This article explains how to meet the high-performance requirements of a base station (BTS) receiver, particularly regarding half-IF spurs. To achieve this, engineers must understand the relationship between a mixer’s IP2 and its second-order response, and then select an RF mixer that satisfies the system's cascading requirements. The mixer's data sheet usually specifies second-order performance using either the second-order intercept point (IP2) or a 2x2 spurious suppression indicator. This paper explores the receiver design process and how to calculate overall half-IF spurs by analyzing these two parameters. As an example, we'll look at the MAX19997A, an active mixer designed for E-UTRA LTE receivers.
Mixer Harmonics
In a superheterodyne receiver, the mixer downconverts the high-frequency RF signal to a lower intermediate frequency (IF). This is known as the downconversion process. If the output frequency is the RF input frequency minus the local oscillator (LO) frequency, it is called low-side injection. Conversely, if the output frequency is the LO frequency minus the RF frequency, it is referred to as high-side injection. Mathematically, the downconversion can be expressed as:
fIF = fRF - fLO = -fRF + fLO
Here, fIF represents the IF frequency at the mixer’s output, fRF is the RF signal applied to the mixer’s RF port, and fLO is the LO signal applied to the mixer’s LO port.
Ideally, the mixer's output signal amplitude and phase are proportional to the input signal, independent of the LO signal. However, due to the nonlinear behavior of the mixer, unwanted mixing products—known as spurious responses—are generated. These spurious components result from interference or noise signals entering the RF port and producing a response at the IF frequency. Such signals may not be within the intended RF bandwidth but can still cause issues due to their high power levels.
The RF filter before mixing cannot always suppress these out-of-band signals sufficiently, leading to additional spurious responses that interfere with the desired IF signal. The mixing process can be generally described as:
fIF = m × fRF - n × fLO = -m × fRF + n × fLO
Where m and n are integers representing subharmonics of the RF and LO frequencies. The resulting spurious products depend on the combination of these values. Typically, the strength of these stray components decreases as m or n increases.
Careful frequency planning is essential to minimize interference that falls into the desired signal band after mixing. For wideband systems, this becomes more challenging, requiring filters to suppress out-of-band (OOB) signals that might fall into the IF band. The IF filter after the mixer helps attenuate spurious responses, but it does not eliminate those already present in the IF band.
Balanced mixers, especially double-balanced ones, are effective in rejecting even-order spurious components. An ideal double-balanced mixer isolates the IF, RF, and LO ports, reducing LO leakage and providing good RF-to-IF isolation. This design improves linearity and reduces the need for aggressive filtering at each port.
Half-IF Spurious Frequency Distribution
The second-order spurious response, often referred to as the half-IF spur, is a particularly complex type of interference. When m = 2 and n = -2, it corresponds to low-side LO injection. Conversely, when m = -2 and n = 2, it refers to high-side LO injection. In high-side injection, the interfering signal is fIF/2 higher than the required RF frequency.
For example, consider an RF center frequency of 2510 MHz (E-UTRA uplink channel number 39790), mixed with a 2860 MHz LO to produce an IF of 350 MHz. A blocking signal at 2685 MHz would generate a half-IF spur at 350 MHz. In low-side injection, the interfering signal is fIF/2 higher than the 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 = 2510 MHz
â— fLO = 2860 MHz
â— fIF = fLO - fRF = 2860 MHz - 2510 MHz = 350 MHz
Calculate the blocking frequency causing the spurious response:
fHALF-IF = fRF + fIF/2 = 2685 MHz
Verify the algorithm for half-IF spurious frequencies:
2 × fLO - 2 × fHALF-IF = 2 × (fRF + fIF) - 2 × (fRF + fIF/2) = 2fRF + 2fIF - 2fRF - fIF = fIF
This confirms that the half-IF spurious frequency produces an unwanted IF signal:
2 × 2860 MHz - 2 × 2685 MHz = 350 MHz
Receiver IP2
If the device data sheet doesn’t directly provide the 2x2 spurious response, it can be derived from the mixer’s IP2 value. Assuming only the fundamental components of RF and LO are applied, and distortion comes solely from the mixer itself, we can estimate the spurious response.
The RF path image rejection filter eliminates unwanted harmonics before the mixer, while the LO path noise filter removes LO-generated harmonics. Strong input signals can lead to distortion or intermodulation products, which are quantified using the intermodulation (IP) calculation. The IP assumes equal input amplitudes for the useful signal and the interfering component. If the LO power remains constant, the order of the IP depends only on the RF signal variation, as the LO is fixed.
Half-IF Spurious Power Level
Using the MAX19997A downconverting mixer as an example, the following specifications can be found in its AC electrical characteristics:
â— RF spurious power = -5 dBm (at 2685 MHz)
â— LO level = +0 dBm (at 2860 MHz)
The typical 2LO - 2RF spurious response is 64 dB lower than the RF carrier level in dBc, which is the 2nd order Intermodulation Rejection Ratio (IMR2).
â— Calculated: PSPUR = -5 dBm + (-64 dBc) = -69 dBm
The MAX19997A’s excellent 2x2 performance translates to an equivalent input IP2 (IIP2) of:
IIP2 = 2 × IMR2 + PSPUR = IMR2 + PRF = 64 dBc + (-5 dBm) = +59 dBm
Similarly, the MAX19985A 900 MHz active mixer provides a 2RF - 2LO spurious response of 71 dBc under similar conditions:
IIP2 = 2 × IMR2 + PSPUR = 71 dBc + (-5 dBm) = +66 dBm
E-UTRA LTE Example
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