**2.3. Receiver MMICs**

Two different types of receivers will be described in this section. Energy detection receivers are implemented for on-off keying (OOK) communications and localization applications, they are conceptually simple and do not require syncronization, but are also sensitive to interference. Correlation detection receivers are introduced to solve this problem, they are more robust to interference, but require accurate timing synchronization with the transmitter. This problem is eliminated in radar applications, because transmitter and receiver are co-located and synchronized with a common reference.

A fully differential UWB low-noise amplifier is a key element for both receivers. The LNA should provide a low noise figure, a high gain, a flat frequency response, and a small group delay variation within the complete frequency range. Another key component is a four quadrant analog multiplier, which performs the squaring operation in the energy detection receiver and the multiplication operation in the correlation receiver. Detailed explanation of these components will be described below.

### *2.3.1. Fully differential UWB low-noise amplifier*

Fig. 13(a) shows the fully differential UWB low-noise amplifier schematic. It consists of a differential cascode, followed by two emitter follower stages as buffers. Input and output are differential as the LNA will be connected to a symmetrical antenna, and shall feed a Gilbert cell type analog multiplier directly, without an unbal circuit. The symmetry of the emitter-coupled pair is achieved by placing identical transistors and passive components in the two branches.

T1 through T4 form the differential cascode which is biased by the stacked current mirror. The primary reason of the cascode configuration is to reduce the Miller effect at the input port, increasing the bandwidth. The shunt-shunt feedback (R1, C1 and R2, C2) further broadens

12 Will-be-set-by-IN-TECH

Normalized PSD / dB

**Figure 12.** Measured time-domain results of biphase modulated impulses with different applied voltages at the data port and the spectrum information of a 200 MHz impulse train with the data port

6.7 GHz from 3.1 - 9.8 GHz, which complies well with the FCC spectral mask for indoor UWB

Two different types of receivers will be described in this section. Energy detection receivers are implemented for on-off keying (OOK) communications and localization applications, they are conceptually simple and do not require syncronization, but are also sensitive to interference. Correlation detection receivers are introduced to solve this problem, they are more robust to interference, but require accurate timing synchronization with the transmitter. This problem is eliminated in radar applications, because transmitter and receiver are co-located and

A fully differential UWB low-noise amplifier is a key element for both receivers. The LNA should provide a low noise figure, a high gain, a flat frequency response, and a small group delay variation within the complete frequency range. Another key component is a four quadrant analog multiplier, which performs the squaring operation in the energy detection receiver and the multiplication operation in the correlation receiver. Detailed explanation of

Fig. 13(a) shows the fully differential UWB low-noise amplifier schematic. It consists of a differential cascode, followed by two emitter follower stages as buffers. Input and output are differential as the LNA will be connected to a symmetrical antenna, and shall feed a Gilbert cell type analog multiplier directly, without an unbal circuit. The symmetry of the emitter-coupled pair is achieved by placing identical transistors and passive components in

T1 through T4 form the differential cascode which is biased by the stacked current mirror. The primary reason of the cascode configuration is to reduce the Miller effect at the input port, increasing the bandwidth. The shunt-shunt feedback (R1, C1 and R2, C2) further broadens

−60 −50 −40 −30 −20 −10 0 10 20

0 2 4 6 8 10 12

FCC indoor mask Measured

Frequency / GHz

(b) Spectrum

−100

connected to ground.

**2.3. Receiver MMICs**

the two branches.

synchronized with a common reference.

these components will be described below.

*2.3.1. Fully differential UWB low-noise amplifier*

systems.

0

0 0.2 0.4 0.6 0.8 1 1.2 1.4

(a) Time domain

Time / ns

Data = 0 Data = 1.2 V

100

Amplitude / mV

200

**Figure 13.** Complete circuit schematic and the chip microphotograph of the fully differential low-noise amplifier.

the bandwidth and improves the input matching simultaneously. Careful selection of input transistor size and adjusting the bias point was done as a compromise between optimum current density for minimum noise figure, noise-matched input impedance and achievable bandwidth. The emitter size of T1 is chosen to be 0.5 *μ*m x 24.7 *μ*m and the emitter current is 5 mA. The wide band noise and input power match were accomplished by the selection of input transistor with suitable biasing and shunt-shunt resistive feedback. A negligible penalty, with a maximum value of 0.2 dB, is achieved within the entire band for not achieving noise match exactly. T5, T6 form a differential emitter follower buffer. The emitter degeneration capacitors are used to improve the buffer bandwidth.

