WO1996002977A1

WO1996002977A1 - Method and apparatus for alias-driven frequency downconversion (mixing)

Info

Publication number: WO1996002977A1
Application number: PCT/US1995/008233
Authority: WO
Inventors: Thomas Land
Original assignee: Stanford Telecommunications, Inc.
Priority date: 1994-07-13
Filing date: 1995-07-13
Publication date: 1996-02-01
Also published as: AU3000295A

Abstract

Frequency conversion is achieved in a receiver by using sample and hold (44), and track and hold for circuits in place of conventional mixers. The invention enhances spectral power efficiency usig the alias-driven frequency translation techniques and is applicable to cover IF ranges from DC to 1 GHz (with sub-Hertz resolution).

Description

METHOD AND APPARATUS FOR ALIAS-DRIVEN FREQUENCY DOWNCONVERSION (MIXING) BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION:

The technique in which alternating electrical currents of different frequencies are mixed so that they modulate each other and produce, in the output components, frequencies equal to the sum and difference of the original frequencies, is called heterodyning and is traditionally achieved using a device most commonly referred to as a mixer. In modern communication systems, the mixer is a fundamental element present in many system designs. Although implementation of a traditional mixer may take one of several forms, a common feature of all traditional mixer implementations is the reliance on excitation of the mixer by a local oscillator (an alternating current source) of some fundamental frequency, f^, to achieve frequency translation of another signal by an amount equal in magnitude to

Traditional frequency translation methods rely upon local oscillator frequencies equivalent to the magnitude of frequency translation desired. When using conventional mixers, it is necessary that the local oscillator frequency be equal to the magnitude of frequency translation desired. Typically, as these local oscillator frequencies become higher, circuit complexity, behavior, and electrical power consumption increase. This is especially true in oscillator implementations supporting a broad frequency tuning range and having requirements for fine frequency tuning resolution, precision, and accuracy.

An object of the invention is to provide frequency downconversion apparatus and method which has a broad IF tuning range, enhanced spectral efficiency, and which simplifies local oscillator requirements; a further object of the invention is to provide frequency downconversion using sample-and-hold and track- and-hold circuits which have wide tuning range, low component- content and higher reliability.

The present invention provides apparatus and methods of RF frequency translation which may be termed Alias-Driven Frequency Downconversion. The invention can be implemented in part or in its entirety in analog signal-, digital signal-, and combined analog/digital signal-processing systems and is most effective when applied in translating a signal to lower frequencies (downconversion) more efficiently than existing techniques using conventional mixer technologies.

The ability to perform frequency translation of bandwidth- limited signals by integer multiples of the local oscillator frequency f^ and with greater spectral power efficiency results in substantial savings in design and implementation complexity, power consumption, size, and cost with simultaneous enhancement to performance and reliability may be realized over implementations based upon traditional methods of frequency translation.

Briefly described, in place of the conventional mixer used for downconversion in RF receivers, the present invention adapts electronic sample-and-hold and electronic track-and-hold circuits to achieve frequency translation to a lower frequency (downconversion) .

The invention is applicable to IF ranges from DC to 1 GHz (with sub-Hertz) resolution) using currently available technology and with substantially no circuit modification. The same broad input bandwidth in a conventionally designed system requires a very complex and broad range frequency synthesizer whose complexity is certain to scale upwards as the range of octaves covered by the synthesizer is increased.

BRIEF DESCRIPTION OF THE DRAWINGS:

The above and other objects, advantages and features of the invention will become more apparent when considered with the following specification and accompanying drawings wherein:

FIG. la is a block diagram illustrating an example of a conventional high-resolution, precision variable frequency translation method using a conventional mixer;

FIG. lb is a block diagram illustrating a variable frequency translation method using spurious frequencies output and filtered from a digitally synthesized sinusoidal oscillator and using a conventional mixer;

FIG. 2a is a block diagram of an ideal electronic sample- and-hold apparatus;

FIG. 2b illustrates the time-domain action of the ideal electronic sample-and-hold apparatus; FIG. 2c is an illustration of the components of the frequency-domain transfer function for an ideal electronic sample-and-hold apparatus;

