An arrangement for compensating errors in a quadrature modulator.

(54) DESCRIPTION Device for compensating for errors occurring in a quadrature modulator (56) QUOTE PUBLICATIONS: US A 4,565,980 (332 / 16R), US A 4 700 151 (332/18) (57) SUMMARY:

A device for compensating errors occurring in a quadrature modulator. The modulator comprises means (1) for generating two quadrature signals (cos a (t), sin a (t)) in the baseband which are each supplied in the modulator, means (2) for generating two high frequency signals (cos wt, sin wt) having a mutual phase difference of 90 degrees, mixer (3,4) for mixing the quadrature signals (cos a (t)), sin a (t)) with the high frequency signals (cos wt, sin wt) and an addition circuit (5) to form the sum of the output signals of the mixers (3,4). Amplitude errors and phase errors are compensated by a special compensation network (11,12, 14, 16, 17, 19, 21) which is inserted into the modulator's low frequency section and which contains adjustable signal generating means (14, 16, 19). Local oscillator leakage is compensated by a second compensation network (13, 15, 18, 20), which is likewise inserted into the modulator's low frequency portion and which also includes adjustable signal generating means (15, 18). The signals (k4, k5) from these latter means (15, 18) are subtracted from the signals appearing in the modulator's two branches.

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The numbers in brackets indicate the international identification code. INlD code The letter within the clamp indicates international document code

459 774

TECHNICAL FIELD

The invention relates to a device for compensating for errors occurring in a quadrature modulator comprising means for generating two quadrature signals in the baseband which are supplied separately in the modulator, means for generating two high frequency signals with a mutual phase difference of 90 degrees, mixers for mixing quadrature signals. the high frequency signals and an addition circuit to form the sum of the mixers' output signals. The errors referred to are amplitude errors, phase errors and errors caused by local oscillator leakage.

BACKGROUND OF THE INVENTION

In a quadrature modulator of the type initially specified, amplitude errors and phase errors can occur, for example, in the means for generating high frequency signals, in the mixers and in the addition circuit. Phase errors can also occur due to different wavelengths on, for example, circuit boards in the modulator. For the desired result to be achieved, the amplitude error and phase error in the modulator should not exceed about 1/2 dB and about 3 degrees respectively. However, it is difficult to achieve low values ​​for frequencies higher than some 100 MHz. For example, the wavelength at 1000 MHz is about 3 dm, which means that 1 degree corresponds to a distance of the order of 1 mm.

Local oscillator leakage occurs due to leakage signals through the mixers of the high frequency signal generating means. Especially when utilizing non-linear transmitter end steps, leakage signals can cause so-called spectrum widening, which in turn can cause interference in adjacent traffic channels.

It is known to try to counteract the occurrence of amplitude and phase errors as well as local oscillator leakage by using components with relatively good properties, which makes the cost however relatively high.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a device which avoids problems caused by amplitude and phase errors as well as local oscillator leakage in a quadrature modulator of the above type. This is accomplished through

459 774 that special compensation networks containing adjustable signal generating means are inserted into the modulator's low frequency portion.

The features of the invention are set forth in the claims.

DESCRIPTION

The invention will be described in more detail in connection with the drawing, in which figure 1 shows a known quadrature modulator and figure 2 shows a quadrature modulator with a device according to the invention.

PREFERRED EMBODIMENT

Figure 1 shows a known quadrature modulator. 1 denotes a means for generating two quadrature signals in the baseband, which are designated cos a (t) and sin a (t), respectively. 2 denotes a means for generating two high-frequency signals, cos wt and their wt, respectively, which have the same frequency but a mutual phase chill of 90 degrees. These signals are each supplied to their mixer 3.4 where they are mixed with the square signals in the baseband. The output signals of the mixers are applied to an addition circuit 5, which at its output gives signals cos wt x cos a (t) + sin wt x sin a (t). This signal can also be called cos (wt-a (t)).

In practice, however, the signal from the addition circuit will be affected by amplitude errors, phase errors and oscillator leakage and can therefore be plotted cos wt x cos a (t) + A x sin (wt + V) x sin a (t) + L x cos ( wt + U). In this expression, A represents the amplitude error, V the phase error, and L x cos (wt + U) the oscillator coating, the errors being only thought to occur in one branch (the lower) of the modulator.