The microphotograph of this differential LNA is shown in Fig. 13(b). Because this design is completely inductor-less, the IC has an extremely small size of 0.37 X 0.38 mm<sup>2</sup> including all bound pads. The lowest available metal layer was placed below the large-sized bonding pads to provide a ground shield, as otherwise the noise figure may be deteriorated by the substrate noise pick-up.

**Figure 14.** Measurement results of the S-parameter magnitudes and single-ended and extracted differential noise figures.

One drawback of the differential configuration is a complex measurement setup. Two identical passive microstrip line UWB baluns are used for differential S-parameter

#### 14 Will-be-set-by-IN-TECH 452 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications <sup>15</sup>

measurement. The influence of the baluns is removed during the calibration process. The measurement is restricted to 3 - 11 GHz due to the operating range of the UWB baluns. Fig. 14(a) shows the measured S-parameters. The non-ideal performance of the UWB baluns introduces ripples in the measured curves. The measurement results show a differential gain of 19.9 dB with a 1.8 dB variation, the input matching has a value of smaller than -7 dB and the output one is smaller than -6 dB in the complete FCC allocated frequency range. The method in [2] is adopted to extract the differential noise figure. First a single-ended noise figure F*single* is measured from port In+ to Out- with the other ports terminated by 50 Ω resistors. Then, by measuring the transducer gain from port In+ to Out- (G31) and Out+ to In+ (G32), the differential noise figure can be extracted as

$$F\_{diff} = 1 + \frac{1}{\mathcal{G}\_{31} + \mathcal{G}\_{32}} (F\_{\text{single}} \mathcal{G}\_{31} - \mathcal{G}\_{31} - \mathcal{G}\_{32}).\tag{5}$$

(a) Circuit schematic (b) Microphotograph

**Figure 16.** Squaring circuit, low-pass filter and buffer of the energy detection receiver.

fabricated receiver IC, it measures 0.43 mm x 0.61 mm, including bond pads.

−0.05 0 0.05 0.1 0.15 0.2 0.25

Amplitude / V

0 2 4 6 8 10 12 14 16

Data pattern to TX

Time / ns

applied to the transmitter and detected signal at the receiver output.

(a) Data sequence

output buffer form low-pass filters with 1 GHz 3 dB bandwidth, which are needed for the envelope detection. The LNA from Fig. 13(a) is added to complete the energy detection receiver, which totally consumes 108 mW. Fig. 16(b) shows the microphotograph of the

For testing the energy detection receiver, a 700 Mbit/s return-to-zero (RZ) impulse train was generated by the impulse generator shown in Fig. 6(a), which has a power comsumption of 7.5 mW at this rate. The transmitter and receiver ICs are seperately mounted on Rogers RO4003C substrates which also carry the dipole-fed circular slot antennas discussed in 2.1.1, and are wire-bonded to microstrip transmission lines feeding the antennas. The two antennas are placed at a distance of 30 cm. Fig. 17(a) shows the input data sequence from a pattern

Amplitude / mV

**Figure 17.** OOK transmission experiment at 700 Mbit/s over 30 cm, data sequence (700 Mbit/s data rate)

generator. The corresponding detected impulse envelopes with a peak amplitude of 40 mV at the output of the receiver IC can be seen in Fig. 17(b). This experiment clearly demonstrates that the simple transmitter/receiver combination can be used to transmit significant bit rates

over short distances. Detailed measured results of the receiver are shown in [23].

0 2 4 6 8 10 12 14 16

Output of the receiver

UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications

453

Time / ns

(b) Output

Fig. 14(b) shows the information of the noise figures. The differential noise figure varies from 2 dB at 3 GHz to 2.9 dB at 10.6 GHz. Small group delay variation within the entire band is required for single-band IR-UWB systems. As depicted in Fig. 15(a), the group delay variation is smaller than 15 ps within the complete band. Fig. 15(b) shows the measured large signal behavior at 7 GHz of this differential amplifier. The input 1 dB compression point is -17.5 dBm. The complete power consumption of this differential LNA is 77 mW.