FIG. 3a is a block diagram of an ideal track-and-hold apparatus;

FIG. 3b illustrates the time-domain action of the ideal electronic track-and-hold apparatus;

FIG. 3c is an illustration of the frequency-domain transfer function components of an ideal electronic track-and-hold apparatus;

FIG. 4a is a block diagram illustrating frequency translation to a lower frequency (downconversion) utilizing the Alias-Driven Frequency Translation method as implemented in an apparatus using an electronic sample-and-hold device;

FIG. 4b graphically analyzes the single-sided frequency- domain donwconversion of the apparatus described in Fig. 4a;

FIG. 5a is a block diagram illustrating frequency translation to a lower frequency (downconversion) utilizing the Alias-Drive Frequency Translation method as implemented in an apparatus using an electronic track-and-hold device;

FIG. 5b graphically analyzes the single-sided frequency- domain downconversion of the apparatus described in Fig. 5a;

FIG. 6a illustrates an architecture and apparatus for Alias- Driven Frequency Downconversion utilizing an electronic sample- and-hold apparatus; FIGS. 6b through 6g graphically analyze in the time domain and frequency domain the signal, f_in, as it progresses through the system presented in Fig. 6a at the three test points labeled TP4 through TP6;

FIGS. 7a and 7b compare the spectral power efficiency of frequency downconversion approaches using the ideal conventional mixer implementation of Fig. la or lb and of the Alias-Driven Frequency Translation implementation using an electronic sample- and-hold apparatus as illustrated in Fig. 6a;

FIG. 8a illustrates an architecture and apparatus for Alias- Driven Frequency Downconversion utilizing an electronic track- and-hold apparatus;

FIG. 8b through 8g graphically analyze in the time domain and frequency domain the signal, f_ln, as it progresses through the system presented in Fig. 8a at the three test points labeled TP7 through TP9;

FIG. 9 compares the spectral power efficiency of frequency donwconversion approaches using the ideal conventional mixer implementation of Fig. la or lb and of the Alias-Driven Frequency Translation implementation using an electronic track-and-hold apparatus as illustrated in Fig. 8a.

DETAILED DESCRIPTION OF THE INVENTION:

Referring to Fig. la, a conventional high resolution, precision variable frequency translation method is disclosed wherein the input F_IF signal is applied through an image reject filter 10 to a conventional mixer 11. A frequency control word F_HC0 is applied along with the clock frequency F_CIJ- to a number controlled oscillator (NCO) 12, the output of which is converted to an analog signal in digital-to-analog converter 13, low pass filtered by low pass filter (LPF) 14 and supplied as the signal from the numerically controlled oscillator F_HCo to a conventional mixer 15. A fixed frequency oscillator 16 supplies a second input to mixer 15 and the output is filtered by bandpass filter 17 so that the output F_HC0 + F_LO J-^s applied as a second input to mixer 11. The output from mixer 11 is passed through bandpass filter 18 as the downconverted signal F_IF - (F_NC0 + F_LO)_*

Fig. lb is a block diagram illustrating a variable frequency translation method using spurious frequency outputs and filtered from a digitally synthesized sinusoidal oscillator and using a conventional mixer. In this system the signal from the numerically controlled oscillator 12' is converted to an analog signal in a high performance digital/analog converter 13' and the signal is filtered in bandpass filter 17' and the resulting output F_CLK +/- F_NC0 is supplied through a high frequency amplifier 19 and applied as a second input to the conventional mixer 11'.

Fig. 2a is a block diagram illustrating an ideal electronic sample and hold apparatus wherein the analog input signal V_N (see Fig. 2b) is applied through an amplifier 20 which has a gain of 1 to electronic sampler switch 21 which is operated by an impulse generator 22 having a sample timing clock F_CLK (see Fig. 2b, top line). The sampled pulses from switch 21 are stored in a storage device, such as capacitor 22 and provided as an analog output through amplifier 23 which has a gain of 1. Fig. 2c (line 1) is a simplified diagram of the ideal electronic sample and hold apparatus shown in Fig. 2a. Fig. 2c (line 2) illustrates an ideal sampler transfer function for the sampler shown in Fig. 2c, (line 1). Fig. 2c (line 3) illustrates a zero-order hold transfer function for the circuit shown in Fig. 2c (line 1).

