Imaging System Front-End at 202GHz Using LO/RF Isolation of

Harmonic Mixer for Illumination

Abdorreza Torabi

School of Engineering Science, College of Engineering, University of Tehran, Tehran, Iran

Keywords: Millimeter-Wave Imaging, Harmonic Mixer, LO/RF Isolation, Horn Antenna, Cassegrain Mirror, Field of

View, Concealed Object.

Abstract: A millimeter-wave imaging system front-end at 202 GHz is developed and presented. To have more compact

and economical transceiver and also due to the finite isolation between the LO and RF ports, a commercial

harmonic mixer acts both as receiver and transmitter. To improve the performance as well as imaging quality

an optomechanical system with ray optics designed configuration of mirrors is presented. Results of the 202

GHz imager for stand-off detection at 2.5m are illustrated.

1 INTRODUCTION

Active radio frequency imaging is utilized in two

main applications; radar cross section (RCS) and

concealed target objects detection. In RCS the wave

reflectivity of desired target is considered, while in

imaging applications of detection, the target objects

are obscured by one or more barriers which are not

transparent (Collins, et al. 1995). Active millimeter

wave (mm-wave) imaging typically points to

frequency range of 30GHz-300GHz (may also

includes lower microwave frequencies (Collins, et al.

1995, Huguenin, et al. 1993, Sheen, et al. 2001,

Sheen, et al. 2010)) is a topic of research interest due

to facts that X-ray or other ionizing radiation imaging

vehicles turns to be inconvenient, unsafe or

ineffective in some practical situations.

Active mm-wave imaging systems are capable of

penetrating common clothing and form an image of

concealed targets like weapons (Appleby and

Anderton, 2007) as well as a person's body.

Moreover, relatively short wavelength of these

systems, high resolution images can be achieved.

Several commercial mm-wave imaging systems have

been presented (TS4 and TS5 by Thruvision, Gen2 by

Brijot). Recent effort lies in developing cost effective

compact and robust systems to be used in the field of

such as airports (García-Rial, et al. 2019).

https://orcid.org/0000-0002-0016-9979

Cold sky radiation is the main source of contrast

for passive imagers at outdoor scenarios, make them

useful to detect and image the thermal emission of the

scene. Furthermore, the amount of attenuation for

passive millimeter wave radiation in poor weather

conditions like fog, snow, rain, dust is less than for

visual or infrared radiation in orders of magnitude

(Spinoulas, et al. 2012). However, at indoor

environment, absence of cold sky radiation make

imager to have higher sensitivity. This requirement

renders passive imagers made for indoor scenarios to

have lower imaging speed and higher overall cost.

One possible solution is to make use of active

illumination of the scene in order to create

radiometric contrast between objects of interest

(Sheen, et al. 2010). Bryllert et al. have developed a

transceiver module for a 3-D imaging radar at 220

GHz that consists of a frequency doubler which also

acts as a subharminic mixer (SHM) based on GaAs

semiconductor membrane technology (Bryllert, et al.

2013). Tang used infrared laser illumination along

with a passive mm-wave imager in order to enlarge

the radiometric contrast between different objects and

background (Tang, 2016). Petkie et al. developed an

imaging system at 640 GHz and concluded that active

mm-wave imaging systems can have large dynamic

ranges even with moderate illumination power

compared to passive imaging (Petkie, et al. 2008).

Torabi, A.

Imaging System Front-End at 202GHz Using LO/RF Isolation of Harmonic Mixer for Illumination.

DOI: 10.5220/0011685400003408

In Proceedings of the 11th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2023), pages 74-77

ISBN: 978-989-758-632-3; ISSN: 2184-4364

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

Figure 1: Block diagram of the implemented imager at 202GHz.

Canavero et al. have investigated most

applicable/suitable type of illumination source for

producing images with enhanced contrast and details

for W band imaging systems at indoor environments

(Canavero, et al. (2013). Finally, Grajal et al. have

demonstrated a stand-off imaging system

incorporating a compact front-end radar at 300 GHz

in which a commercial-of-shelf (COTS) SHM is used

as a transceiver (Grajal, et al. 2017).

In this paper active mm-wave imaging system at

202GHz with the use of COTS harmonic mixer for

both illuminating the scene and receiving the

reflected wave due to finite isolation between mixer's

LO and RF ports is presented. Using mono-static

power relation for desired configuration minimum

required antenna gain is derived for given minimum

detectable signal to noise ratio (SNR), noise

equivalent power (NEP) of the receiver and selected

harmonic mixer. To provide required radiation gain,

a cassegrain antenna configuration with focusing

point of the reflector system at 2.5m from front is

implemented to improve the performance of the

radiation of the horn antenna. A scanning system

including a scanner and secondary inclinating and

rotating mirror is used to spirally scan of the desired

field of view (FoV). The main advantages of the

proposed system are lower cost compared to

conventional active imaging systems, simple

hardware architecture and compactness due to

integration of transmitter and receiver modules. To

exclude very noise frequency noise like Flicker noise

contribution, amplitude (AM) modulation of the LO

signal is used. The results of the imaging show

excellent performance of the presented mm-wave

imager.

2 SYSTEM ARCHITECTURE

AND ACTIVE MODE

COMPUTATIONS

The mm-wave imager is a total power radiometer in

202 GHz frequency band. Block diagram of the mm-

wave imager is shown in Figure 1 which consists of

RF part including transceiver system,

optomechanical part including cassegrain mirror and

scanning system and finally data gathering part

including data acquisition and image formation and

processing.

