CA2126821A1 - Wide band microwave modulator arrangements - Google Patents

Abstract

ABSTRACT

Wide Band Microwave Modulator Arrangements A wide-band microwave modulator arrangement for an information transmission system, such as a video signal distribution system, in which signals from a microwave carrier signal source are split and directed over a first path comprising an attenuator and a second path comprising phase shift means and a balanced modulator, the output signals from the two paths being combined for transmission.
Signals from the carrier signal source may also be directed over a third path including phase shift means and a second balanced modulator, amplified output signals from the first two paths being combined with output signals from this third path before transmission.

Description

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, Wide-Band Microwave Modulator Arrangements The present in-/ention relates to wide-band microwave modulator arr~mgements.
According to the present invention a wide-band microwave modulator arrangement comprises a microwave carrier signal source, means to direct signals from said source over a first path including attenuator means and over a second path 5 including phase shift means and a balanced modulator arrangement to which wide-band modulation signals are arranged to be supplied, and means to combine signals from the outputs of said first and second paths for transmission.
~` A video signal distribution system incorporating a wide-ba~d rnicrowave modulator arrangement in accordance with the present invention wi11 now be described `~ 10 by way of example with reference to the accompanying drawings, of which:-Figure 1 shows schematically a transmitter station of the system, ~igure 2 shows schematically in more detail one form of the up convertor or . modulator of Figure 1, Figure 3 shows schematically in more detail a second form of th~ up convertor 15 or rnodulator of Figure 1, Figure 4 shows schematically a receiver station of the system, ` 1 Figure S shows schematically a return path transmitter of the system, a~d .~j Figures 6 and 7 illustrate diagrammatically range/loss characteristics of the -.~ system.

Referring first to ~igure 1, the first part of the t~ansmitter consists of a source 1 of rnillimetre wave power sufficient to drive the modulator/upconverter. I~is may : ~ be a stabilised GUNN oscillator, synthesised source or any convenient medlod ~own ~, to the ar~ with stability and spectral purity adequate for the application. The source ,, is to be provided with an electronic tuning port capable of frequency shift modulation up to the rate necessary for the return path application. Tlle output of the source is taken via a low pass filter 2 (E~igure 2) to limit the level of even order harmonics of the source frequency applied to the modulator.
The modulator is shown in Figure 2. The input signal is split into two paths by means of a -3dB, 180 hybrid coupler 3. The carrier reference path 4 is fed via a microwave iso1ator 5 to the input port of a second reversed -3dB, 180 coupler 6.
The modulation path 7 takes the output from the second port of the coupler 3 andfeeds this signal via a phase shifter 8 and a level set attenuator 9 to the bal~mced }û modulator 10. The modulator match is isolated from the modulation path 7 by means of microwave isolators 11 and 12 on its input and output ports. The modulation port of the modulator 10 is fed with the desired amplitude modulated intelligence at ~he same frequency and channel spacing as that of the cable video signal by way of afilter 13. The action of the modulator is to impose amplitude modulation of the 15 rnicrowave carrier at the modulation rates of dle complete band vf cable video signals.
To provide high linearity of modulation across the working bandwidth both the amplitude and phase of the superimposed modulation may be adjusted by means of the level set attenua~or 9 and the phase shifter 8 in the modulation path 7. By this means '.~, ` ~ ~ the superposed modulation may be adjusted so as to exactly represent pure amplitude 20 modulation across the working bandwidth of the modulator. Furtherrnore, if the input signal to the modulator 10 be prediseorted in both amplitude and phase so as to :`,.j compensate for the inherent residual non-linearity of the transmitting amplifier chain, 3 then the modulator controls may be used to optimise the overall performance of the ,, ' chain by means of the inherent capability of the modulator to provide any i :;1 combination of modulation between pure phase modulation and pure amplitude modulation. The modulator is therefore capable of introducing asymmetry into the spectrum S(o~) to compensate for the effects of the following system transfer function, K(ic~). If so desired, an additional modulator 14 may be operated in parallel, as 5 shown in Figure 3, so as to provide feedforward of a pro]portion of the modulated -. carrier to be summed with the output at a suitable point 15 in the system so as to provide additional compensation of non-linearity.
` The output of the modulator 10 is fed via a band limiting filter to a linear amplifier chain 16 with sufficient gain to raise the output power to the desired 10 transmission level. The amplifier chain may comprise of any suitable type available, solid state or thermionic, providing that the amp1itude and phase linearity is .~ sufficiently good to provide negligible levels of distortion of the applied signal. The ` ~ level of composite triple beats between individual channels of the applied signal ;~ should be -56dB. with ~espect to the peak carner power in the transmitter.
The output of the amplifier chain 16 is fed via a microwave isolator (not shown) to the ~ransmitting antenna. The antemla type is dependant on application and will provide for the desired polarisation of the tiransmitted wave, preferably circular , polarisation. Circular polarisation is chosen to elirninate the effects of multipath reception in the receiver causing ghosting on the demodula~d video signal. The :,20 reception of transrnitted energy via two or more paths of different lengths, due to reflection and re-radiation, leads to interfering signals in the receiver whose phase differs from that of the energy received via the direct path. Where the system is . ., used for area disttibution it will be impossible to avoid the reflection and re-radiation ~-of some energy due to the existence of buildings and other structures within tihe .,~

