TW201731263A - Chirp generator - Google Patents

Abstract

A chirp generator comprises a direct digital synthesiser operable to generate a frequency chirp, wherein the frequency chirp is restricted to span less than one octave, a carrier signal that has a higher frequency than the frequency chirp, a mixer for mixing the frequency chirp and the carrier signal to produce a signal that has a lower sideband that has a rising frequency response and an upper sideband that has a falling frequency response, a filter for selecting one of the sidebands and rejecting the other 10 sideband, and at least one component that has a frequency response that compensates for the frequency response of the selected sideband thereby to provide a substantially flat frequency chirp.

Description

Continuous frequency conversion signal generator

The present invention relates to a continuous variable frequency signal generator. In particular, the present invention relates to a continuous variable frequency signal generator for use in radar applications.

Many radars and some spectrum analyzers use sweep or continuous frequency conversion signals with output frequencies in the microwave or (sub)millimeter range. Gradually, these radars require a broadband continuous frequency conversion signal with 10% or more of the bandwidth to achieve fine range resolution. It is also desirable that the continuous variable frequency signal generator circuit produces a continuously variable frequency signal that is very linear, has good amplitude flatness, low phase noise, and has a very short time duration. In some applications, successive continuous frequency conversion signals are also required to be phase coherent (for example, to allow Doppler processing). These different requirements may conflict with each other as the required bandwidth increases.

Gradually, direct digital synthesis is used to generate high linearity, low phase noise, repeatable short duration continuous length continuous frequency conversion signals. When using direct digital synthesis, there are two special ways to generate wideband continuous frequency conversion signals: (i) upconverting the direct digital synthesis continuous variable frequency signal output to the microwave carrier, or (ii) using the direct digital synthesis as a sweep The reference is shifted to control a microwave voltage controlled oscillator in a phase locked loop. The former provides lower phase noise, but because of the limited range of output frequencies that can be obtained from direct digital synthesis of continuous frequency conversion signals, and the existence of spurious spurs in the direct digital synthesis output spectrum, the available bandwidth Subject to limit. The latter provides a wider bandwidth, but because of the effective loop bandwidth of the phase-locked loop, there is a higher phase noise and less dynamic flexibility. The output of the continuous variable frequency signal generator is often at the microwave frequency and can be used directly or multiplied to a higher frequency. Although the frequency multiplication processing will detract from the fidelity of continuous frequency conversion signals, it is still the preferred source of good quality continuous frequency conversion signals. In many applications, including sub-millimeter wave 3D imaging radars for personnel screening, direct digital synthesis with up-conversion is preferred.

Direct digital synthesis of continuous frequency conversion signals, while providing many advantages, still presents two special drawbacks that are not conducive to their use for broadband continuous frequency conversion signal generation. First, the amplitude response of the direct digital synthesis follows the sinc(sin x/x) response, so the continuous frequency conversion signal will have a falling frequency response. The lack of amplitude flatness results in a range point response impairment of the continuous variable frequency signal radar. Second, the direct digital synthesis output spectrum contains discrete spurious signals at multiple frequencies that are connected to the output frequency. When the direct digital synthesis frequency is continuously frequency-converted, the spurious signals will move up and down in the output spectrum, which will actually contaminate the desired signal.

Some direct digital synthesis devices include a 1 sinc compensation filter that eliminates the need for compensation flatness, but most direct digital synthesis devices do not, especially for higher clock speed devices. Direct digital synthesis of controlled voltage controlled oscillators and phase-locked loops avoids the problems of flatness and spurious signals, but suffers from other disadvantages previously mentioned. As above, the equalization filter can be used to flatten the DDS roll-off, but it is not the case, because such filters are inherently lossy. Some broadband radar systems that use direct digital synthesis appear to accept these impairments as inevitable impairments, but their performance is degraded.

According to the present invention, there is provided a continuous variable frequency signal generator comprising: The digital synthesizer is operable to generate a frequency continuous frequency conversion signal, wherein the frequency continuous frequency conversion signal is limited to span less than one frequency multiplied; a frequency higher than the frequency continuous frequency conversion signal of the carrier signal; a mixer, Mixing the frequency continuously variable frequency signal and the carrier signal to generate a signal having a lower sideband and an upper sideband, the lower side having a rising frequency response and the upper side having a falling frequency response; a filter for Selecting one of the sidebands and rejecting the other sidebands; and at least one device having a frequency response that compensates for the frequency response of the selected sideband to provide a substantially flat frequency continuous frequency conversion signal.

