Figure 3.24 Chip microphotograph of the proposed integrated current-mode transmitter front-end.
1.5 × 1.1 mm2 including testing pads.
The measured environment setups of the fabricated transmitter are described as fol-lows. On-wafer probing measurement is adopted to verify the performance of the pro-posed current-mode transmitter front-end. One GSG RF probe with the pitch of 100 µm, one GSGSG RF probe with the pitch of 100 µm, and two 6-pin dc probes with the pitch of 150 µm are applied to probe the testing pads. As shown in Fig. 3.25(a), the S-parameters are measured to analyze the matching characteristics of baseband input port and RF output port by the network analyzer, Agilent E8364B, which can characterize the S-parameter performance from 10 MHz to 50 GHz. To measure conversion power gain and linearity, two signal generators Agilent E8257D are used to provide two baseband signals for the DUT. The LO signals are generated by the on-chip VCO. The spectrum
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alyzer Agilent E4448A is used to monitor the spectrum to verify the linearity and conver-sion power gain of the proposed transmitter. The measurement environment is illustrated in Fig. 3.25(b). Furthermore, the characteristics of the on-chip VCO is also measured and analyzed through the spectrum analyzer. The noise figure of the transmitter is mea-sured by the spectrum analyzer Agilent E4448A embedded with noise figure option. The broadband noise source of Agilent 346CK01 is used. The measurement environment is illustrated in Fig. 3.25(c).
The measured tuning curve of the integrated VCO is shown in Fig. 3.26. The on-chip VCO provides the frequency of the LO signal from 20.8 GHz to 22.7 GHz as the control voltage VT2 is varied from 0 V to 2 V. The measured phase noise is –108 dBc/Hz at 10-MHz frequency offset from 22.7 GHz. The measured matching characteristics of the IF input and RF output port are shown in Fig. 3.27. The return loss of the IF input port is below –20 dB within the frequency range from 170 MHz to 1.2 GHz. The return loss of the RF output port is below –10 dB within the frequency range from 21.2 GHz to 22.8 GHz.
Under the 1-V supply voltage, the proposed current-mode double-balanced up-conversion mixer dissipates a very small amount of power of 3.1 mW. The VCO, transformer-based VCO buffer/repeater, and baseband current buffer/repeater circuits dissipate 2.2 mW, 3.3 mW, and 3.1 mW, respectively. The total power dissipation of the integrated current-mode transmitter front-end is only 11.7 mW.
Before verifying the linearity performance of the proposed current-mode transmit-ter front-end, the losses from the RF cables, probes, adaptors, 180◦ phase shifter, and power combiner are measured. These sources of losses in the measuring environment are compensated when analyzing the measurement data. The measured and post-simulated conversion power gains versus the IF input powers are shown in Fig. 3.28. In the mea-surement, the IF frequency is at 200 MHz, and the LO frequency is at 20.8 GHz where VT2
is equal to 0 V. The double-sideband RF outputs are observed at the frequencies of 21 GHz (upper sideband, USB) and 20.6 GHz (lower sideband, LSB). The measured conversion power gain is about –5 dB where the losses from on-chip transformer and output matching network are included. The measured input 1-dB compression point (IP−1dB) and output 1-dB compression point (OP−1dB) are –22 dBm and –28 dBm, respectively. Furthermore, the average single sideband (SSB) noise figure of the transmitter front-end is around 12.7
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V A
V A
V A
DC Power Supplies
Network Analyzer on-wafer probing
BB+
BB−
RF
(a)
DUT
GHz dBm
GHz dBm
V A
V A
V A
Power Combiner
DC Power Supplies
RF Transformers
Spectrum Analyzer BB+
BB−
BB Signal Generators
on-wafer probing RF
(b)
Figure 3.25 The environment setup of (a) S-parameter measurement, (b) linearity mea-surement, and (c) noise figure measurement.
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V A
DUT
Figure 3.25 The environment setup of (a) S-parameter measurement, (b) linearity mea-surement, and (c) noise figure measurement (Con’t).
0.0 0.4 0.8 1.2 1.6 2.0
Figure 3.26 Measured tuning range of VCO.
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0 250 500 750 1000 1250 1500 1750 2000 -26
Return Losses of IF Port [dB]
Frequency [MHz]
Return Losses of IF Port
(a)
15.0 17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0 -20
Return Losses of RF Port [dB]
Frequency [GHz]
Return Losses of RF Port
(b)
Figure 3.27 Measured matching characteristics of (a) IF port, and (b) RF port.
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-60 -50 -40 -30 -20 -10 0 CG, TT, Post-simulated CG, FF, Post-simulated CG, SS, Post-simulated
Conversion Power Gain (CG) [dB]
PIF,IN [dBm]
Figure 3.28 Measured and post-simulated power conversion gain versus IF input power from one-tone testing result.
dB. The LO suppression of the proposed current-mode transmitter front-end is only 16 dB.
The performance of intermodulation is measured by two-tone testing. Two IF inputs with the same signal power level are at the frequencies of 150 MHz and 250 MHz. The LO frequency is also tuned to 20.8 GHz where VT2is equal to 0 V. The measured results under these conditions are represented in Fig. 3.29. It reveals that the proposed mixer has the measured input 3rd-order intermodulation intercept point (PIIP3) and output 3rd-order intermodulation intercept point (POIP3) of about –9.6 dBm and –14.6 dBm, respectively.
As the LO frequency is increased, the measured power conversion gain decreases because of the reduced output magnitude of the VCO at the higher frequency and therefore the reduced LO current signal. As shown in Fig. 3.30, the measured conversion gain of the mixer is degraded from –5 dB to –14 dB as the LO frequency is increased from 20.8 GHz to 22.7 GHz.
As shown in Fig. 3.28, the measured conversion power gain is –5 dB which is close to the simulated results in the process corner SS. The gain is about 5 dB smaller than the sim-ulated results in the process corner TT. It is because the process variations make the bias
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Figure 3.29 Measured 3rd-order intermodulation from two-tone testing result.
0.0 0.5 1.0 1.5 2.0
Conversion Power Gain (CG) [dB]
VT2 [V]
VCO Tuning Curve
VCO Frequency [GHz]
V
DD=1V
Figure 3.30 Measured conversion power gain versus LO frequency.