IL132802A - Low spurious homodyne receiver - Google Patents

Abstract

An offset frequency homodyne receiver for receiving at least one input signal, comprising: (a) a limiting amplifier (2) for amplifying a sample signal to produce an amplified signal; and for each at least one input signal; (i) a respective first mixer (7) for mixing said each input signal with a reference signal (4) to produce a respective intermediate signal, and (ii) a respective second mixer (5) for mixing said respective intermediate signal with said amplifier signal to produce a respective output signal. 296 ו' בניסן התשס" ד - March 28, 2004

Description

Spurrious Homodyne *pttl Vb n Low Spurious Homodyne Receiver Low Spurious Homodvne Receiver Background of the invention 1. Field of invention The present invention relates to the field of wideband electromagnetic (and also of other wave phenomena like optical or acoustical) signals receivers used in any application of which but not limited to are communication, electronic warfare, radar, medical instrumentation, and parameter measurement systems. The novel method enhances the performance of wideband frequency offset Homodyne receivers by lowering the amount of spurious signals in a controlled manner. 2. Description of the related art Systems involved in wideband reception of electromagnetic signals such as electronic warfare, radar and communication systems are often required to measure accurately the amplitude and phase of the signal or a multiplicity of signals originated from the same source (i.e. Direction Finder (DF), Digital Frequency Discriminator (DFD), etc). The two major categories of receivers, which are being used in such systems, are wide instantaneous band receivers, and narrow instantaneous band scanning receivers. In many applications the specification of the system dictates the use of a wide instantaneous band receiver, which is the domain of the invention. Within the wide instantaneous band receivers there are different options: 1 A Direct Receiver, for which all the signal processing (amplification, filtering, dynamic range control or compression, amplitude and phase measurements) are being made by wideband components. It has the advantage of simplicity, but the drawbacks of high cost and inferior performance in parameter measurements 2. Autocorrelating or Homodyne Receiver, in which a sample of the signal is being mixed with the signal, producing a baseband signal, which may be processed by supposedly simple and cheap video and digital circuits. The major drawbacks of this approach are the DC offset and low frequency interferers which are hard to be removed from the measurement even by calibration over a wide dynamic range, and the strong dependence of the mixing (especially IQ mixing) process on the signal frequency 3. Offset frequency Homodyne Receivers, in which the signal is being mixed with a frequency shifted sample of itself, producing a single usually low frequency signal, preserving parameters of the original signal. This approach lends to single low frequency measurement- components with their superior performance and lower cost. The drawback of this approach stems from the spurious signals, which are generated during the mixing process These spurious signals contaminate the signal and limit the use of the method especially for systems with high dynamic range and high accuracy. This invention introduces a novel method for lowering the spurious signals of such receivers as much as may be accepted in the system.

The basic form of an offset frequency Homodyne receiver is illustrated in FIGURE 1. The input signal SI having a frequency f¾ (designated as Slffk] ) is being sampled by a sampler device (1) The sampled signal is amplified to fixed amplitude by a limiting amplifier (2). Then it is shifted in frequency by mixer A (3) and a local oscillator (4) having a frequency fL. In the example the conversion in frequency is up, thus the signal at BB' has a frequency fR + fL. In this example and all subsequent examples the choice between up-conversion and down-conversion is arbitrary, and may be changed without loosing generality of any method. The signal at BB'. SBB-(fk+fL] is sufficiently strong and constant in amplitude over all the input dynamic range, and it functions as the LO driving mixer B (5). The product of mixer B after mixing SlffjJ with SBB-tfR+ J is a signal at frequency ¾L- '/ The ideal description of the process is altered by the actual function of any mixer The output of any mixer contains spurious signals having frequencies which are any combination of the input frequencies multiplied by whole numbers: where fa is the input frequency, ft is the LO frequency, Μ,Ν are whole numbers. The magnitude of any spurious signal depends on the type of mixer, the signals themselves and usually decrease as jM|+|N] increase. It is customary to consider spurious signals up to the third order.

Referring back to the basic receiver of FIGURE 1, mixer A produces a desired signal at frequency fu+ft S2[fR+fi], along with a signal of equal strength at frequency fR-fL S3[fR-fil, and leakage signal at f¾: S4[f j. The other spurious signals are either out of band thus they will be filtered out by the band-pass filter (6), or too weak to be considered. These signals mixed with Sl(fk) in mixer B produce the following signals: S5 - S1*S2 = S5[(fR+ft fc] = S5[fL]. S6 - S1*S3 = S6[fR-(firfL)] = S6[fiJ and other less important signal S5 is the desired signal. S6 is a spurious signal having the same magnitude as S5. It is clear that a spurious signal of such magnitude renders the receiver useless.

