In recent years, with the promotion of third- generation (3G) wireless networks in Japan (IMT-2000) , Europe (UMIST) and the United States (CDMA2000) , users of low cost, low power consumption and small form factor required by 3G mobile phones Equipment (UE) becomes important. The direct down-conversion receiving structure realized by silicon process and circuit design technology is a promising system solution for the highly integrated platform of 3G mobile phones. This article presents a commercial fully integrated zero-IF receiver solution for 3G radio ( Figure 1) . The receiver input second- order intercept point (IIP2) is widely discussed because it is a key performance indicator of direct conversion receivers. The results of measurement, simulation and calculation are given here. Figure 1 Direct conversion receiver IC for 3GPP FDD mobile phone radio  Figure 2 2nd order intermodulation distortion caused by two -tone blocking in a zero-IF receiver  Figure 3 CCDF blocked by UL reference channel and DL 16 channel Direct conversion receiver structure As shown in Figure 1 , the direct conversion or zero-IF receiver structure is the way to realize the complete on-chip integration of the receiver, and the direct demodulation signals are baseband I and Q signals. In 3G WCDMA FDD ( full duplex ) working mode, only an external duplexer is needed to separate the RX and TX parts. Also, post- LAN RF filters are required in FDD radios to suppress out-of-band blocking and transmitter leakage in the demodulator input. In the zero-IF receiver IC , the channel selection of the baseband is realized by the on-chip low-pass filter. This is followed by channel filtering, and the I/Q signal in the baseband is amplified by a variable gain amplifier (VGA) before being digitized by the analog baseband part of the radio modem IC . 2nd order distortion effect In a zero-IF receiver, the second- order intermodulation product (IM2) is a source of interference, and these components in the baseband channel of the receiver must be minimized. In a zero-IF receiver, after the front-end 2nd order non-linear demodulation of the AM signal, the resistance difference component falls into the baseband. Since these second order IM2 component is blocked envelope consisting of a squared term, so the bandwidth of the baseband these undesirable spectral components may reach twice the amplitude of the envelope blocking bandwidth. IM2 components depend on the desired signal modulation bandwidth in the baseband, so these IM2 components will partially or completely lead to receiver interference tolerance. The IM2 distortion component discussed here occurs in the downconverter of the zero-IF receiver. This is because the low-frequency IM2 component in the LNA is usually filtered out by AC coupling or band-pass filtering between the LNA and the mixing unit . There are multiple IM2 component generation mechanisms in a zero-IF receiver . However, there are mainly two sources of IM2 : RF self-mixing: This is caused by the non-ideal hard-switching IV characteristics and spurious coupling of the conversion stage in the mixer of the zero-IF receiver, which causes the RF signal to leak into the LO port. Downconverter RF stage 2nd order nonlinearity and LO stage switching mismatch: When the I/Q mixer input of the zero-IF receiver introduces strong CW or modulation blocking, the mixer transconductance or RF stage active device The second- order nonlinearity will produce low-frequency IM2 components. IIP2 formula derivation The weak nonlinear characteristic of the receiver front end can be expressed as: ∧         (1) The input signal of the receiver ( see Figure 2) is expressed as the total two-tone power equal to A2/R . The second- order distortion component of the receiver front end is:      (2) The total output IM2 component ( including total DC offset ) in (f1+f2) and (f1-f2 ) is expressed as:   (3) The total power related to the system impedance R in the output IM2 component ( Equation 3) is calculated as follows:            (4) By definition, at the IIP2 power level, the total input signal power is equal to the total power in the output IM2 component ( Equation 4) , divided by the gain factor |a1|2 can be written as:                 (5) According to the total two-tone input power equal to P2T=A2/R , the total power level of the IM2 component related to the receiver ( Equation 4) can be expressed as:                               (6) IM2 component of the total power level of 4 noted equation, which is a DC of 50% - IM2 component (3dB), the f1-f2 25% (- 6dB ) component, f1 + f2 of 25% (- 6dB ) IM2 component composition. Therefore, the power level of the IM2 component in f1-f2 can be derived from Equation 4 and Equation 6 : (7) Wherein the power level of each tone (f1 or f2 in the P1T) is 50% of the total two-tone power. Effective low frequency IM2 component In 3GPP WCDMA wireless communication, the