In RF part, a scalar horn antenna with 10 dB gain

at boresight direction is considered. Harmonic mixer

of Radiometer Physics (RPG FS-Z220) is used which

works in 16

LO harmonic number with LO

frequency 12.6 GHz and LO power of 15 dBm which

is provided by Signal Generator HP8340. RF to IF

conversion loss is 30dB and nearly 26dB isolation

between LO/RF is measured and used in

computations. Down converted signal at IF frequency

amplified with the use of typical low noise amplifiers

so that the signal power reaches detection limit of

square low detector (HP 8474E). A function

generator is used to AM modulate the signal deriving

mixer’s LO port. A lock-in amplifier synchronized to

the LO modulating signal to detect the down-

converted signal buried in large background noise.

System noise temperature is given by

ys rec A

TTT=+

(1)

where

rec

T and

T are receiver and antenna noise

equivalent temperature respectively. The antenna

noise temperature is typically negligible compared

Imaging System Front-End at 202GHz Using LO/RF Isolation of Harmonic Mixer for Illumination

to the receiver noise temperature. For

rec

T we have

Frec M M

TTT+= (2)

where

T is mixer noise temperature,

is RF to

IF conversion loss factor and

T is LNA noise

temperature.

Using overall system noise temperature, noise

equivalent power (NEP) of the system at the specified

bandwidth

(10 MHz is considered) is given by:

sys

NEP kT B=

(3)

where k is the Boltzmann constant. Typically, mLO

to RF leakage in passive radiometer is regarded

undesirable. In this work we make use of the mLO

(m=16) to RF leakage as an additional means to the

mm-wave imaging system to illuminate the scene and

therefore configure an active mm-wave imager. The

resulting active image can serve an additional

information besides the primary passive image

(García-Rial, et al. 2019, Grajal, et al. 2017). In this

way, this setup represents a mono-static configuration

in which a single antenna is used for both illuminating

the scene and receiving the reflected waves. In order

to get a detection criterion in active mode, utilizing

mono-static radar equation and assuming that the

receiver is able to detect only vertical polarization:

()

22 02

max

εσλ

(4)

where

is backscatter coefficient,

max

is the

antenna gain boresight direction (neglecting

sidelobes contributions),

is free space wavelength,

is the distance of object and antenna and

P is

radiated power from the antenna and

is the

efficiency of the antenna aperture (typical: 80%). By

assuming

ource

P as derive power at LO port of mixer

and

/mLO RF

I as isolation factor of LO/RF ports, we

have

/tsourcemLORF

PPI= . Then the received power

should be satisfied inequality;

PNEP≥Ω , where

is the minimum detectable SNR. Substituting from

the equation derived above we have:

()

22 02

max

source mLO RF sys

IkTB

εσλ

≥Ω

(5)

To help inequality (5) to be established one can make

the right side of (5) as small as possible and the left

side as large as possible. For selected harmonic mixer

as well as other harmonic mixers,

ource mLO RF

PI has its

own certain amount and does not have much changes.

Smaller

required more sensitive and low noise

receiver which directly increase the total cost of the

system. But an efficient approach to have smaller

minimum detectable SNR is AM modulating of the

LO signal and making IF detector synchronized with

it (Figure 1). Frequency of modulation can be chosen

in [200KHz-800KHz] for

10MHzB = . Therefore

very low frequency noise sources like Flicker noise

would not contribute and make

smaller. In the left

side of (5), it is focused on the

max

. As illustrated in

Figure 1 and 2, a cassegrain antenna configuration is

used to redirect the incoming radiation from the scene

into the horn. This configuration has a circular field

of view (FoV) of 40cm in diameter focusing point of

the reflector is set at a distance of 2.5m from the front

panel. The location of the horn and the parameter of

two reflecting mirrors, are designed using ray optics

relations. The scanning of the FoV is performed via

synchronized rotation and inclination of the

secondary mirror which provides spiral scanning of

the scene. It is proved that this configuration of

optical mirrors with horn located in the aperture

(Figure 2) makes the main lobe of radiation pattern of

the horn more directive and conclusively improve

max

3 RESULTS

Figure 3 shows indoor experimental results of the

implemented mm-wave imaging system at 202 GHz.

They show someone standing at 2.5m in front of the

system with concealed metallic gun simulator and

pierced plate which their images are appropriately

formed and perfect detection is done with the use of

active illumination of power leakage LO/RF of the

mixer.

4 CONCLUSION

In this paper, with the use of finite isolation between

mixer's LO and RF ports for illuminating the scene,

active mm-wave imaging system at 202GHz is

presented. An optomechanical system with ray

optics designed configuration of mirrors is also

presented. Stand-off imaging of the target in 2.5m

PHOTOPTICS 2023 - 11th International Conference on Photonics, Optics and Laser Technology

Mirror 1

fixed

Mirror 2

Rotation-inclination

Scanner

Image formation

Lock in Amp

Horn

Antenna

(a)

Scalar horn

Power Source

12.6 GHz

Isolator

(b)

Figure 2: (a) Front and (b) behind view of the imager.

Figure 3: Results of imaging at 202 GHz with proposed

imager, metallic gun simulator and metallic pierced plate.

is implemented with scanning system including a

scanner and inclinating/rotating mirror. Such a

configuration of mirrors improve the gain of the horn

antenna located on the aperture of the system due to

dual reflection mechanism to satisfy the mono-static

radar equation required for detection. The results of

the imaging show excellent performance of the

presented mm-wave imager.

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Imaging System Front-End at 202GHz Using LO/RF Isolation of Harmonic Mixer for Illumination