Homodyne receiver with SSB modulator in the LO channel is illustrated in FIGURE 2. Here, the basic receiver has been modified, with mixer A being replaced by a Single-Side-Band suppressed carrier modulator: SSB (7). The SSB is a type of mixer, which ideally produces only one signal having either the sum of the frequencies of the input signals or the difference frequency. The first is referred to as upper sideband modulator and the latter is a lower sideband modulator (unlike a simple mixer, which produces equally the two products) There are many implementations and names to the SSB, but the resultant function is the same, thus in the following explanations the SSB is defined by its function rather than specific implementation. The most important figures of merits of a SSB contributing to the spectral purity of the Homodyne receiver are the amount of the undesired sideband suppression and carrier suppression relative to the desired sideband. The other spurious signals suppression is also of importance in several cases. Apparently, introducing a SSB in place of mixer A should null out the undesired signal produced by the other sideband. Nevertheless actual non-ideal performance of the mixers should be analyzed. As an example we take a high performance SSB modulator producing in cut BB' the spectral components SI to S7 illustrated in the left-hand portion of FIGURE 3. The sideband rejection is -25 dB, carrier rejection -20 dB, Insertion Loss 0 dB (normalized signals). Other spurious signals on the figure are typical. LO power to the mixers +10 dBm, IP3 of mixer B: +15 dBm, LO frequency fi-o, input power of Sl[f ] at BB* cut: -50 dBm. The input signal S8 is superimposed on the figure, illustrating all the input signals to mixer B. The output spectrum of mixer B at cut CC is illustrated in the right hand portion of FIGURE 3. A filter around fLo eliminates the DC and unwanted harmonic spurious signals, but S9 and S 10 are stray signals, which can not be filtered out being at the same frequency as the desired signal Si 1. SI 1 is the mixing product of S5 and S8, S9 is the mixing product between S5 and S4, while S10 is due to S5 and S6. S 12 is a low-level spurious signal resulting from high-order intermodulations. The level of S9 and S10 is much higher than the signal, and it is constant (does not depend on the signal strength). It is obvious that this configuration has a very limited useful dynamic range, over which large errors exist due to the spurious signals.

An Image-Reject Mixer (IRM) is a type of mixer functioning as a special type of down-converter. The IRM selects either all the frequencies above the LO frequency and rejects (attenuates) all the frequencies below the LO frequency, or vice-versa. In the first case it is called upper-sideband IRM, while the latter is lower-sideband IRM. The sideband rejection is the relative attenuation between a signal in the desired band to a signal of the same level in the rejected sideband. There are different implementations of the IRM, but in the following discussions we will not be limited to a specific implementation. Any reference to IRM is to its resultant function.

Replacing mixer B with Image-reject Mixer (IRM) will attenuate the spurious signal S10 by the amount of Image Rejection, for example by 25 dB, but the dominant spurious signal S9 will not be changed, rendering the receiver almost useless.

Reference Patent 1 introduces a complicated solution to remove the spurious signals. In the method as depicted in FIGURE 4, both the signal and its replica are being shifted in frequency by different amounts, preferably by a 2: 1 ratio, before being mixed down together. The analysis made in reference 2 shows that all the spurious signals of the first order do not fall on the same frequency of the desired signal, thus they are readily filtered out. The reference also claims to achieve a 36 dB spurious rejection experimentally, without stating the dynamic range.

This invention introduces a novel different approach that is simpler, yet the amount of spurious signals may be controlled to any practical purpose.

Summary of the Invention 1. The present invention solves the problem of spurious signals in offset frequency Homodyne receivers. The basic idea is to shift the frequency of the signal in the signal path (using SSB modulator) rather than in the LO path with its constant high power. This configuration produces spurious signals, which are relative to the signal rather than at constant high power in the former art. The use of an Image Reject mixer in the last conversion stage further reduces the spurious signals. The relative power of the spurious signal to the desired one is the sum in dB of the sideband suppression of the modulator and the image rejection of the IRM. This allows for reduced specifications of the modulator and mixer, with the possibility to alleviate the demands from one of them at frequencies where the other component has better performance. The receiver benefits from using low performance components still achieving high overall performance. This is done using only single modulation as opposed to the last mentioned prior art, which has a complicated costly dual frequency modulation scheme. 2. The performance improvements over the prior arts are most significant, enabling the use of Homodyne reception for wideband, wide dynamic range, high accuracy systems, using single frequency conversion with low-performance components. 3. In another embodiment of the invention, a multiplicity of channels sharing a common local oscillator and signal sample, form a multi-channel Homodyne receiver. 4. In another embodiment of the invention, a filtering means in the frequency or time domains is inserted in the Limiting Amplifier path. A band pass (or low pass or high pass) filter will pass only the signals in a selected band, and will improve the receiver sensitivity. A band stop filter will reject interfering signals. The filters may be of any type, fixed or variable. A switch will shut down reception at specific time intervals. This feature is relevant in the single channel receiver, but it is most important in the multi-channel receiver since filtering may be implemented only on the single common channel, instead of on each and every channel. 5. In another embodiment of the invention, a frequency selective filter fixed or tunable is inserted between the SSB modulator and the IRM, improving the spurious rejection of the circuit.

