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HAMADA et al.: 300-GHz-BAND 120-Gb/s WIRELESS FRONT-END BASED ON InP-HEMT PAs AND MIXERs 11
Fig. 23. CG improvement by the RF–IF isolation obtained by the analysis.
Fig. 24. Simulated CG of the designed mixer with and without the RF–IF
isolation.
294 GHz. The measured S-parameters of the LO PA are shown
in Fig. 28. The small signal gain is 15 dB at 270 GHz. Because
the output power of the extender is around −7dBm,theLO
PA is cascaded to provide sufficient LO power to the mixer.
The dependence between the CG and LO power is shown
in Fig. 29. The CG is saturated when the LO power is larger
than 5 dBm. In the following measurement, the LO power
was set to 7.5 dBm. The band-pass-filter (BPF) is used to
cut out the spurious emission and to apply the desired LO
signal of 270 GHz to the mixer. The measured CG for both
up-conversion (U-C) and D-C is shown in Fig. 30(a). The CG
is −15 dB for both U-C and D-C in the IF of 0–10 GHz. The
3 dB BW for the USB is very broad, 32 GHz. The measured
CG is approximately 5-dB lower than the simulation results
shown in Figs. 24 and 25. We consider that this is caused by
the IF packaging loss (1–2 dB, IC-to-V-connector junction),
RF packaging (1 dB, WG propagation loss inside the mixer
module), and the inaccuracy of the transistor model made from
the low-frequency (10 MHz–110 GHz) measurement data. The
linearity for the U-C at the IF of 21 GHz was also measured,
as shown in Fig. 30(b). The measured OP1dB is −16.5 dBm.
The LO-to-RF leakage of the mixer was also measured using
the power meter, VDI PM5. The input LO power of the mixer
(after the 270-GHz BPF shown in Fig. 27) and LO leakage
measured at the RF port of the mixer are 7.5 and −16.4 dBm,
respectively, as shown in Fig. 31. Therefore, LO leak sup-
pression is 23.9 dBc. This is not that bad for the single-
ended mixers compared to the value for balanced ones in
300-GHz-band (19.6 dBc in [35]).
V. TRX
AND DATA TRANSMISSION EXPERIMENTS
The proposed TRX is constructed using the RF PAs,
mixers, and LO PAs discussed in Section IV. To locate the
RF in our target frequency band, which is in the un-allocated
frequency of above 275 GHz, the LO frequency, IF, and RF are
Fig. 25. Simulated CG of the mixer module with ridge couplers for with and
without the RF–IF isolation. (a) Simulation model. (b) Simulation results.
set to 270, 20, and 290 GHz, respectively, i.e., the USB is used
as the RF signal. The characteristics of each transmitter (TX)
and receiver (RX) are discussed in this section.
A. TX
The schematic of the TX is shown in Fig. 32(a). It consists
of the individual WG modules of RF PAs, mixers and LO
PAs described in Section IV. As described in Section IV-B,
the mixers have substantial LO-to-RF leakage of −16.4 dBm
where the LO input power is 7.5 dBm at 270 GHz. This
LO leakage is almost the same as the mixers OP1dB of
−16.5 dBm shown in Fig. 30(b). Therefore, the HPF module
is used to cut out the LO leakage between the U-C mixer
and RF PA to avoid the undesired effect of the LO leakage
such as the saturation of the RF PA and the intermodulation
distortion. This HPF also cuts the image signal of the LSB
from the mixer output. The CG and I–O characteristics were
measured using the same measurement setup in Fig. 27. Note
that in the TX measurement, the WR3.4 WG-type attenuator
(ATT) with the attenuation of 14 dB was inserted between the
RF PA and the WR3.4 extender to protect the extender from
the large output power of the RF PA. The insertion loss of the
ATT is subtracted after the measurement. Fig. 32(b) shows
the CG of the TX in the USB. The frequency response of the
TX CG reflects those of the CG and the gains of the mixer
and PA. It gradually increases as the frequency increases and
takes its peak valu e of around 3 dB around 290–295 GHz and
degrades after 296 GHz. The 3-dB BW is 17 GHz. The I–O
characteristics at the RF and IF of 291 and 21 GHz are shown
in Fig. 32(c). The measured OP1dB of the TX is 1.5 dBm,
which matches the theoretically calculated OP1dB of 1.6 dBm
using the formula [36] in the cascaded form of the mixer
(OP1dB =−16.5 dBm) and RF PA (OP1dB = 6.8dBm)
described in Figs. 19(a) and 30(b).
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