Figure 2 shows a quadrature modulator comprising a device according to the invention. Bodies with the equivalent in Figure 1 have been designated in the same way as before. 14, 16 and 21 denote adjustable signal generating means which generate signals denoted k1, k2 and k3. The square signal cos a (t) is multiplied by the signal k1 in a multiplication circuit 11, and the square signal sin a (t) is multiplied by the signal k2 in a multiplication circuit 17. The output of the multiplication circuit 17 is subtracted from the output signal

459 774 from the multiplication circuit 11 in a subtraction circuit 12. A signal k4 generated in an adjustable signal generating means 15 is subtracted from the output signal from the subtraction circuit 12 in a subtraction circuit 13. In this, the signal kl x cos a (t) - k2 x sin a (t) is thus generated. - k4.

The square signal sin a (t) is also applied to a multiplication circuit 19, in which it is multiplied by a signal k3 from an adjustable signal generating means 21. A signal k5 generated in an adjustable signal generating means 18 is subtracted from the output signal of multiplication circuit 19 in a subtraction circuit 20. thus the signal k3 x sin a (t) - k5.

At the output of the addition circuit 5, the signal (k x cos a (t) - k2 x sin a (t) k4) x cos wt + (k3 x sin a (t) - k5) x A sin (wt + V) + L x is obtained. cos (wt + U). As before, A denotes the amplitude error, V the phase error or L x cos (wt + U) the oscillator leakage. Developed the expression kl x cos a (t) x cos wt -

- k2 x sin a (t) x cos wt - k4 x cos wt + A x k3 x sin a (t) x sin wt x cos V + + A x k3 x sin a (t) x cos wt x sin V - A x k5 x sin wt x cos V -

- A x k5 x cos wt x sin V + L x cos wt x cos U - L x sin wt x sin U.

From the above expression, two equations containing k4, k5 and L can be set up.

-k4 x cos wt - A x k5 x cos wt x sin V + L x cos wt x cos U = 0 (1)

-A x k5 x sin wt x cos V - L x sin wt x sin U = 0 (2)

By solving these equations, values ​​can be determined for k4 and k5, with which the oscillator leakage can be compensated.

From equation (2), k5 = -L x sin U / (A x cos V).

If this is inserted in equation (1), k4 = L x (cos U + sin U x sin V / cos V).

If the expressions as above containing k4, k5 and L are eliminated from the expression representing the signal at the output of the addition circuit 5, k x cos a (t) x cos wt - k2 x sin a (t) x cos wt + A x k3 x sin a (t) x sin wt x

459 774 x cos V + A x k3 x sin a (t) x cos wt x sin V. If k is set equal to cos V, k2 is set equal to his V and k3 is set equal to 1 / A, the second and fourth terms are removed. each other. The star cos V x cos a (t) x cos wt + sin a (t) x sin wt x cos V = cos V x (cos wt x cos a (t) + sin wt x sin a (t)) = cos V x cos (wt - a (t)).

So when s as on the factor cos V, this expression is equal to the ideal expression derived in connection with Figure 1 without regard to amplitude errors, phase errors and local oscillator leakage. Since V represents the phase error, and this may be assumed to be small, the value of cos V is in practice somewhat less than one.

Thus, with a device according to the invention and with appropriately set values ​​of the signals k-k5, the remaining error in the output of the quadrature modulator is only a constant amplitude factor which is slightly less than one. However, this factor has no significance in practice.

The device thus consists of a compensation network which contains adjustable signal generating means and which is inserted into the modulating frequency part of the modulator. However, this compensation network can be said to consist of two separate networks, one of which compensates for amplitude and phase errors, and the other compensates for local oscillator leakage. The networks can thus function independently of each other.

A practical realization of the compensation network may consist of operational amplifiers and trimmable resistors. The setting of signals k1-k3 can be made in the following way. If a (t) is selected to (w ^ χ t), the modulator's ideal output signal becomes cos (wxt - w_m xt), ie it contains only the frequency w - w_m· In practice, however, phase errors and amplitude errors cause another frequency component to be formed. This can be made to disappear with the help of a spectrum analyzer and iteratively change the signals k1-k3. Similarly, an undesirable frequency component caused by local oscillator leakage can also be caused to disappear by iterative modification of signals k4 and k5.

459 774