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antenna radiation pattern. The use of a high gain antenna for the receiver system only provides partial reduction of the effect as the antenna sidelobes have finite gain.

Furthermore, antenna gain and therefore beamwidth, is lirnilted by engineering costs.

If a circular polarised wave is reflected from an arbitrary scatterer, the reflected wave 5 is of opposite rotation to the original transmitted wave. Th;s provides an additional -30dB. of isolation from the first reflected wave. Under conditions of precipitation, such as fog or rain, the water droplets in the atmosphere form spherical reflectors ~ capable of producing severe multi-path scattering, due to continual re-reflection and `1 the effects of variable electrical perrnifflvity along the link path. Circular polarisation .~
lO consisti of the vectoral rotation of both E and H fields in a direction and radial velocity set by the geometry of the transmitting anlenna. The receiving antenna must exactly replicate the geometry of the transmitter so æ to maximise the energy taken up from the rotating field. All other polarisations, differing in geometry, radial velocity and relative phase delay, are rejected in some degree by the receiving antenna 15 system. Maximum rejection is achieved for waves of opposite radial velocity or fixed polarisation where the field cornponents are an~phase to the standing wave on the receiving antenna. Where the anteMa is immersed in a radiated field a proportion ` ~ of whose energy is of the correct polarisation for that antenna and the rest consists of arbitrary and random polarisation, the antenna acts as a pola~isation filter, rejecting 20 those components whose phase and amplitude do not reinforce the average wave on the antenna.

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The system provides the facility for the use of a retum pa~h using orthogonal complex modulation. In this application, both frequency modulation and amplitude modulation are superimposed on the carrier radiated by the main transmitter. The use s of double sideband amplitude modulation for the video frequency channels means t'nat the transmitter phase and amplitude linearity has to be strictly controlled over the full working bandwidth. In the case of amplitude modulation, both the amplitudes and phases of the upper and lower sidebands with res"ect to the carrier amplitude and 5 phase are identical. In the case of pure phase modulation, the amplitudes of the upper and lower sidebands are identical and the phases differ by 180. Therefore, if a detector sensitive only to envelo,pe amplitude variation and a detector sensitive omy . to phase variation of the carrier are used with a carrier carrying both for.ns of modulation intel~igence and the system linea~ity is such that no cross modulation of 10 AM to PM or PM to AM terms occurs, both forrns of modulation may be used without interference. The modulation rate and modulation index of the frequency modulation must be set so that higher order products of the frequency modulated wave do not fall into the AM video frequency baseband. This ~i nits the deviation and modulation rate of the FM signal to sideband levels not exceeding -26dBc at 15 modulation rates up to 50kBaud. To limit the production of nigher order sidebands i ~`~and to maximise dynamic range, matched fil~e;s are used at the trans~,nitter and the receiver for the frequency modulated signa'l. The difference between the bandwidth of the receiver FM intermediate frequency amplifier and the AM intermediate frequency amplifier corresponds to 21dB improvement in signal to noise ratio. This 20 is sufficient to compensate for the lower level of intelligence modulated on the FM
channel out to the maximum working range of the transmitter.
~`Frequency modulation is applied to the source by the use of a direct electronic `~tuning port in the preferred application. Modulation can also be applied via a synthesised source if so desired by means of direct or indirect modulation of the ` :`
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: ~' ':. , ' ' : ' .j 6 ~, reference frequency or the upconversion of the frequency modulated carrier onto a ;~ subcarrier within the synthesis loop.