**Figure 15.** Measured results of group delay versus frequency and gain depending on the input power.

### *2.3.2. Energy detection receiver*

The core of energy detection receivers is a squaring circuit. Fig. 16(a) displays the squaring circuit based on a Gilbert cell four quadrant multiplier comprising two differential stages in parallel with cross-coupled output, complemented by a low-pass filter and a differential output buffer. The squaring operation is realized by connecting the same signal to both inputs of the Gilbert cell. The signal fed to the lower pair of the Gilbert cell is taken directly from the LNA output transistors, while the signal fed to the top quad is passed first through the emitter follower buffer. Both paths introduce almost the same group delay. Thus, the two branches of the input signal arrive simultaneously at the multiplier, ensuring an exact squaring operation. The load resistors (R1, R2) of the Gilbert cell, together with shunt capacitors (C1, C2) of the 452 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications <sup>15</sup> 453 UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications

**Figure 16.** Squaring circuit, low-pass filter and buffer of the energy detection receiver.

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measurement. The influence of the baluns is removed during the calibration process. The measurement is restricted to 3 - 11 GHz due to the operating range of the UWB baluns. Fig. 14(a) shows the measured S-parameters. The non-ideal performance of the UWB baluns introduces ripples in the measured curves. The measurement results show a differential gain of 19.9 dB with a 1.8 dB variation, the input matching has a value of smaller than -7 dB and the output one is smaller than -6 dB in the complete FCC allocated frequency range. The method in [2] is adopted to extract the differential noise figure. First a single-ended noise figure F*single* is measured from port In+ to Out- with the other ports terminated by 50 Ω resistors. Then, by measuring the transducer gain from port In+ to Out- (G31) and Out+ to In+ (G32), the

Fig. 14(b) shows the information of the noise figures. The differential noise figure varies from 2 dB at 3 GHz to 2.9 dB at 10.6 GHz. Small group delay variation within the entire band is required for single-band IR-UWB systems. As depicted in Fig. 15(a), the group delay variation is smaller than 15 ps within the complete band. Fig. 15(b) shows the measured large signal behavior at 7 GHz of this differential amplifier. The input 1 dB compression point is -17.5 dBm.

Gain / dB

**Figure 15.** Measured results of group delay versus frequency and gain depending on the input power.

The core of energy detection receivers is a squaring circuit. Fig. 16(a) displays the squaring circuit based on a Gilbert cell four quadrant multiplier comprising two differential stages in parallel with cross-coupled output, complemented by a low-pass filter and a differential output buffer. The squaring operation is realized by connecting the same signal to both inputs of the Gilbert cell. The signal fed to the lower pair of the Gilbert cell is taken directly from the LNA output transistors, while the signal fed to the top quad is passed first through the emitter follower buffer. Both paths introduce almost the same group delay. Thus, the two branches of the input signal arrive simultaneously at the multiplier, ensuring an exact squaring operation. The load resistors (R1, R2) of the Gilbert cell, together with shunt capacitors (C1, C2) of the

(*FsingleG*<sup>31</sup> − *G*<sup>31</sup> − *G*32). (5)

−30 −25 −20 −17.5 −15 −10

Gain at 7 GHz

Input power / dBm

(b) Gain

differential noise figure can be extracted as

*2.3.2. Energy detection receiver*

Group delay / ps

*Fdi f f* = 1 +

3 4 5 6 7 8 9 10 11

Simulated Measured

Frequency / GHz

(a) Group delay

The complete power consumption of this differential LNA is 77 mW.

1 *G*<sup>31</sup> + *G*<sup>32</sup>

> output buffer form low-pass filters with 1 GHz 3 dB bandwidth, which are needed for the envelope detection. The LNA from Fig. 13(a) is added to complete the energy detection receiver, which totally consumes 108 mW. Fig. 16(b) shows the microphotograph of the fabricated receiver IC, it measures 0.43 mm x 0.61 mm, including bond pads.

