GB2209639A - Solid state high frequency single side band transmitter - Google Patents

Abstract

A transmitter is described in which envelope feedback control is used. Phase modulation is avoided by driving the power amplifier stage from a low impedance source so that non-linear current occasioned by interelectrode capacitance of the solid state amplifier devices, has no or little effect on the drive voltage waveform fed thereto. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO HIGH FREOUENCY SINGLE SIDE B RANTD TRANSMITTERS This invention relates to High Frequency (HF) Single Side Band (SSB) transmitters. The HF band is over-crowed and out-of-band emissions are extremely undesirable. Limits on such emissions are defined only by recommendations at the present time but are likely to be enforced more strictly in the future. It is possible, using valved transmitters, to improve linearity so that the unwanted emissions are - 40 dB relative to signal strength but such transmitters are inefficient and expensive. Linearisation of solid state HF SSB transmitters is difficult and complex (and hence expensive) using the known Polar Loop and Cartesian Loop Methods.

Envelope feedback has been used for many years in the linearisation of medium frequency amplitude modulated (AM) transmitters. In principle, it can be applied to HF SSB transmitters, although the nature of the modulation requires an amplitude detector of high linearity over a wide dynamic range.

Proponents of the Polar Loop and Cartesian Loop methods have claimed that envelope fedback is an unsatisfactory method of linearisation when applied to HF SSB solid state transmitters due to the non-linear (voltage dependent) feedback capacitance of high power bipolar or Field Effect Transistor (FET) output devices. This non-linearity causes AM to phase modulation (PM) conversion resulting in spurious phase modulated side bands. Such PM sidebands are not seen by the detector circiut and hence remain uncorrected by the envelope feedback loop.

It is an object of the present invention to provide a solid state transmitter for HF SSB transmissions, wherein the aforesaid disadvantage is overcome or minimised.

According to the present invention, there is provided a solid state transmitter, for high frequency single sideband transmissions using envelope feedback, wherein the input to a power amplifier stage of the transmitter is arranged to be of low impedance , whereby the drive voltage waveform input to the power amplifier stage is unaffected by the non-linear current flow occasioned by the interelectrode capacitance of semi-conductor amplifying devices of the amplifier stage.

The invention will be described further, by way of example, with reference to the accompanying drawing in which: Figure 1 is a block diagram of a simulated High Frequency Single Side Band solid state transmitter; and Figures 2a and 2b are graphical representations of the output of the test-bed transmitter.

Referring to the drawing, there is shown a test-bed arrangement for simulating a High Frequency Single Side Band (HF SSB) solid state transmitter. An element 10 is an exciter and represents a SSB modulated signal source. The element 10 would normally include all the circuitry necessary for transducing information to be transmitted and for amplitude modulating the same using a carrier wave subsequently translated to the appropriate transmitting frequency.

Filter means may be provided in the element 10 for reducing the amplitude modulated signal to single sideband, suppressed carrier The SSB modulated signal from the element 10 is fed to a splitter 11. From a first output of the splitter 11, the frequency modulated signal is fed to a PIN diode attenuator 12 wherein it may be attenuated to a greater or lesser extent as hereafter described. The appropriately attenuated SSB modulated signal is fed to a driver 13.

The driver 13 acts as a very low impedance source by transforming the signal input thereto.

The output of the driver 13 is fed to a power amplifier stage 14 in which the power amplifiers are field effect transistors (FETs). As is known, the gate/drain depletion capacitance of an FET gives rise to a non-linear current flow. Due to the low impedance of the source (the driver 13), this non-linear current flow has little effect on the gate drive voltage waveform (as compared with the effect which would be present if the driver 13 presented a high impedance source).

Phase modulation sidebands are thereby avoided.

The output of the power amplifier stage 14 is fed through a "thruline" wattmeter 15 to an attenuator load 16 and spectrum analyser 17 representing an antenna load to the transmitter.

In order to improve linearity of response of the transmitter, envelope feedback is provided.

A second output from the splitter 11 feeds the SSB modulated signal from the element 10 to a first demodulator 18. The output of the demodulator 18 is supplied to a signal processor 19 and, after processing, provides a drive control output to the amplifier 14 to provide open loop dynamic bias control.

Simultaneously, the output of the amplifier 14 is sampled and the sample is fed to a second demodulator 20. The output of the first and second demodulators 18, 20 enable the respective signal envelopes to be compared in the processor 19. An output representative of the difference between the envelopes is fed to the attenuator 12 to reduce or increase the attenuation of the frequency modulated signal passing therethrough in a sense such as to minimise the envelope difference, the difference between the outputs of the first and second demodulators 18, 20.

As can be seen in Figure 2, in which the figures 2a and 2b are graphical representations of a "transmitted" two-tone signal, the sidebands, though present, are some 55dB down on the wanted signal and only some 5 to 10 dB above the general noise level (-65dB).

The invention has been described in detail with reference to a particular test-bed example. It will be obvious to replace the element 10 in the manner aforesaid and the load 16 and analyser 19 with the necessary elements to provide a fully operative transmitter.

Similarly, each of the elements forming the transmitter, with the exception of the driver 13, may be conventional and selected from equivalent elements well known in the art. The driver 13 may be a conventional pre-amplifier to which a transformer or other low impedance output stage is added to ensure that the source impedance of the power amplifier stage 14 is very low.

Other variations are possible within the scope of the present invention as defined in the appended claims.

Claims (8)

1. A solid state transmitter, for high frequency single sideband transmissions using envelope feedback, wherein the input to a power amplifier stage of the transmitter is arranged to be of low impedance, whereby the drive voltage waveform input to the power amplifier stage is unaffected by the non-linear current flow occasioned by the inter-electrode capacitance of semi-conductor amplifying devices of the amplifier stage.

2. A transmitter as claimed in claim 1 wherein the amplifying devices comprise field effect transistors whose drain/gate capacitance occasions the non-linear current flow.

3. A transmitter as claimed in claim 1 or 2 further including a driver stage whose output to the power amplifier stage is transformed to form the low impedance input thereto.

4. A transmitter as claimed in claim 1,2 or 3 further including first and second demodulators for determining the signal envelope respectively before and after the power amplifier stage.

5. A transmitter as claimed in claim 4 wherein the first and second demodulators are arranged to operate at the final transmit frequency of the transmitter.

6. A transmitter as claimed in claim 5 or 6 wherein a signal representative of the difference between the signal envelopes detected by the demodulators is applied to an attenuator before the power amplifies stage in such sense as to minimise the envelope difference.

7. A transmitter as claimed in claim 5,6 or 7 wherein an output of the first demodulator is used to apply open loop dynamic bias control to the power amplifier stage.

8. A solid state transmitter substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.