The filter can be adapted to select the lower sideband and reject the upper sideband, and the at least one device has a frequency response that compensates for the rising frequency response of the lower sideband.

The filter can be adapted to select the upper sideband and reject the lower sideband, and the at least one device has a frequency response that compensates for the falling frequency response of the upper sideband.

The at least one device having a falling frequency response can include an amplifier. In general, these devices have a falling frequency response.

An isolator may be disposed between the mixer and the lower sideband filter to suppress reflection of the energy of the upper sideband.

A frequency multiplier can be configured to multiply the bandwidth of the substantially flat frequency continuous variable frequency signal. In this case, an isolator and a bandpass filter may be provided behind the frequency multiplier to remove unwanted frequency components from the frequency multiplier.

10‧‧‧Fixed frequency reference clock

12‧‧‧Direct digital synthesizer

14‧‧‧Low-pass filter

16‧‧‧Mixer

18‧‧‧Local oscillator

20‧‧‧Underside

22‧‧‧Upside

24‧‧‧Isolator

26‧‧‧Bottom bandpass filter

28‧‧‧Amplifier

30‧‧‧Frequency multiplier

32‧‧‧Isolator

34‧‧‧Bandpass filter

36‧‧‧Response of the bandpass filter on the upper sideband

LO‧‧‧Local oscillator leakage

The aspects of the present invention will now be described by way of example only with reference to the accompanying drawings in which: FIG. 1 is a block diagram of a continuous variable frequency signal generator; 2 shows various signals at different positions in the continuous variable frequency signal generator of FIG. 1; and FIG. 3 shows the response of the upper sideband band pass filter used in the continuous variable frequency signal generator of FIG.

Figure 1 shows an up-converted direct digital synthesis continuous variable frequency signal generator 10. The continuous variable frequency signal generator has a direct digital synthesizer 12 for creating a waveform from a fixed frequency reference clock 10. The maximum frequency from the direct digital synthesizer is typically limited to 40% of the clock frequency. The frequency range of the continuous variable frequency signal generated by the direct digital synthesizer 12 is limited to just less than one frequency multiplied by the control software. This prevents the secondary resonance wave of the signal from entering the continuously frequency-converted frequency band.

The direct digital synthesizer is coupled to a low pass filter 14. This is adapted to conduct the highest frequency signal that the direct digital synthesizer is hopping, and rejects the main spurious signal at (f_CLK-f_OUT), the clock frequency and its resonant wave, and appears in the clock. The frequency and the aliased output of the direct digital synthesizer above its resonant wave. The low pass filter is directly connected to the intermediate frequency input of a mixer 16. The signal input to the mixer 16 is shown in Figure 2A. This has a falling frequency response that follows the sinc envelope. This signal mixes the output of a higher frequency local oscillator 18 at the mixer 16.

The output of the mixer 16 is a signal having both a sum term and a difference term. This is shown in Figure 2B. At this stage, the signal has two frequency bands, a side band 20 and an upper side band 22. The lower sideband 20 has a rising frequency response. The upper sideband 22 has a falling frequency response.

At this output, the mixer 16 is directly connected to an isolator 24, which is isolated The device 24 is directly connected to the lower sideband bandpass filter 26. The lower sideband bandpass filter 26 is adapted to conduct the lower sideband 20 and to block the local oscillator leakage LO and upper sideband 22 of Figure 2B. The isolator 24 is configured to minimize signal amplitude chopping by suppressing reflection of the rejected upper sideband.

An amplifier 28 is directly coupled to the lower sideband bandpass filter 26. The amplifier 28 essentially has a falling frequency response. The significance is that when the sideband signal 20 passes through the amplifier 28 at the rising frequency, it is automatically frequency compensated to flatten the response. This is shown in Figure 2C.

