Quadrature compensation for orthogonal signal channels

IIIIIIIIIIIH

US005400363A

United States Patent [19] [li] Patent Number: 5,400,363

Waugh et al. [45] Date of Patent: Mar. 21,1995

1 2

is undesirable, and tends to degrade the fidelity of the

QUADRATURE COMPENSATION FOR received data signals.

ORTHOGONAL SIGNAL CHANNELS SUMMARY OF THE INVENTION

BACKGROUND OF THE INVENTION 5 The aforementioned problem is overcome and other

~ . .. . , , . . „ . . advantages are provided for a system employing or

The invention disclosed herein is a subject inven- 7 • i t. i ■ \- e

^ »> J „ . . .,.oc n/mn o I. thogonal signal channels with earner regeneration for

tion under NASA Contract. NAS5-33000, Subcon- t, °. , r , , . . . .. e &, .

t NT Fiannn.Tiq'in signal channels by introduction of quadrature com

ta~ .?".," . pensated carrier reference signals. The quadrature com

This invention relates to a signal processing system 1Q tion ides for adjustment of a phase offset

employing orthogonal signal channels with carrier re- the carrief reference signals a lied t0 ortho generation, such as a QPSK (quadrature phase shift onal signal cnannels t0 mailltain a desired cophasal keymg) demodulator, operative with an mput data sig- relationship between a carrier reference and a correnal which is a composite of two orthogonal signal com- sponding signal component in each of the orthogonal ponents, the demodulator having biphase and quadra- 15 gig^ channels. The foregoing operation of the inventure channels for extraction of inphase and quadrature tion is independent of the manner in which the carrier is components of the data. More particularly, the inven- regenerated. For example, the carrier can be regenertion relates to a QPSK demodulator having a variable ated by a process including a squaring of an input signal phase offset circuit at an output of an oscillator for in the case of a two-phase signal, or by use of a second adjusting a phase difference between reference carrier 20 squaring to provide the fourth power of the input signal components to the two channels to compensate for a in the case of a four-phase signal, by way of further deviation in orthogonality of the components of the example, the carrier can be regenerated by cross multiinput data signal. plication of inphase and quadrature signal channels, as is The Costas loop is a well-known form of a demodula- accomplished in well-known fashion in a Costas loop, tor for reception of a QPSK signal. A basic form of the 25 The invention is particularly useful for reduction of Costas loop is shown in FIG. 1 of Waters, U.S. Pat. No. crosstalk in systems wherein there is a cross multiplica4,344,178. A mathematical description of an input com- tion of signals of the orthogonal channels, such as in a posite signal having inphase and quadrature compo- Costas loop QPSK demodulator, nents is disclosed in Col. 1 of Ryan, U.S. Pat. No. The principles of the invention are demonstrated, in 4,092,606. The incoming QPSK signal, to be processed 30 the case of a preferred embodiment of the invention, for by the demodulator, is composed of two orthogonal a Costas loop demodulator wherein a signal of each data components, namely, an inphase (I) component and a channel is multiplied by the signal of the other channel, quadrature (Q) component. Mathematically, the two tMs bdnS a cross channel multiplication, to produce a signals may be represented as two sinusoidal waveforms product for each channel which serves as an output wherein one of the waveforms is a sinusoid and the 35 for each channel. The output signals of the two other is a cosinusoid. Each component is modulated J*3TM618 subtracted from each, other to provide a separately with data, by shifting a phase of the compo- lo,°P error for contro1 °/ a VC° for regeneration nent bv 180 degrees earner to serve as a reference for demodulating

The Costas loop is provided with two signal chan- ^ °f *w0 °*hoS°na! ^ ^ c«rier is aP"

nels, an I channel and a Q channel. The input signal is 40 Phfed t0 a phase-offset device to provide two earner

r* u it. i , -it. it. • i. • i reference signals which are offset from each other by a

split between the two channels with the mphase signal . , ,° rr,n , ~. . c

4.u- _4. 4.JU4.1.T1.1 J 4.1. nominal value of 90 degrees, the offset earner reference

component being extracted by the I channel, and the . i • i-J4. ^ r,, , ,

., . r . * j i_ .j. /-v signals being apphed to respective ones of the channels,

quadra hire signal component being extracted by the Q ^ a/xm^ ^ theVmvention) the phase-offset

channel. The two channels provide signals which are 45 deviceisautomaticauyadjusted by a further loop of the

combined to provide a loop error signal for dnving a a compensation lo t0 adjust the of

voltage controUed oscillator (VCO). The VCO outputs hase offset bg^ween ^ ^ ... ... m CQr.

