This article describes how to meet the requirements of a high-performance base station (BTS) receiver for the half-IF spurs. To achieve this goal, the engineer must understand the relationship between the mixer's IP2 and the second-order response, and then select the RF mixer that meets the system's cascading requirements. The mixer data sheet represents second-order response performance in the form of a second-order intercept point (IP2) or a 2x2 spurious suppression indicator. This paper describes the receiver design and how to determine the overall half-IF spurs by introducing the relationship between these two parameters. Take the IP2 and 2x2 relationship of the MAX19997A as an example. This is an active mixer for E-UTRA LTE receivers.
Mixer harmonic
In a superheterodyne receiver circuit, the mixer converts the high frequency RF signal to a lower intermediate frequency (IF), a process known as downconversion. In the mixer, if the output frequency is the RF input frequency minus the local oscillator (LO) input frequency, it is called low-side injection (LO frequency is lower than RF frequency); if the output frequency is LO frequency minus RF frequency, it is called High side injection. The down conversion process can be expressed by:
fIF= fRF - fLO= - fRF+ fLO
Where fIF is the intermediate frequency of the mixer output port; fRF is the RF signal applied to the mixer RF port; fLO is the LO signal applied to the mixer LO port.
Ideally, the mixer's output signal amplitude and phase are proportional to the amplitude and phase of its input signal, independent of the LO signal. Under this assumption, the mixer amplitude response is linear with the RF input signal and also independent of the LO signal amplitude.
However, due to the nonlinear nature of the mixer, undesirable mixing products will be produced, known as spurious responses. The spurious response is caused by an interference or noise signal input by the mixer's RF port and produces a response at the IF frequency. Interference signals arriving at the RF input port may not be within the specified RF bandwidth, but can also cause problems. This type of signal usually has a high enough power. The RF filter before mixing cannot implement enough attenuation to cause additional spurious response, which directly affects the required IF signal. The mixing principle can be expressed as:
fIF= m fRF -n fLO= - m fRF + n fLO
Note that m and n are integer subharmonics of the RF and LO frequencies, and the combination of spurious products in the lattice is generated by mixing. Typically, the magnitude of these stray components decreases as m or n increases.
The corresponding RF input frequency range is known, the frequency is carefully planned, and the appropriate IF and corresponding LO frequency are selected. Careful planning of the frequency is important because it helps to reduce interference that falls into the effective signal band after mixing, and these sources of interference directly affect receiver performance. For wideband systems, it is more difficult to avoid spurious mixing products during frequency planning, and filters are needed to suppress out-of-band (OOB) RF signals that may fall into the IF band. The selectivity of the IF filter after the mixer is limited to only allowing the pass of the effective signal frequency, thereby attenuating the spurious response before the signal enters the final detector (after the mixer). The IF filter does not attenuate spurious responses within the IF band.
Many types of balanced mixers will reject spurious components with m or n being even. An ideal double balanced mixer rejects all harmonic components of even or m or n (or both). The IF, RF, and LO ports in the double balanced mixer are isolated from each other to minimize LO leakage and provide inherent RF to IF isolation. The double balanced mixer design provides the best linearity and reduces the filter attenuation requirements for each port.
Half-IF spurious frequency distribution
The second-order spurious response (called half-IF, 1/2 IF) is a very tricky special spurious signal. In the mixer, when m = 2, n = -2 is called low-side LO injection; when m = -2, n = 2, it is called high-side LO injection (Figure 1). For high-side injection, the input frequency that produces the half-IF spurious response is fIF/2 higher than the required RF signal frequency.
For example, the required RF center frequency is 2510 MHz (E-UTRA uplink channel number 39790). The RF frequency is mixed with the 2860 MHz LO frequency to produce an IF frequency of 350 MHz. In this example, 2685MHz is an undesired signal (or blocking signal) that produces a half-IF spur component of 350MHz. For low-side injection, the input frequency that produces the half-IF spurs is fIF/2 higher than the required LO frequency.
Figure 1: E-UTRA high-side LO injection example showing the required fRF, fLO, fIF and undesired fHALF-IF frequency distribution.
Assumption:
â—fRF center frequency = 2510MHz
â—fLO= 2860MHz
â—fIF = fLO- fRF= 2860MHz - 2510MHz = 350MHz
Calculate the blocking frequency that causes the spurious response:
fHALF-IF= fRF+ fIF/2 = 2685MHz
Check the algorithm to verify half-IF blocking or spurious frequencies:
2 &TImes; fLO - 2 &TImes; fHALF-IF = 2 &TImes; (fRF + fIF) - 2 &TImes; (fRF+ fIF/2) = 2fRF+ 2fIF- 2fRF- fIF= fIF
This causes the half-IF spurious frequencies to produce unwanted IF spurious signals:
2 × 2860MHz - 2 × 2685MHz = 350MHz
Receiver IP2
If the device data sheet does not directly give a 2x2 spurious response, it can be derived from the mixer's IP2 indicator. Assume that only the fundamental components of RF and LO are applied to the mixer port, and harmonic distortion is only produced by the mixer itself.
The image rejection filter of the RF path rejects any unwanted RF amplifier harmonics at the front of the mixer; the noise filter of the LO path rejects the harmonics generated by the LO injection. Strong input signals can produce distortion or intermodulation products at the input or output of the device or system. These products can be quantified by calculating the intermodulation (IP). The input intermodulation calculation assumes that the amplitude of the useful signal is the same as the input amplitude of the interfering signal component. If the mixer LO power remains constant, the order of the IP or distortion product is only determined by the multiplication of the RF (instead of the LO multiplication), because we only consider the variation of the RF signal, and the order represents the amplitude of the distortion product. The speed of increase as the input level rises. For example, due to the squared relationship, the magnitude of the 2nd order intermodulation (IM) product increases by 2 dB as the input signal increases by 1 dB.
Half-IF spurious power level
The following discussion uses the MAX19997A downconverting mixer as an example. The following specifications can be found in the AC electrical specifications of the data sheet:
â—RF spurious power is -5dBm (2685MHz)
â—LO level is set to +0dBm (2860MHz)
The typical 2LO-2RF spurious response is 64dB lower than the RF carrier level in dBc; 64dBc is the 2nd order Intermodulation Rejection Ratio (IMR2).
â— Calculated: PSPUR = -5dBm + (-64dBc) = -69dBm.
The MAX19997A's excellent 2x2 performance forms the following equivalent IP2 performance (IIP2) at its input:
IIP2 = 2 × IMR2 + PSPUR = IMR2 + PRF
= 2 × 64dBc + (-69dBm) = 64dBc + (-5dBm)
= +59dBm
Similarly, the MAX19985A 900MHz active mixer provides a typical 2RF - 2LO spurious response, which is equal to 71dBc under similar conditions:
IIP2 = 2 × IMR2 + PSPUR = IMR2 + PRF
= 2 × 71dBc + (-76dBm) = 71dBc + (-5dBm)
= +66dBm
E-UTRA LTE example
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