from the power supply was first regulated to 3.3V by a LDO and then filtered by a choke. The resulting noise is above the set limit ov 80mV on the oscilloscope.

This design with choke → LDO was chosen for the circuit. The noise can be reduced more by introducing an LC filter or ferrite beads directly at the radar module or amplifiers.

Update PCB

Ist von der Größe so ausgelegt, dass es in ein 1457K1201 - Hammond Mfg. passt. Daten und Simulationen update ich im Github wenn die BOM fertig ist.