Brief Description of the drawings Further details are explained in the detailed description of the invention.

FIG. 1 illustrates the prior art of a basic offset frequency Homodyne Receiver. FIG. 2 illustrates the prior art of a SSB modulator in a basic Homodyne Receiver.

FIG. 3 illustrates the typical signals at different cuts of Figure 2.

FIG. 4 illustrates the prior art of dual frequency modulation Homodyne Receiver.

FIG. 5 illustrates the first embodiment of the present invention.

FIG. 6 illustrates the signals present at different cuts of Figure 5.

FIG. 7 illustrates a second embodiment of the invention comprising of a multiplicity of channels, fed by a common LO and signal sample.

Detailed description of the present invention The invention illustrated in figure 5 consists of a signal sampler device (1) amongst which but not limited are couplers, splitters, hybrids, spatial samplers like antennas or a multiplicity of antennas etc. The sampled signal is amplified to a fixed high level power over the whole dynamic and frequency range by a limiting amplifier (2). The signal out of the limiting amplifier at cut DD' serves as the LO signal of an IRM (5). The signal path consists of a SSB (7), which is driven by a Local Oscillator (4). The output of the SSB at cut BB' feeds the IRM, which produces an IF output (at cut CC) at the frequency of the Local Oscillator, having a phase and amplitude relative to the input RF signal. This signal is further filtered by filter (6), eliminating out-of-band spurious signals and a DC component. The practical implementation of the circuit should contain other components, which are common practice and knowledge in the art of receivers, and are not shown on FIGURE 5 since they do not have any significance referring to the essence of the invention. Such components may include amplifiers, isolators, attenuators, filters etc., providing enough isolation, gain, power, filtering and matching, as dictated by standard engineering to obtain full performance. In order to explain the circuit function we will follow the following example. FIGURE 6 illustrates the important signals in different cuts of the circuit of FIGURE 5. The performance of the mixers is typical of medium performance mixers. In the example the LO power is lOdBm, RF input power is -50 dBm, signal SO at frequency fk out of the Limiting amplifier has constant power of 10 dBm, SSB modulator sideband rejection: -15 dB and carrier rejection: -20 dB. IRM Image rejection: -15 dB, Insertion Loss of mixers. 0 dB (normalized signals). The left-hand portion of FIGURE 6 depicts the signals S 1 to S7 in cut BB'. The right-hand of FIGURE 6 depicts the signals at cut CC\ S8 is the desired signal, and is the mixing product of SO and S5. S9 is an in-band spurious signal, and is the mixing of S3 with SO. S3 has been already attenuated by 15 dB by the SSB, and is further attenuated by 15 dB by the IRM, producing S9, which is 30 dB below the desired signal. Si 1 is out-of-band spurious signal, and is the mixing product of S6 and SO. S12 is out-of-band spurious signal, and is the image rejected product of S2 and SO. S 13 is out-of-band signal, and is a product of S7 and SO S14 is out-of-band signal, and is the image rejected product of SI and SO. S10 is a higher order intermoduiation product between low-level signals, and is negligible compared to the dominant spurious signal S9. The DC component and all the spurious signals except of S9 and SlO will be eliminated from the output by filter (6). The overall result is an offset frequency Homodyne reception with low spurious content, using low performance SSB modulator and IRM, and simple single frequency offsetting scheme.

In another embodiment of the invention illustrated in FIGURE 7, a multiplicity of signal channels provide a multi-signal Homodyne reception, where all the IF signals follow the relative amplitude and phase of the input RF signals. The Local oscillator signal is split by a splitter device (8), feeding the LO port of the SSB modulators (7) of the individual channels. The signal sample may be taken as described in the first embodiment of the invention, or by a combination of the samples from the channels. In FIGURE 7, the signal sample is shown as a separate input The signal sample is amplified to a constant high-level signal by a limiting amplifier (2), then it is split by a power splitting device (9) to feed the LO port of the IRMs (5) of the individual channels Again, additional provisions of good engineering practice in the field should be included in every practical implementation, without limiting the extent of the invention. Such multiplicity of channels is required in applications like Direction Finders, Frequency discriminators, Polarization Diversity measurements etc.