serious interference to the receiver input is not the dual-tone type, but the broadband digital modulation blocking part. Therefore, it is very important to estimate the effective low-frequency component of the modulation block in order to obtain a receiver IIP2 that meets the bit error rate performance requirements . This requires understanding the characteristics of modulation blocking. Especially its non-constant envelope, this is because it transforms the RF blockade to baseband, including the square term of the envelope. In 3G standard test cases 7.3.1 and 7.6.1 , two main modulation blockings in 3GPP WCDMA receivers are given . The first test case 7.3.1 specifies the minimum sensitivity required for BER<10-3 when the transmission uplink (UL) signal is at the maximum power level (+24dBm) at the antenna . The second test case 7.6.1 stipulates that for the minimum received signal at the antenna connector for BER>10-3 , the modulation downlink (DL) blocking is -44dBm , which is 15MHz away from the desired signal , and the UL power transmitted at the antenna is In the case of +20dBm . The 3GPP standard document A.1 table shows the structure of the reference measurement channel (12.2kbps) for transmitting UL signals at the antenna of the 3GWCDMA mobile phone . It consists of a dedicated physical data channel (DPDCH) and a dedicated physical control channel (DPCCH) . In the radio modem part, both DPDCH and DPCCH channels are extended to 3.84Mcps , calibrated to an appropriate power ratio (DPCCH/DPDCH=-5.46dB) , HPSK encoding and 1.92MHz square root cosine (RRC) filter ( roll-off factor a=0.22) ) Filtering. In addition, the forward channel modulation block ( offset by 15MHz from the desired channel ) consists of the common channel ( calibrated in Table C.7 ) and 16 dedicated data channels ( calibrated in Table C.6 ) required for the test . The signal is QPSK mixed code, extended to 3.84Mcps , coded and filtered with RRC filter ( similar to UL signal ) . The signal -3dB bandwidth is equal to 3.84MHz ( at RF) , and 99% of the total signal power is within the 4.12MHz bandwidth (-6dBBW) . In order to understand the envelope characteristics of modulated UL transmission signals or modulated DL16 channel signals and estimate the effective IM2 components of these signals in WCDMA zero-IF receivers , first study the power statistics of each signal expressed by the complementary distribution function (CCDF) . CCDF gives the peak-to-average power ratio (PAR) of the relationship between signal and probability . Figure 3 shows the ADS (Advanced Design System) simulation CCDF of the UL transmission signal and the DL16 channel signal . Note that in Figure 3 based on a 0.1% probability of a UL reference channel transmitting DPDCH , PAR is 3.1dB . In addition, the DL blocker ( offset at 15MHz ) containing 16 dedicated communication channels has 8.4dB PAR ( at a probability of 0.1%) , which is almost equivalent to a Gaussian noise signal. The effective low frequency IM2 component estimation shown below is different from the two standard test cases because the PAR between the two different blocking components is different. The simulation template of ADS IM2 that studies the modulation and blocking IM2 component at the input of WCDMA zero-IF receiver is shown in Fig . 4 . The IM2 component is filtered by the RRC filter, which is matched with the base station transmitter RRC filter. The total low-frequency IM2 component measured in the simulation is within the desired oHz~2.06MHz baseband signal frequency band, which is half of the 99% power bandwidth of the RF signal . Figure 5 and Figure 6 respectively show the WCDMA UL reference measurement channel (12.2kbps) and the WCDMA DL 16 channel blocking the simulated IM2 component spectrum output by the baseband of the zero-IF downconverter . In the ADS template, for simulation, a modulation blocking power of 0dBm and a zero-IF downconverter IIP2 equal to +30dBm are used . For the 0dBm WCDMA UL transmission signal integrated within the desired signal passband of 1KHz...2.06MHz , the total low-frequency IM2 component power is equal to -43.7dBm . The DC offset caused by the second- order nonlinearity is 5mV , which is equivalent to generating -33dBm at 50 Ω ( Figure 5) . In addition, for the power level of the total IM2 component blocked by the 0dBm WCDMA DL 16 channel , the integral within the desired signal passband of 1KHz...2.06MHz is equal to -33.1dBm . The total DC offset caused by the second- Phenolic Paper Tube 3520 Phenolic Paper Laminated Tube is formed by hot rolling, baking insulation paper impregnated with phenolic resin under heat and pressure. It is class E (120℃) insulation with high mechanical property. It has good dielectric property in dry conditions.
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Effective IM2 component evaluation of dual-tone and WCDMA modulation blocking
introduction