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