Fig. 3a is a block diagram of an ideal track-and-hold circuit in which an analog input signal V_1N is amplified by amplifier 30 (which has a gain of 1) and output is sampled by electronic sampling switch 31, which receives track/hold switch control signals from source 32. The output is stored on charge storage capacitor 33, passed through amplifier 34. Fig. 3b (line 1) shows the track (complement) and hold time intervals "T" and "H" which have a time period T₈. Fig. 3b (line 2) shows the time domain action of the track-and-hold apparatus of Fig. 3a. The input analog signal V_1H is shown as a sinusoidal wave (light trace) and the analog output V_ouτ (heavy trace) tracks the input signal during the track periods (T) and holds the last value during the hold periods (H). Thus, at 35-1 the output trades the portion of the sine wave during track period Ti and holds the last valve 35-2 during period H₂. During track period T_z the signal tracks the sine wave at 35-3 and holds the last value 35- 4, etc.

Fig. 3c (line 1) illustrates the frequency domain transfer function components of an electronic track-and-hold circuit and lines 2-4 illustrate the sampler transfer function (line 2), the hold transfer function (line 3) when T_H _< T₃ and the track transfer function (line 4) when T_τ is less than T_s.

Referring to the system shown in Fig. 4, the signal on antenna 40 is amplified by broad-band amplifier 41 and its output S_IN(τ) is applied as the input signal to band-limiting filter 42 which has a bandwidth equal to or less than 1/2 the sampling frequency f_s of local oscillator 43 (which in this embodiment is a square wave source). Sampling clock signals f_β from source 43 are applied to sample-and-hold circuit 44 (which ha the form shown in Fig. 2a). The output of sample-and-hold circuit 44 is filtered by low pass filter 45 which has a center frequency which is about 1/2 of the sampler clock frequency f,, and the filtered output is supplied to a utilization device 46 for further processing. Low order aliases 47 and higher order aliases 48 are shown in Fig. 4b (line 2) (before low pas filter 45) and the output signal to the utilization device 46 is shown in Fig. 4b (line 3) .

Fig. 5a shows a receiver system similar to Fig. 4a but using a track-and-hold circuit 50 for producing the alias-driven frequency translation. In Fig. 5b (lines 1-4), graphically illustrates the single-sided frequency f_a while Fig. 5 (line 2) illustrate the spectral output prior to the low pass filter 45' hold function (T_H hold time); Fig. 5 (line 3) shows the spectral output (prior to low pass filter 41) track function τ = track time, and Fig. 5 (line 4) shows the spectral output signal SQ_UT (f)-

Fig. 6a shows the architecture for the alias-driven downconversion system of this invention using the sample-and-hold system described earlier, and Figs. 6b through 6g illustrate time and frequency domain aspects of the signal at test points TP_4/ TP₅ and TP₆.

Figs. 7a and 7b provide graphical comparisons of spectrals of the system of Fig. la, lb (test point 3) with system of the present invention (test points in Fig. 6a).

Fig. 8a shows the architecture for the alias-driven downconversion system of this invention using the electronic track-and-hold system described earlier herein with the waveform at test points TP₇, TP_β and TP₉ being illustrated in figs. 8b through 8g.

Finally, Fig. 9 compares the spectral efficiency of frequency downconversion of the present invention with conventional mixer systems of the prior art. ADVANTAGES OF THE INVENTION:

Traditional frequency translation methods rely upon local oscillator frequencies equivalent to the magnitude of frequency translation desired. When using conventional mixers, it is necessary that the local oscillator frequency be equal to the magnitude of frequency translation desired. Typically, as these local oscillator frequencies become higher, circuit complexity, behavior, and electrical power consumption increase. This is especially true in oscillator implementations supporting a broad frequency tuning range and having requirements for fine frequency tuning resolution, precision, and accuracy. In simplifying local oscillator requirements, the method of Alias-Driven Translatio. of this invention offers several distinct advantages over the conventional method. These advantages are: Simplified tunable high resolution local oscillator design.