a earner reference signal for demodulation of the sinu- respondence ^ ^ of nase deviation from

soidal components of the composite mput signal. A true orthogonality of the respective carriers of the two

phase shifter is employed at an output tenmnal of the 50 orthogonal signals of the composite signal applied to an

oscillator to provide both mphase and quadrature refer- mput terminal of the coStas loop. The compensation

ence earner signals for use by the I and the Q channels. loop k implemented by summing together the output

The foregoing form of demodulator provides satisfac- product signals of the two channels to develop a phase

tory operation in the usual situation wherein the inphase correction signal which commands the phase-offset

and the quadrature signal components are truly orthog- 55 device to alter the phase offset from its nominal value of

onal, that is, wherein the sinusoidal components are 90 degrees in accordance with the aforementioned

shifted in phase by 90 degrees relative to each other. pnase deviation from orthogonality of the inphase and

However, there are situations wherein the source of the the quadrature signals.

composite input signal may be somewhat imperfect In the case wherein the two signal carriers are truly resulting in the production of a composite signal 60 orthogonal, the summing together of the output prodwherein the two components deviate in their orthogo- Uct signals of the two channels provides for a fixed nality. For example, if such a defective composite signal value of phase correction signal which commands the is applied to the foregoing Costas loop, it is observed phase offset device to maintain a nominal value of phase that it is impossible to align the inphase and the quadra- offset. However, in the event that there is a deviation ture reference carrier signals with the components of 65 from true orthogonality, then each of the output prodthe composite input signal. As a result, some crosstalk is uct signals of the two channels includes an additional observed in the demodulated signals extracted from the component, described mathematically in terms of multiI and the Q channels of the demodulator. The crosstalk plication of trigonometric terms, which varies in accor3 4

dance with the amount of phase shift by which the two loop QPSK demodulator which has been modified, in

channels deviate from orthogonality. This additional accordance with the invention, to include an adjustable

component causes the phase correction signal to vary in phase offset unit for producing phase-corrected carrier

value based on the deviation from orthogonality. reference signals.

In the preferred embodiment of the invention, the 5 __T__TrYM

phase-offset device is constructed as a microwave cir- DETAILED DESCRIPTION

cuit comprising a 90 degree hybrid unit, a voltage con- With reference to the FIGURE, there is shown a

trolled attenuator (VCA), a power divider, and a delay system 10 for generation of carrier reference signals for

line. The hybrid unit has a main input port and an auxil- operation with inphase (I) and quadrature (Q) data

iary input port (the latter usually referred to as an iso- 10 channels 12 and 14, such as that employed in a Costas

lated port). The power divider applies input power to ioop Qpsk demodulator, the system 10 further com

both of the input ports with a relatively large fraction of prising an adjustable phase-offset unit 16 operative in

the power being applied to the main input port and a accordance with the invention for providing inphase

relatively small fraction of the power being applied to md quadrature reference carrier signals. In the pre

the auxiliary input port. The power divider is connected 15 ferred embodiment of the invention, the system 10 is

directly to the main input port, and is coupled via the operative with microwave signals at a carrier frequency

VCA to the auxiliary input port. The VCA includes a of approximately 370 megahertz (MHz). The composite

control terminal for receipt of the phase-correction signal at a system termmai j8 mciudes a

command signal. The hybrid device mciudes two out- sinusoidal component and a cosinusoidal component

put ports, each of which outputs one-half of the power 20 wherdn each component may be modulated separately

inputted at the mam input port and, wherein, a first of ^ a data si ^ For e 1C) the data si d be

the output ports introduces a 90 degree phase lag and a a A ^ ... a lo^c ^ of Q fa resented

second of the output ports mtroduces a 180 degree fe & ^ shjft of zerQ d ^ a ^ ^ Qf j fa

phase lag relative to the phase of the signal inputted at represented by a phase sUft of 180 degrees ^ CQm.

