Upconversion

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4.2 Upconversion Block (Block B)

The design of block B to solve the problem proposed is depicted in figure 4.1. (See section 1.4)

Figure 4.1 – Architecture to be used to construct block B, which was exhibited in the general architecture of the LTE filter, presented in figure 1.2.

LIV Block B1 architecture is represented in figure 4.2.

Figure 4.2 – Scheme that exhibits the operation of block B1, that is included in figure 4.1. In the band 2 pathway, A pair consisting of Mux 1->2 followed by a Mux 2->1 is also present, both after HP Band 2 Sub Band 2 and before Mux 2->4. Another pair of muxes, exactly like the one just described, is placed after Band 2 Sub Band 3 and before Mux 2->4. These are not shown due to lack of space, because of visualization purposes.

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For the image band rejection, it is enough to take advantage of the already made division of the LTE bands into 3 major bands, and to design an image rejection filter for each major band. These filters are already projected, since they are exactly equal to the 3 filters inside block A1 in the down conversion block, HP1, HP2 and HP3, respectively, by band.

However, the problem of the variable LO tone rejection is more complicated and entails a further division of the LTE spectrum into smaller bands than the major 3 already existent.

In this way, band 2 was further divided into 3 sub bands and band 3 was split into 2 sub bands. Band 1 remained unchanged. This was necessary so the order of the LO rejection filters obtained was reasonable and the resultant filters’ response would be stable enough, even with the possible variations of the functional component values (capacitance, inductance).

The first and last muxes conduct the signal through the image rejection filter of the correspondent major band the signal is in.

In band 2 and 3, a pair of muxes also lead the signal into the sub band specific LO tone rejection filters. In band 1, the signal is directly headed to the respective LO tone rejection filter.

Just like in the downconversion block, an isolation of at least 80 dB, between any route of the chosen path and its parallel routes, is good practice. The importance of this 80 dB isolation has mainly to do with the filters’ different phase responses and is described in section 2.2.

In the band 1 path, a pair of muxes assures a minimum of 80 dB isolation between the band path selected and the other 2 that remain “open”. In band 2 and in band 3 paths, there are muxes with the objective of providing a minimum 80 dB isolation between the chosen sub band path and the other sub band paths that stay “open”.

This level of isolation is allways achieved except, in band 3, where a minimum isolation of 78 dB is obtained. This value is only 2 dB below the required isolation and concerns the worst typical case scenario. It was considered to be good enough. The alternative would be to insert 4 more muxes (2 for each sub band path) that would add a minimum isolation of 39 dB.

Lastly we would like to call to the attention, the arrow entering the block B1 and terminating right at its entrance in figures 4.1 and 4.2. This arrow actually connects to all attenuators and muxes in block B1, since the control unit must be in charge of the path and sub path choice decisions as well as control the digital step atenuators to avoid the amplifiers to enter into power compression. This is not represented for obvious reasons related to visualization purposes.

4.3 Upconversion Block SNR

At the beginning of the upconversion chain, the minimum signal level in the frequency domain, found by simulation of the entire Downconversion plus FPGA chain in Matlab was -99.97 dB dBW/Hz (LTE signal initially centered in 2350 MHz, filters’ components with maximum deviation from ideal values).

The simulation was run for the frequencies of 800MHz, 1500MHz, 1800 MHz, 2350 MHz and 2585MHz. The maximum level of the signal was -94.64 dB/Hz (LTE band initially centered in 2350 MHz. components with ideal values).

Note that in the simulation performed, it was chosen a fixed digital gain in the FPGA block and because of the fact that the attenuation of the analog filters is changeable with frequency, the signal

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present at the start of the Upconversion block, is also variable with frequency. Different power spectrums and signal ripples are obtained by varying the center frequency of the 20 MHz LTE band.

Nevertheless, it was guaranteed that the total power of such signal is always below -7 dBm to not drive the first mixer of the Upconversion circuit into compression. If a new simulation yields a smaller minimum value for the signal level at block B’s entry, that difference has to be taken into account in the signal to noise ratio degradation in the upconversion block.

The noisefloor was already calculated in section 2.3 and has the value of ≅ −203,8 . The initial SNR at the entrance of block B is: = − =− 99.97— 203.8 =

= 103.83 = 80 + 23.83 = + _Á

To achieve the 80 dB of SNR sought, the maximum SNR degradation must be of 23.83 dB.

Two cases were considered: (i) the SNR degradation in an “optimum case” and (ii) an SNR degradation in a very bad case. These are defined in section 2.3.2.

Again, sections of the circuit were considered for the calculations of the SNR degradation. Those sections are defined exactly as in section 2.2 of the thesis and the same parameters are taken into account to calculate each section’s SNR degradation.

In the upconversion block, the number of sections varies with the band path followed. The first 3 sections are independent of the band in which the signal initially was.

The SNR degradations of these 3 sections and the overall SNR degradation, at this point in the circuit, are shown in table 4.1

SNR Degradation Attenuation Comparison Sections Optimum

Case Worst Case A max without attenuators

Section 1 7.45 dB 8.05 dB 8.05 dB

Section 2 2.3 dB 2.3 dB 7 dB

Section 3 1.82 dB 5.55 dB 14.95 dB

Accumulated 11.57 dB 15.9 dB 30 dB

Table 4.1 – SNR degradations of the first 3 sections of block B as well as accumulated SNR degradation . The attenuations on the right are present, so the reduction in SNR degradations due to employing amplifiers+attenuators can be evaluated.