In another embodiment of the invention, a filtering means in the frequency or time domains is inserted in the Limiting Amplifier path (component (2) of FIGURE 5 and FIGURE 7). A band pass (or low pass or high pass) filter will pass only the signals in a selected band, and will improve the receiver sensitivity. A band stop filter will reject interfering signals The filters may be of any type, fixed or variable. A switch will shut down reception at specific time intervals This feature is relevant in the single channel receiver, but it is most important in the multi-channel receiver since filtering may be implemented only on the single common channel, instead of on each and every channel. This feature not only enhances the performance of the receiver, but also it is less complex and more cost effective than other choices like filtering on every channel. Furthermore, any calibration made between the channels is kept, even after the insertion or tuning of any filter in the common path.

In another embodiment of the invention, a frequency selective filter fixed or tunable is inserted between the SSB modulator and the IRM, improving the spurious rejection of the circuit.

A laboratory prototype of a 6-18 GHz triple channel Homodyne Receiver utilizing the method has been built and tested. The rejection specification of the SSB modulator and IRM has been -15 dB. A spurious content of -30 dB has been demonstrated, rendering a peak differential phase error of less than 3.6 degrees. o References Patent Documents , U S A. Patent No. 5,661,485 Thomas B Manuel ' Other Publications William B. Sullivan, Microwave Homodyne Receivers, Applied MICROWAVE & WIRELESS WINTER 1995.

James Bao-Yen Tsui, Microwave Receivers and Related Components. John Wiley and Sons, 1986, p. 59.

Stephen E. Liplsky, Microwave Passive Direction Finding, John Wiley and Sons,

Claims (12)

132802/2 WHAT IS CLAIMED IS:

1. An offset frequency homodyne receiver for receiving at least one input signal, comprising: (a) a limiting amplifier for amplifying a sample signal to produce an amplified signal; and (b) for each at least one input signal: (i) a respective first mixer for mixing said each input signal with a reference signal to produce a respective intermediate signal, and (ii) a respective second mixer for mixing said respective intermediate signal with said amplified signal to produce a respective output signal.

2. The receiver of claim 1 , wherein each said first mixer is a single sideband mixer.

3. The receiver of claim 1 , wherein each said second mixer is an image reject mixer.

4. The receiver of claim 1 , further comprising: (c) a signal sampler for producing said sample signal from the at least one input signal.

5. The receiver of claim 4, wherein said signal sampler is operative to produce said sample signal from one of the at least one input signal.

6. The receiver of claim 4, wherein said signal sampler is operative to produce said sample signal from a combination of the at least one input signal.

7. The receiver of claim 1, further comprising: (c) a local oscillator for producing said reference signal.

8. The receiver of claim 1 , further comprising: (c) for each at least one input signal, a respective bandpass filter for filtering said respective output signal.

9. The receiver of claim 1, further comprising: (c) a filter for filtering said amplified signal prior to said mixing of said amplified signal with said at least one intermediate signal.

10. The receiver of claim 1 , further comprising: (c) for each at least one input signal, a respective frequency selective filter for filtering said respective intermediate signal prior to said mixing of said respective intermediate signal with said amplified signal.

11. An offset frequency homodyne receiver for receiving a plurality of input signals, comprising: 132802/2 (a) a limiting amplifier for amplifying a sample signal to produce an amplified signal; (b) for each input signal: (i) a respective first mixer for mixing said each input signal with a reference signal to produce a respective intermediate signal, and (ii) a respective second mixer for mixing said respective intermediate signal with said amplified signal to produce a respective output signal; (c) a local oscillator for producing said reference signal; and (d) a splitter for distributing said reference signal among said first mixers.

12. An offset frequency homodyne receiver for receiving a plurality of input signals, comprising: (a) a limiting amplifier for amplifying a sample signal to produce an amplified signal; (b) for each input signal: (i) a respective first mixer for mixing said each input signal with a reference signal to produce a respective intermediate signal, and (ii) a respective second mixer for mixing said respective intermediate signal with said amplified signal to produce a respective output signal; and (c) a power splitter for distributing said amplified signal among said second mixers. 67897 TE L - AVI V I SRA EL