emam inpu po\ ■ *• posite input signal at terminal 18 is applied via a signal

„ f ... °T P°? f flitter 20 to both of the channels 12 Zd 14. By way of the hybrid unit to provide for an optional amount of r , . , .... -„ , * \ * _ ,j, , . \, , , „ j u ^ u u • J example, the signal sphtter 20 may be constructed as a fixed delay between the signals outputted by the hybrid i_ • J r i. unit. For a specific carried frequency of the signals in ^ ^rowave umt having zero degrees of phase the hybrid unit, the delay unit introduces a correspond- 30 ^b*TM the tW° °UtpUt P°rtS 22 md 24 °f the ing phase shift, proportional to the product of carrier sp_,er. „ , , , „„ ^ frequency and the delay between the signals outputted J** mPhase data <*ann^ 12 uprises & ^ by the hybrid unit for the following reason. A true P^er operative as a phase detector 26, a second multininety-degree phase shift is obtained between the output Pber ^ a ^P3?s fl}ter 30 30(1 a *BP* amplitude terminals of the hybrid unit for the case wherein no 35 ltaater 3Z Similarly, the quadrature data channel 14 signal is applied to the auxiliary input terminal. How- comprises a first multiplier operative as a phase detector ever, application of signals to both input terminals of the ^ a second multiplier 36, a low-pass filter 38 and a hybrid unit, even though the auxiliary input signal be amplitude hmiter 40. An mphase earner refermuch smaller than the main input signal, introduces a ence K provided to the phase detector 26 via line phase offset from the desired nominal value of ninety 40 42 from the phase-offset unit 16, and a quadrature cardegrees between the two output signal of the hybrid rier reference signal is provided to the phase detector 34 unit. The phase offset is adjustable over a range of val- via line 44 from the phase-offset unit 16. The system 10 ues phase shift about the nominal value of phase offset. further comprises two summers 46 and 48, two loop The delay unit is operative to cancel the nominal value mters 50 and 52, and a voltage-controllable oscillator of phase offset so as to restore the desired nominal value 45 (VCO) 54.

of ninety degrees. There are two parts to the operation of the system 10.

The two signals outputted by the hybrid unit are &1 the first part of the operation, the system 10 operates

applied to respective ones of the data channels of the m a manner analogous to that disclosed in the aforemen

Costas loop. The relatively small signal inputted at the tioned U.S. Pat. No. 4,344,178 wherein the phase detec

auxiliary input port of the hybrid unit has the effect of 50 tors 26 and 34 effectively multiply the components of

altering the amount of phase offset between the two the composite signal by the quadrature carrier reference

output ports of the hybrid unit. The amount of the offset signals on lines 42 and 44, respectively, to produce

depends on the relative magnitudes of the signals input- product signals which are filtered by the filters 30 and

ted at the two input ports of the hybrid unit. Alteration 38, respectively, to produce base band I and Q demodu

of attenuation of the VCA by the phase-correction 55 lated output signals on lines 56 and 58, respectively.

command signal changes the relative magnitudes of the Since, in each of the channels 12 and 14, the carrier

two input signals to the hybrid unit with a consequent reference signal is in phase with only one of the compo

shift in the value of the phase offset between the two nents of the input signal, and in quadrature with the

carrier reference signals outputted by the hybrid unit to other of the two components, the detector 26 and the

the data channels. This provides for full compensation 60 filter 30 serve to output only the I data while the detec

of deviation of the input signal channels from orthogo- tor 34 and the filter 38 serve to output only the Q data.

nality. The filters 30 and 38 remove high-frequency compo

„„„ „„„ , ^ „„„ T„ nents of the multiplication operation from the outputted

BRIEF DESCRIPTION OF THE DRAWING data. ^ signalsPon Me g/^ 58 are applied 4 the

The aforementioned aspects and other features of the 65 limiters 32 and 40, respectively, to provide sign inputs

invention are explained in the following description, to the multipliers 28 and 36. The I data on line 56 is also

taken in connection with the accompanying drawing, applied to the Q channel multiplier 36 to provide a wherein the sole FIGURE shows circuitry of a Costas cross-channel multiplication and, similarly, the Q data