The next sections are dependent on the LTE band chosen and correspondent path that is followed.

They will be presented in the following order, Band 1 signals, Band 2 signals and Band 3 signals, repectively in tables 4.2, 4.3 and 4.4.

SNR Degradation Attenuation Comparison Sections Optimum Case Worst Case A max without attenuators

Section 4 1.92 dB 8.3 dB 14.4 dB

Section 5 1.96 dB 2.3 dB 15.7 dB

Total 15.55 dB 26.5 dB 58.4 dB

Table 4.2 – SNR degradations of the specific sections transversed by signals belonging to Band 1, as well as the total SNR degradation in block B for those types of signals. The attenuations on the right are present, so the reduction in SNR degradations due to employing amplifiers+attenuators can be evaluated.

The SNR at the end of block B, for signals in band 1, ranges between 77.98 dB (80 + 24.48 - 26.5) and 88.93 dB (80 +24.48–15.55).

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SNR Degradation Attenuation Comparison Sections Optimum Case Worst Case A max without attenuators

Section 4 1.92 dB 9.45 dB 21.15 dB

Section 5 2.09 dB 4.9 dB 7.6 dB

Section 6 1.23 dB 8.15 dB 16.45 dB

Section 7 1.87 dB 2.3 dB 7 dB

Total 18.68 dB 40.7 dB 82.2 dB

Table 4.3 – SNR degradations of the specific sections transversed by signals belonging to Band 2, as well as the total SNR degradation in block B for those types of signals. The values correspond to the pathway of band 2, sub band 3, where the SNR degradation is maximized. The attenuations on the right are present, so the reduction in SNR degradations due to using amplifiers+attenuators can be evaluated.

The SNR at the end of block B, for signals in band 2, at worse, range between 63.68 dB (80 + 24.48 - 40.8) and 85.8 dB (80 +24.48–18.68). Like already stated, this values correspond to sub band 3 of band 2. In sub band 2, the SNR degradation is slighly lower. In sub band 1, despite existing one less section, the SNR improvement at the end of the chain is just, between 1.29 dB in the optimum case and 4.36 dB in the worst case, which is a notable but small, improvement.

SNR Degradation Attenuation Comparison

Sections Optimum Case Worst Case A max without attenuators

Section 4 6.86 dB 13.96 dB 23.96 dB

Section 5 2.3 dB 2.3 dB 17.11 dB

Section 6 0.57dB 2.1 dB 7 dB

Total 21.3 dB 34.26 dB 78.07 dB

Table 4.4 – SNR degradations of the specific sections transversed by signals belonging to Band 3 as well as the total SNR degradation in block B for those types of signals. The values correspond to the pathway of band 3, sub band 2, where the SNR degradation is maximized. The attenuations on the right are present so the reduction in SNR degradations due to using amplifiers+attenuators can be evaluated.

The SNR at the end of block B, for signals in band 3, at worse, range between 69.57 dB (80 + 23.83 - 34.26) and 82.53 dB (80 +23.83–21.3).

4.4 Up Conversion Process 4.4.1 First Step

The first step in the upconversion process is represented in figure 4.3.

Figure 4.3 – Schema of the process of upconversion of the 20MHz bandwith LTE band from 60MHz to a center frequency of 600MHz. The image band produced is marked with a cross. The intended upconverted band is marked with a circle.

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From figure 4.3, it is made clear that a high pass filter is going to be needed; this filter has the same exact response, as depicted in figure 2.25. The bands are the ones in figure 2.24.

The 540MHz local oscillator tone inserted in the process must also be rejected at this stage. This way the tone is rejected always at the same frequency which is low enough to allow the minimum desired attenuation of 80 dB to be possible. For simulation purposes, a band reject chebyshev filter is used to simulate a Notch at 540 MHz. The filter response is in figure 4.4.

Figure 4.4 – Ideal reponse of Band Rejection Filter to attenuate the LO leaked tone in 540 MHz.

The filter actually projected is shown in figure 4.5.

Figure 4.5 - Band Rejection Filter to attenuate the LO leaked tone in 540 MHz.

The constituent components of the filter are detailed in table a2.7 in the appendix, section 2.3.

4.4.2 Second Step

The second step in the upconversion process is represented in figure 4.6.

Image Bands Rejection

The image bands generated in this upconversion step depend on the LTE band chosen. Then again, to accomplish simplicity of the system architecture and to gain flexibility, the image band of the whole major bands was taken into account. The image band to consider is, therefore, only dependent, on if the LTE band selected is within band 1, band 2 or band 3.

The necessary HP filters (one for each of the 3 bands), required to filter out the image bands produced, are exactly the same as the ones exhibited in figures 2.14, 2.18 and 2.22 for band 1, band 2 and band 3 respectively. See section 2.5.1.

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Figure 4.6 – Schema of the process of upconversion from the 600 MHz centered, 20MHz bandwidth band, back to the higher frequency that varies with the LTE band chosen.