Figs. la and lb illustrate conventional approaches to high- resolution, precision tunable frequency translation. In the architecture of Fig. la, high-resolution, precision, and tunability are achieved using a numerically controlled oscillator. Since the tunable range of a numerically controlled oscillator (NCO) implemented as in Fig. la is restricted to frequencies below 0.5*_clk (theoretical Nyquist limit) and approximately 0.4*^ (practical implementation limit), where F_clk represents the clock rate of the NCO and which itself has practical limitations, the NCO output is frequently augmented by heterodyning with a precision oscillator in order to shift the tunable range of the numerically controlled oscillator into usable range of local oscillator requirements. Using this approach for frequency translation provides a frequency tuning bandwidth restricted to the lesser of the tuning range of the NCO, and the filter bandwidth of Filter-B. Disadvantages of this implementation architecture include: narrow tuning range, high component count, and lowered reliability.

Fig. lb illustrates an approach to synthesizing high- resolution, precision frequencies for local oscillator implementation by selectively bandpass filtering for a high order frequency spur generated by a numerically controlled oscillator at the output of a digital-to-analog converter. Using this method for frequency translation provides a frequency tuning bandwidth restricted to the lesser of the tuning range of the NCO and the filter bandwidth of the bandpass reconstruction filter, Filter-A. Because of the amplitude degradation associated with higher order spurious response out of the NCO, it is often necessary to provide signal amplification to the selected frequency spur. Disadvantages of this implementation architecture include: a narrow tuning range restricted by the need for a bandpass reconstruction filter, increased local oscillator phase noise and the increased power consumption necessary to support the high frequency amplifiers (both due to the degraded amplitude of the selected frequency spur), and the need for a high-performance digital-to-analog converter featuring the high output slew rates and fast settling time necessary to generate the desired high frequency spurious response and to suppress undesired frequency spur generation.

As described above and illustrated in the drawings, downconversion according to this invention provide a broad IF tuning range with enhanced spectral power efficiency.

The invention has a broad input signal range resulting in circuit simplification compared to conventional approaches when building a system with equivalent functionality. For example, the invention can be used to cover IF ranges from DC to 1 GHz (with sub-Hertz resolution) using currently available technology and with substantially no circuit modification.

While preferred embodiments of the invention have been described and illustrated, it will be appreciated that other embodiments, adaptations and modifications of the invention will be readily apparent to those skilled in the art.

WHAT IS CLAIMED IS:

Claims

1. In a receiver system having an antenna RF amplifier coupled to said antenna for receiving intelligence modulated RF signals and producing a received intelligence modulated (RIM) RF signal and means for downconverting said RIM RF signal for use by a utilization circuit, the improvement in said means for downconverting comprising: first filter means connected to receive said RIM RF signal and having a first filter output, aliasing circuit means including electronic switch means connected to said first filter output, and a storage means connected to said switch means, means for producing signal, and switch control signal means for applying a control signal f, to said electronic switch to connect and disconnect said RIM RF signal to said storage means, a low pass filter means having a center frequency fc <_ 1/2 Fs connected to said storage means, and means connecting said low pass filter means to said utilization circuit.

2. The receiver system defined in claim 1 wherein said aliasing circuit means is a sample-and-hold circuit.

3. The receiver system defined in claim 1 wherein said aliasing circuit means is a track-and-hold circuit.

4. In a receiver system having an antenna RF amplifier coupled to said antenna for receiving intelligence modulated RF signals and producing a received intelligence modulated (RIM) RF signal and means for downconverting said RIM RF signal for use by a utilization circuit, the improvement in said method for downconverting comprising: enhancing spectral power efficiency using alias-driven frequency translation of received intelligence modulated RF signals, and low pass filtering the aliased signal.

5. The invention defined in claim 4 wherein the downconversion is in the IF ranges from DC to 1 GHz (with sub- Hertz) resolution).

1