The main receiver is shown in Figure 4. The signals are received by the antenna whose gain is chosen according to the application and which provides for the S depolarisation of the circular polarised wave back to a TEM wave on transmission line to feed the receiver mixer. For applications where the receiver is operated in conditions of high signal strength, a variable pad attenuator is incorporated between the antenna and the mixer port. A single superheterodyne is used to downconvert the received signal to an intermediate frequency commensurate with the bandwidth 10 requirements of the complete signal. The output of the intermediate frequency amplifier is fed to a synchrodyne demodulator using an injection locked l:)ielectric Resonator Oscillator (DRO) the output of the DRO being phase coherent with the carrier of the double sideband signal to within ~1-20. The output of the DRO is fed ~ to two synchronous dernodulators, using high level FET mixer technology. One .~ 15 channel is fed in phase with the IF signal, to provide AM demodula~ion. The other is fed through a gQ phæe shifter, to provide demodulation of the F~ close to carrier `~ signal and may also be used, in conjunction with a fre~uency lock loop, to provide a frequency steering voltage to a varactor on the DRO. This will allow ~he .$ demodulator oscillator to be aligned with signals outside its injection locking 20 bandwidth, at which point, injection locking and phase alignment will occur. The ~` ~ combination of the narrow bandwidth (high quality factor or 'Q') of the oscillator resonator and the use of amplitude limitation of the fundamental IF frequency fed to , ~ the locking port of the DRO, serve to strip off the modulation sidebands of the IF
:~ signal, to leave a clean carrier reference for the demodulators.
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'> 1 P~S/4591 Both upper and lower sidebands are detected and summed then passed through a low pass filter to produce the cable video signal as originally modulated onto the carrier at the transmitter. This signal, after suitable amplification and buffering is directly distributed into the cable network. The amplitude oF the signal at the detector S is monitored by an automatic gain control system which adjusts the gain of the intermediate frequency amplifier to ensure that the detector is operated with a signal ' input sufficient to maintain the synchronous detector in its most linear region of - operation.
The frequency demodulation channel output is fed to a low pass filter and 10 video amplifier. The demodulated digital signal is amplified and buffered before feeding to the microprocessor controller for that section of the cable network.
' To provide a method whereby the first local oscillator provides the correct ;, frequency input to the first mixer, an output is taken from the DRO at 12GEIi and ;~ mixed with the signal from a 4.25GHz offset synthesis loop. The upper sideband 15 output of this mixer is taken, via a bandpass filter and a}nplified to the correct power - . level for the local oscillator port of the first mixer. Thus, any movement of the signat ~, in the IF passband is compensated, firstly by the movement of the 12GHz oscillator towards the direction of the frequency shift and secondly by the movement of the effe tive lF frequency in the direction necessary to counter the shift. This effectively ` 1 20 doubles the available tuning range for a given oscillator deviation, making the best . possible useage of the available injection locking bandwidth.
To allow the system to provide infonnation regarding the use by subscribers . of the cable vi~eo signal, a r~turn path transmitter is integrated into each main receiver site. The transmitter is shown in Fig. 5. A simple frequency modulated GIJNN

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8 PS/459t oscillator source is integrated into an E-plane waveguide structure containing a `i microwave reflective PIN attenuator and isolators. The output of the E-plane waveguide structure containing a microwave reflective PIN attenuator and isolators.
The output of the E-plane waveguide is fed to a high gain ~mtenna whose bore sight 5 is aligned back to the main transmitting site. The GUNN oscillator is thermally stabilised by means of an integrated temperature controlled heater to reduce thermally induced frequency drift. In norrnal operation, the GUNN oscillator runs continuously ~ with the PIN attenuator in the O~ state, providing -26dB of isolation of the output ., ~ .
signal. As the power output of the GUNN oscillator at the antenna is of the order of 10 ~16dBm, then radiated power in this state is circa -lldBm. This power level is insufficient to activate the receiva at the main transmitter site or to give substantial leakage into the main receiver via the antenna sidelobes. The frequency of operation of the return path system is chosen so as to fall outside the working bandwidth of the cable video modulation on the main transmitted carrier. The receiver at the main ~ ~
`~ 15 transmitter site is tuned so as to be sensitive to the ~eturn path transrnission only. ~ ~ ;
Interrogation signals are transmited from the main transmitter using the narrow band .~ . , digital frequency modulation capability of the rnicrowave source. These suitably . coded signals are picked up by the main reseivers at each remote site and are ;;~j demodulated and fed to the microprocessor controller at that location. If ~e decoded 20 signal is recognised by the controller, then ~e return path transrnitter is energised, the PIN attenuation is reduced to minirnum and a handshaking sequence is transmitted, ~
back to the main site. The main site controller can now exchange inforrnation with .:
: the remote receiver site, each message being subject to a handshaking protocol. As each message contains an identification sequence that completely identifies the context .
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of the following information, then the main site can multiplex messages or instructions to make the best possible use of the a~/ailable transmit time on the return path loop.
The system as above delineated is designed to make the best possible use of the double sideband band signal in providing frequency and space diversity. As is S well known in the application of FM systems for area distlibution, multiple sources and antennas lead to no-capture areas in which the received signals at a given point are severely distorted due to multipath beats. This phenomenon may be avoided by the use of spread spectn~n techniques where sufficient frequency diversity is applied ~ to ensure that a reasonably phase coherent signal is always present at the receiver.
;` 10 Such techniques, which involve rapidly switching the transmitter output frequency, are 3. not applicable to cable video signals where the effective signal modulation rates can exceed SWMHz. However, a double sideband signal provides, where the modulation rate is substantial, a degree of ~equency diversity in real time. The upper and lower sidebands in amplitude modulation, are in phase with each other and the carrier `"'`lqF
5 frequency. Therefore, if the subcarrier groups, carried as amplitude modulation are ` 3 sufficiently spaced either side of the carrier, the process of demodulation compensat~s ~ for frequency dependant amplitude and phase changes along ~he translmission path.

