Choosing an Architecture
The IF-sampling receiver architecture presents a clear argument why this level of integration is best done with SiP technology. Each component has unique and demanding requirements and therefore must rely on an optimized process. Consequently, there is no common process today. More significantly, the signal filtering at the IF must be very good to ensure out-of-band rejection of unwanted signals that could impact base station performance. Today this filtering is accomplished with surface acoustic wave (SAW) filters in hermetic ceramic packages. This type of filter is integrated in the LTM9005.
The LTM9005-AB can be used with an RF front end, for example, to build a complete UMTS band uplink receiver. Minimum performance of the receiver is detailed in the 3GPP TS25.104 V7.4.0 specification (Medium Area Base Station in Operating Band I, four carriers). An RF front end consists of a ceramic diplexer, along with one or more low noise amplifiers and ceramic bandpass filters. Here is an example of typical performance for such a frontend:
- Rx frequency range: 1920 to 1980 MHz
- RF gain: 17 dB maximum
- AGC range: 20 dB
- Noise figure: 1.6 dB
- IIP2: +50 dBm
- IIP3: 0 dBm
- P1dB: -9.5 dBm
- Rejection @ 20 MHz: 2 dB
- Rejection @ Tx band: 95 dB
The direct conversion receiver architecture, while less common than IF-sampling, offers several advantages over the traditional superheterodyne and provides the most potential for eventual monolithic integration. It eases the requirements for RF front-end bandpass filtering, as it is not susceptible to signals at the image frequency. The RF bandpass filters need only attenuate strong out-of-band signals to prevent them from overloading the front end. Also, direct conversion eliminates the need for IF amplifiers and bandpass filters. Instead, the RF input signal is directly converted to baseband.
Direct conversion does, however, come with its own set of implementation issues. Since the receive LO signal is at the same frequency as the RF signal, it can easily radiate from the receive antenna and violate regulatory standards. Regardless, the LTM9004-AC can be used with an RF front end to build a similar UMTS band uplink receiver, using the same 3GPP TS25.104 V7.4.0 specification (Medium Area Base Station in Operating Band I, four carriers) discussed earlier. An RF front end will again consist of a diplexer, along with one or more low noise amplifiers (LNAs) and bandpass filters. In this case, the variable adjustment for automatic gain control (AGC) will be placed in the RF domain in order to minimize gain or phase variation between the in-phase and quadrature channels. As before, the typical performance for such a front end is similarly consistent with the 3GPP standard:
- Rx frequency range: 1920 to 1980 MHz
- RF gain: 23.5dB maximum
- AGC range: 20dB
- Noise figure: 1.6dB
- IIP2: 50dBm
- IIP3: 0dBm
- P1dB: –9.5dBm
- Rejection at 20MHz: 2dB
- Rejection at Tx band: 96dB
Unwanted baseband signals can also be generated by second-order nonlinearity of the receiver. A tone at any frequency entering the receiver will give rise to a DC offset in the baseband circuits. The 2nd order nonlinearity of the receiver also allows a modulated signal, even the desired signal, to generate a pseudo-random block of energy centered about DC. Regardless, it is suitable for many applications today and holds promise for the future for many reasons – such as the potential for integration.
It is not always possible to achieve significant integration using the traditional, monolithic approach, especially when performance requirements are high. When semiconductor processes are incompatible, functions can still be integrated without compromising performance. New types of receivers prove that macro base station performance can be achieved in a fully integrated and compact package. Perhaps in time, the performance limitations can be overcome allowing a common semiconductor process for all of the functional blocks in the signal chain. Until then, the pressure for integration continues, with SiP technology providing significant performance and size advantages.
About the Author
Todd Nelson is module development manager at Linear Technology Corporation.