DIGITALLY REFERENCED RF FEEDBACK CARTESIAN LOOP
Original Publication Date: 1999-Jun-01
Included in the Prior Art Database: 2002-Aug-06
In order for base stations to support the complex base-band Cartesian feedback entails the sampling modulation schemes and multi-carrier operation of the station output, down-converting to base-band, demanded by modern systems, it is necessary to and comparing the down-converted signal with the improve the intermodulation performance of the lin- base-band input. Figure 1 shows a block diagram ear amplifier chains. One commonly used method for a typical Cartesian feedback loop. is base-band Cartesian Feedback. Conventional RF Out 0 RF Synth - Indicates Base Sand Signals - Indicates RF & IF Signals -- Indicates Local Oscillator Signals Fig. 1 Conventional Base-band Cartesian Feedback There are two main shortcomings of the Car- tesian feedback loop as it is used today in paging and land-mobile applications. First, the de-modula- tor is in the reverse path, and therefore any distor- tion introduced by this circuitry will not be correct- ed. This can significantly degrade the station's close-in noise performance. Also, any imbalances in magnitude or phase of the analog I and Q lines will result in the presence of an unwanted sideband, or "image". This image can raise havoc in simul- casting systems as well as cause symbol ringing. One possible way to improve both of these issues is to modulate in the digital domain, up-band to the desired transmit frequency, and couple this signal into the feedback path of a Cartesian feedback loop. The Cartesian loop is kept active, but the base-band input is zero. Doing so results in the block diagram shown in Figure 2. Note that the de-modulator is now in the forward path, and therefore distortion that it introduces will be corrected by the loop. Also, since the reference path is digital, it will not have an I/Q imbalance and therefore any image gen- erated should be minimal. Initial testing of this technique resulted in a noise-floor improvement of over 10 dB within the unity gain points, and an improvement of over 30 dB on the image level. Figure 3 shows a potential scheme for utilizing this technique. The digital reference path in this figure represents a high speed digital modulator, but vari- ous other digital schemes, e.g. DDS or direct DSP to the DAC, may be used as well.