Amplifier 28 is directly coupled to a frequency multiplier 30 which multiplies the flat response from amplifier 28 to multiply the signal bandwidth. This is shown in Figure 2D. A second isolator 32 and bandpass filter 34 are used behind the frequency multiplier 20 to clear the output signal and reject any unnecessary leakage terms from the multiplier 20 (for example, at frequencies f and 3f) Leakage). This provides a clean, flat output with the desired, extended bandwidth.

Although the amplifier of Figure 1 is shown to substantially flatten the response (see Figure 2C), in practice, the response can be planarized by the combined effect of all of the devices behind the lower bandpass filter.

The system of Figure 1 is adapted to output a substantially flat, frequency multiplied signal based on the lower sideband 20 output from the mixer 16. However, the invention is equally applicable to providing a substantially flat, frequency multiplied signal based on the upper sideband 22 output from the mixer 16. In this case, the lower sideband bandpass filter 26 of Figure 1 is replaced by an upper sideband bandpass filter. Figure 3 shows the response 36 of the upper sideband bandpass filter. As will be seen from this figure, the upper sideband bandpass filter allows the upper sideband 22 to pass and blocks the lower sideband 20 and local oscillator leakage. Because of the side band 22 has a falling response relationship, and the device behind the upper sideband bandpass filter is selected to compensate for this response. In particular, the amplifier, the frequency multiplier, the isolator, and the bandpass filter 34 are selected to compensate for the falling response of the upper sideband.

In fact, in both embodiments of the invention, the devices used to flatten the response must be selected dependent on their own frequency response. This can be achieved by measuring or calculating the frequency response. A common method for measuring the frequency response of such devices is to use a scalar or vector network analyzer or an adjustable signal source plus a spectrum analyzer.

Those skilled in the art will appreciate that variations of the disclosed configurations may be employed without departing from the invention. Accordingly, the above description of specific embodiments is presented by way of example only and not for the purpose of limitation. It will be apparent to those skilled in the art that minor modifications can be made without significantly altering the operation.

10‧‧‧Fixed frequency reference clock

12‧‧‧Direct digital synthesizer

14‧‧‧Low-pass filter

16‧‧‧Mixer

18‧‧‧Local oscillator

24‧‧‧Isolator

26‧‧‧Bottom bandpass filter

28‧‧‧Amplifier

30‧‧‧Frequency multiplier

32‧‧‧Isolator

34‧‧‧Bandpass filter

Claims (8)

A continuous variable frequency signal generator comprising: a direct digital synthesizer operable to generate a frequency continuous frequency conversion signal, wherein the frequency continuous frequency conversion signal is limited to span less than one frequency; a frequency is higher than the frequency continuously a carrier signal of the frequency conversion signal; a mixer for mixing the frequency continuous frequency conversion signal and the carrier signal to generate a signal having a lower side band and an upper side band, the lower side having a rising frequency response and the upper side band Having a falling frequency response; a filter for selecting one of the sidebands and rejecting the other sidebands; and at least one device having a frequency response that compensates for the frequency response of the selected sideband, thereby providing a substantially Flat frequency continuous frequency conversion signal. A continuous variable frequency signal generator according to claim 1, wherein the filter is adapted to select the lower sideband and reject the upper sideband, and the at least one device compensates for the rise of the lower sideband Frequency response of the frequency response. The continuous variable frequency signal generator of claim 1, wherein the filter is adapted to select the upper sideband and reject the lower sideband, and the at least one device compensates for the lower sideband Frequency response of the frequency response. A continuous variable frequency signal generator according to any one of claims 1 to 3, wherein a low pass filter is disposed between the direct digital synthesizer and the mixer to remove unnecessary comparisons. High frequency spurious output signal. A continuous variable frequency signal generator according to any one of claims 1 to 3, wherein the at least one device having a falling frequency response comprises an amplifier. A continuous variable frequency signal generator according to any one of claims 1 to 3, comprising an isolator between the mixer and the sideband filter. A continuous variable frequency signal generator according to any one of claims 1 to 3, further comprising a frequency multiplier for multiplying the bandwidth of the substantially flat frequency continuous variable frequency signal. A continuous variable frequency signal generator according to item 4 of the patent application scope includes an isolator and a band pass filter for removing leakage from the frequency multiplier.