GB2471516A - Upconverter circuit suitable for electronic countermeasure - Google Patents

Abstract

A circuit for up-converting an input baseband RF signal to a GHz frequency range comprises a digital pulse generator whose output is connected to an input of an active mixer whose other input is arranged to receive the baseband RF signal. The output of the active mixer is coherent with the baseband RF signal and comprises the baseband signal sampled by a pulse train generated by the digital pulse generator. The pulse train has controllable pulse width. The mixer may be a Gilbert cell mixer. It is disclosed that a commercially available XOR gate may be used as the mixer as it is known that such a gate incorporates an active mixer in the form of a double-balanced Gilbert cell. The pulse generator and the mixer may be fabricated on a single integrated circuit using high cut-off frequency bipolar technology, such as InP HBT. The output signal comprises a comb of frequencies which may be filtered and amplified for use as an electronic countermeasure (ECM) in order to jam radar signals.

Description

UP-CONVERTER CIRCUIT

This invention relates to an electronic radio frequency up-converter. Although it is suitable for different applications, it is particularly well adapted to radar and electronic attack systems. Such systems usually process the signal at a lower carrier frequency than that used for transmission and reception and so require coherent up and down conversion of the signal frequency. Based on the sampling principle, this invention offers complementary operation to the track-and-hold (or sample-and-hold) process, which is used for frequency down-conversion.

Conventional frequency up-converters for this purpose have used multiple stage mixers, fast-tuning oscillators and filters, and have been expensive, heavy and large, with rather poor reliability and maintenance costs.

The present invention provides an integrated circuit for frequency up-converting an input baseband RF signal, comprising a digital pulse generator whose output is connected to an input of an active mixer whose other input is arranged to receive the baseband RF signal, whereby the output of the active mixer is coherent with the baseband RF signal and comprises the baseband signal sampled by a pulse train generated by the digital pulse generator.

The invention, when compared with previous up-converter circuits, can show improvements in reconfigurability, simplicity, low power consumption and integrability.

It can have a good linearity of signal response, and need only consume low input power. It can be made light in weight and small in size. It does not require the use of broadband passive circuit components such as couplers and distributive elements.

There is no need for a high power conversion clock.

The benefit of using the same semiconductor technology for the pulse generator and for the active mixer makes the up-converter circuit more integrable. It can be implemented in a single MMIC, monolithic microwave integrated circuit, with a high cut-off frequency bipolar technology, such as InP HBT.

In order that the invention may be better understood, embodiments of the invention will now be described by way of example only, with reference to the accompanying schematic drawings, in which: Figure 1 illustrates in functional terms an integrated up-converter circuit embodying the invention, with waveforms representing amplitude against frequency of the baseband signal and the output RF signal; Figure 2 is a circuit diagram of the up-converter circuit of Figure 1; Figure 3 is a circuit diagram of an experimental realisation of the up-converter circuit of Figure 2; Figure 4 is a graph showing the output power in dBFS of the up-converter circuit of Figure 3, obtained with a 2GHz sampling frequency with a 2Ops pulse width and a baseband signal at 250MHz; and Figure 5 is a circuit diagram of an alternative experimental realisation of the up-converter circuit of Figure 2.

As shown in Figure 1, the up-converter circuit embodying the invention comprises a digital pulse generator whose input receives a continuous signal at radio frequency from a local oscillator, and whose output is connected to an input of an active mixer, whose other input receives a baseband signal, at a frequency substantially lower than that of the pulse generator. This sampling of the baseband signal generates high side and low side images around every multiple of the sampling frequency i.e. of the frequency of pulses of the digital pulse generator. These integral multiplies of this output frequency are illustrated as upward arrows in the graphs representing the output RF signal and the input baseband signal in Figure 1. The images of the baseband signal are illustrated as shaded regions over a spread of frequencies, above and below each sampling frequency multiple.

The baseband RE signal may be the output of a digital radio frequency memory, DRFM, replaying a stored portion of a signal derived from an incoming radar transmission. The output of the up-converter, which comprises pulses of RF at a comb of frequencies, may be filtered in a band pass filter (not shown) and amplified, for example in a travelling wave tube, before being transmitted as an electronic counter measure, ECM in order to jam the incoming radar signal. The pulsed RE output is coherent with the baseband signal.

The preferred implementation of the up-converter circuit is shown in Figure 2, in which the digital pulse generator or output driver is an integrated circuit logic gate with an output buffer and a differential output. This output is fed to the active mixer which is a conventional double-balanced Gilbert cell mixer. The other input of the active mixer is a baseband signal, provided in differential form. The output of the active mixer is also in differential form, in this example.