> For testing the energy detection receiver, a 700 Mbit/s return-to-zero (RZ) impulse train was generated by the impulse generator shown in Fig. 6(a), which has a power comsumption of 7.5 mW at this rate. The transmitter and receiver ICs are seperately mounted on Rogers RO4003C substrates which also carry the dipole-fed circular slot antennas discussed in 2.1.1, and are wire-bonded to microstrip transmission lines feeding the antennas. The two antennas are placed at a distance of 30 cm. Fig. 17(a) shows the input data sequence from a pattern

**Figure 17.** OOK transmission experiment at 700 Mbit/s over 30 cm, data sequence (700 Mbit/s data rate) applied to the transmitter and detected signal at the receiver output.

generator. The corresponding detected impulse envelopes with a peak amplitude of 40 mV at the output of the receiver IC can be seen in Fig. 17(b). This experiment clearly demonstrates that the simple transmitter/receiver combination can be used to transmit significant bit rates over short distances. Detailed measured results of the receiver are shown in [23].

#### 16 Will-be-set-by-IN-TECH 454 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications <sup>17</sup>

### *2.3.3. Correlation detection receiver*

Coherent detection receivers are based on the cross-correlation realized by feeding the received signal and the on-chip generated template impulse into a wideband analog four-quadrant multiplier and subsequent low-pass filtering. Fig. 18(a) shows the block diagram of the correlation receiver. The multiplier-based correlation is done in the RF domain, which leads to an energy efficient solution by omitting power-hungry wideband ADCs. In a radar setup, the transmit and receive clocks need to be phase adjusted, which in practice is done by a DDS board. The complete schematic of the UWB analog correlator circuit

(a) Microphotograph (b) Correlation

cross-correlation of the received impulse with the template impulse.

**Figure 19.** Chip micrograph of the fully integrated correlation receiver and the measured normalized

455

UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications

novel front-end concept based on a merged impulse generator/low noise amplifier, shown in Fig. 20(a) is proposed. In this design, the input of the differential low-noise amplifier is tied

(a) Diagram (b) Microphotogragh

**Figure 20.** Block diagram and chip photo of the proposed monostatic UWB radar front-end.

transmitted pulse is barely visible and will not influence the further processing.

together with the output of a buffer amplifier following the impulse generator. An external monoflop and a bandgap reference circuit ensure that the LNA is disabled during the impulse emission. The LNA bias is recovered after the impulse has been transmitted, and it returns to full gain within 1.5 ns. The added parasitics of the buffer are included in the design of LNA. Fig. 20(b) shows the fabricated IC. In the experimental test, the antenna terminal is connected to two short coaxial cables, each of which feeds into a 10 dB attenuator shorted at the far end. An approximately 1 ns delay is generated by the coaxial cable and attenuator, corresponding to a distance of 30 cm in air. The measured time domain trace at the output of the differential LNA can be seen in Fig. 21. The significant common-mode transients due to the bias switching are completely invisible owing to the balanced setup. The result clearly shows the received impulse echo. Due to a high isolation of the 'cold' low-noise amplifier, the crosstalk from the

**Figure 18.** Architecture of the correlation receiver system and the schematic of the correlator with a true multiplier, a low-pass filter and a buffer.

can be seen in Fig. 18(b). The core of the correlator is again a Gilbert cell which acts as a wide-band multiplier with proper template impulse amplitude applied to the switching quad. Capacitively shunted resistive emitter degeneration results in the necessary gain flatness over the whole UWB frequency band. The low-pass filters are formed by the load resistors (R1, R2) of the Gilbert-cell with the shunt capacitors (C1, C2) of the buffer, the same as shown for the case of the energy detection above. The correlator consumes 35 mW.

The complete receiver IC, including the differential LNA, the correlator and the template impulse generator, is shown in Fig. 19(a). It measures 0.43 X 0.61 mm2 and consumes a total DC power of 130 mW. To demonstrate the correlation performance of the receiver, the receiver IC was connected to a dipole-fed slot antenna, and placed at a distance of 20 cm from the transmitter. A small offset frequency of 100 Hz was introduced between the transmitter and receiver clocks, making the template impulses continuously sweep through the received signal. The measured cross-correlation can be seen in Fig. 19(b). More details of the correlation receiver can be found in [24].