.; If this is combined with spatial diversity at the transrmitter antennas, then the !; , resultant system is largely immune to ~e effects of non-capture within its area of ~ 20 application due to Fbeats produced by path length differences. Where a large area rmust ., be served, more than one transmitter must be used to ensure full coverage. To arrange that all the transmitters within the service area have no sensible o~erlap in ~'F their radiation patterns or are completely synchronised in such a way as to compensate ` ~ for path length differences is impractical and results in a high degree of systern ;~..', ~i g 2 1 redundancy and extra cost.
A further effect that is alleviated by the above system is that of varying path length phase and amplitude change due to changing atmospheric conditions due to water vapour, pressure and temperature. These have a marked effect on phase delay, in particular at the microwave frequencies used in this system, producing time variant modulation of both chrominance and level signals in a frequency modulated system.
Providing that phase delay equalisation in the modulation frequency band is used for the modulation applied to each transmitter in the net~,vork then the amplitude 3 modulated system is not affected by path length changes in electrical permittivity. -As the above system has been designed to allow the reception of multiple transmitted signals at a single receiver without the production of significant levels of intermodulation due to path length delay; this capability may be used to overcome the accepted inverse square-law losses from a single radiating transrnitter and single `~ receptor.
Figure 6 shows the accepted picture of the spherical geometry associated with an isotropic source and a single receiver. As the ~ansmitter is assumed to transrnit equal energy to all portions of the surrounding spherical sur~ace, ~hen the dec~ease in energy received at a single point receiver on that surface, at a given distance R, corresponds to the ratio between the fixed receptor area and the total area o~ the 20 spherical surface. That is, from the given geometry, proportional to l/R .
Next, consider ~igure 7, where the transmitted energy, equal to that in the previous example, is distributed over a spherical surface at the same range R and radiates inwards to the single receptor at the centre of dle spherical geometry. It is :!~

obvious that the inverse square law does not apply in this case of inverse spherical geometry and that the only losses associated in the integrated energy at the point receptor are those due to resistance losses in the transmission medium. It follows that any system of directed radiators, arranged so as to beam energy at a constant range to a single receptor, has an improvement in performance directly proportional to the increase in effective area of the total radiating surface. If the total radiating area can be made equal to the equivalent to the corresponding spherical surface at that . range, by means of the sum of the antenna gains of all the source antennas, together with the receiver antenna gain, then the path length loss is purely resistive.
Any practical system that sums the received power at a point from multiple 3 sources will outperform a single source system. The combination of multipaths in a cellu1ar network, together with the concornitant equalisation of modulation phase at each transmitter, provides the optimal cellular solution.
For efficient functioning of the above system, the operational bandwidth of 15 both the transmitter and the receiver must be large, to ensure that radio fequency phase ;~ distortion is kept to a minimum. lhe R~ characteristics of all transmitters and receivers in the network must also be matched to ensure that phase ripple across the , modulation bandwidth does not degrade the intermodulation performance of the ; system. However, this is in line with normal microwave system pra~ise at these ~`3 ~' 20 frequencies.

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Claims (3)

1. A wide-band microwave modulator arrangement for an information transmission system comprising a microwave carrier signal source, means to direct signals from said source over a first path including attenuator means and over a second path including phase shift means and a balanced modulator arrangement to which wide-band modulation signals are arranged to be supplied, and means to combine signals from the outputs of said first and second paths for transmission.

2. A wide-band microwave modulator arrangement in accordance with Claim 1 wherein signals from said source are directed over a third path including a further phase shift means and a further balanced modulator arrangement to which said wide-band modulation signals are arranged to be supplied, signals from the output of said third path being arranged to be combined with the outputs of said first and second paths for transmission.

3. A wide-band microwave modulator arrangement in accordance with Claim 2 wherein the combined signals from said first and second paths are arranged to be amplified before the signals from the third path are combined therewith.