The circuit shown in Figure 2 represents one type of logic gate used to produce a pulse train. Various alternatives are possible using logic gates or comparators, based on a high cut-off frequency bipolar technology, typically lnP HBT. The Gilbert cell mixer is preferably realised in this same semiconductor technology. Other bipolar semiconductor technologies could be used in place of lnP HBT, and other Ill-V compounds could replace the lnP, The pulse train signal and the baseband signal are multiplied in the broadband active mixer to produce the RF output signal, typically in the range 2 to 18GHz. As mentioned above, optionally the output signal is filtered to select one or more of the required frequency multiples of the sampling frequency. In the time domain, the output signal is a pulse train modulated in amplitude; and in the frequency domain, the baseband signal is transposed around multiples of the sampling frequency. These images are attenuated by the frequency shape of the pulse.

An implementation of the circuit of Figure 2 is shown schematically in Figure 3. In the practical circuit of Figure 3, a commercially available exclusive or, XOR, gate is used, with appropriate terminations, since it is known that such a gate incorporates an active mixer in the form of a double-balanced Gilbert cell. A continuous signal from the local oscillator is fed through a saturated broadband amplifier, with a typical frequency range of 2 to 18GHz, and an amplification of typically 45dB, to generate a square wave signal, which is fed through a differentiator to a bias tee with a DC offset from earth.

The output of the bias tee is a digital pulse train fed to one of the differential inputs of the XOR gate, which has a broadband frequency range of up to 50GHz: The corresponding negative differential input is provided by a 500 terminator. The differential input buffer of the XOR gate is then used as a comparator to remove negative pulses and generate a digital pulse train. The DC offset controls the pulse width.

The other main differential input of the XOR gate is taken from a 1800 power splitter, operable in the frequency range 50MHz to I GHz, whose input is the baseband signal.

The frequency of the baseband signal may range from 1 MHz typically to 1 GHz.

The output of the XOR gate is in differential form, and one differential output is terminated at 50Q. The other provides the up-converted output.

Although in this example an XOR gate is used, as such gates are commercially available, it will be appreciated that the invention may be implemented using an active mixer in any form, such as a Gilbert cell, to which is fed the digital pulse signal from e.g. the broadband amplifier in saturation.

Figure 4 is a graph of output power, in dBFS, against output frequency, in GHz, obtained typically with a 2GHz sampling frequency and 2Ops pulse width, for a baseband signal of 200MHz.

Figure 5 is a circuit diagram of an alternative to Figure 3, with the pulse generator and the active mixer in the same semiconductor technology. The sinusoidal clock signal is split into two in-phase signals. One is delayed by a few picoseconds relative to the other one, and these signals are connected to the inputs of an AND gate as single-ended inputs. The AND gate generates a differential pulse train which is used in one of the XOR gate inputs. The pulse width is close to the differential delay between the split signals from the power splitter. The other XOR gate input receives the baseband signal in differential mode. Clearly, the Gilbert cell is again the only essential element of the active mixer.

To provide the highest integrability of the up-converter circuit, all its components are implemented in a single MMIC with a high cut-off frequency bipolar technology.

Claims (9)

1. CLAIMS1. An integrated circuit for frequency up-converting an input baseband RE signal, comprising a digital pulse generator whose output is connected to an input of an active mixer whose other input is arranged to receive the baseband RF signal, whereby the output of the active mixer is coherent with the baseband RF signal and comprises the baseband signal sampled by a pulse train generated by the digital pulse generator.
2. 2. A circuit according to Claim 1 in which the active mixer is a double-balanced Gilbert cell.
3. 3. A circuit according to Claim 1 or Claim 2, in which the digital pulse generator and the active mixer are implemented in the same semiconductor technology.
4. 4. A circuit according to Claim 3, in which the semiconductor technology is lnP HBT.
5. 5. A circuit according to Claim 3, in which the semiconductor technology is a Ill-V technology.
6. 6. A circuit according to any preceding claim, in which the digital pulse generator and the active mixer are implemented in high cut-off frequency bipolar semiconductor technology.
7. 7. A circuit according to any preceding claim, having a broadband frequency range of at least 2 to 18GHz.
8. 8. A circuit according to any preceding claim, constituting a monolithic microwave integrated circuit, MMIC.
9. 9. An integrated circuit substantially as described herein with reference to the accompanying drawings.