Single Tone Technique for Measuring IM Distortion

a. Field of the Invention

Broadly speaking, this invention relates to the measurement of distortion. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for measuring the intermodulation distortion generated by microwave repeaters, and the like.

B. Discussion of the Prior Art

Multi-channel, microwave transmission systems employing suppressed-carrier, single-sideband, amplitude-modulation have been proposed as a replacement for existing frequency-modulation systems. Due to the nature of amplitude-modulation, such systems will be more sensitive to non-linearities in the microwave repeaters than are the FM systems, for example. Of particular concern are third order IM distortion products which tend to fall within the passband of the transmission system and, hence, are potentially troublesome. Accordingly, whenever a single-sideband microwave route is placed in service it will be necessary to implement a testing procedure providing for routine field measurements of the linearity of the microwave repeaters along the route to insure that distortion of the amplitude-modulated signal is held to a minimum.

One way to perform this routine maintenance procedure would be to selectively take each repeater along the route out of service, and then subject the repeater to any of several, known IM testing procedures. Alternatively, the novel, swept-frequency testing procedure described in my co-pending patent application, H. Miedema U.S. Pat. No. 4,048,559 issued Sept. 13, 1977, may be employed.

All such testing procedures require that the repeater under test be taken out of service, which complicates scheduling of the test procedure. Also, the generation and insertion of multiple test-tones, and the selection of the desired intermodulation products, requires complicated and expensive hardware. Further, the high-level test-tones employed create a potential interference problem in adjacent systems.

Clearly, what is needed is a testing procedure which can be performed on a repeater in situ, and preferably while the repeater is in service, under actual working conditions. This testing procedure will advantageously be less complicated, and the testing equipment less expensive, than that required heretofore.

Fortunately, these and other problems have been solved by the instant invention. Briefly, the invention comprises a testing procedure which employs only one test-tone. This considerably reduces the amount of hardware required and, in addition, reduces the potential for interference since the single test frequency can be located near the center of the passband where it does not interfere with the message load.

One skilled in the art will perceive that the use of a single-frequency test-tone makes it necessary to find some means for separating the non-linear part of the tone output from the repeater from the linear part. The difficulty, of course, is that both parts have the same frequency. Fortunately, this separation is readily accomplished in the instant invention by amplitude-modulating the incoming message signal, but not the single-frequency test-tone.

Another important aspect of the invention is that the measurement sensitivity is increased over the known systems by using two synchronous detectors, driven in phase quadrature, and by the selection of the largest of the two detector outputs. This novel approach eliminates the uncertainty introduced by the unknown phase of the reference signal which drives the synchronous detectors, without at the same time causing the serious deterioration in the signal-to-noise ratio which would have resulted had the unknown phase of the reference signal been eliminated by using the conventional technique of taking the sum of the squares of the outputs from the synchronous detectors.

The invention and its mode of operation will be more fully understood from the following detailed description, when taken with the accompanying drawing which depicts an illustrative circuit arrangement for practicing the invention.

When a single-frequency test-tone is added to the payload spectrum transmitted by a single-sideband transmission system, for example the 6 GHz system known in the industry as AR-6A, intermodulation between the test-tone and the rest of the spectrum causes a non-linear phase shift of the test-tone due to AM/PM conversions as well as an amplitude reduction due to gain compression. It can be shown that for a third order non-linearity in the transmission system, both the non-linear phase shift and the amplitude reduction are proportional to the total message power and to the degree of third order non-linearity present in the system. I have discovered that these combined effects can be used to measure transmission non-linearity.

Measuring non-linearity in this manner has the advantage that no second test-tone is required; i.e., only one frequency slot is required rather than three. Advantageously, the test-tone can be located near the center of the passband to minimize interference into adjacent FM systems. Since the measurement depends only on the total signal power and not on the shape of the signal spectrum, it is possible to use a signal that will minimize interference, such as the message load itself, for in-service testing or, if that is not convenient, a noise load for out-of-service testing. The sensitivity obtained with a single test-tone of +20 dBm, assuming a total signal power of +27 dBm and a message level modulation of 0.5 dB peak-to-peak, is equivalent to that obtained with two tones of +19 dBm each in a two-tone test.

The measurement technique disclosed herein requires the ability to distinguish between linear and non-linear components of a single-frequency test-tone. To separate these components, the level of the incoming signal is modulated periodically by a small amount (e.g., 0.5 dB) during the measurement. The circuit arrangement is such that this modulation affects only the non-linear part of the test-tone output, not the linear part, and thus permits a separation of the two. The amplitude of the non-linear part can now be retrieved and is a direct measure of the non-linearity of the system. It can be shown that the sidebands created by the small degree of amplitude-modulation applied to the input signal during the testing process, do not contribute measurably to the background noise level of the payload.

As shown in the drawing, the incoming signal, illustratively an FDM multiplex signal which has priorly been translated up to a 70 MHz IF frequency, is applied to a modulator 10 having a modulation frequency .omega..sub.m. The output of modulator 10 is applied to the input of a microwave transmitter 11 that typically includes a travelling wave tube, or the like. The output of transmitter 11 is connected to a microwave antenna (not shown) in the normal manner.

A generator 12 generates a single-frequency test-tone of frequency .omega., which test-tone is added to the FDM signal applied to the input of transmitter 11, via a coupler 13. The output of generator 12 is also applied to a first synchronous detector 14 and, via a 90.degree. phase shifter 16, to a second synchronous detector 17, as shown. A portion of the output from transmitter 11 is tapped-off by a coupler 18 and applied, via a bandpass filter 19, to the input of synchronous detectors 14 and 17. The output of detector 14 is applied, via a bandpass filter 21, to a comparator 22. In like fashion, the output of detector 17 is applied, via a second bandpass filter 23, to another input of comparator 22. A third coupler 24 taps off a further portion of the output of transmitter 11 and applies it to a square-law detector 26. The output of detector 26 and the output of comparator 22 form the inputs to a second comparator 27. As will be explained below, coupler 24, detector 26 and comparator 27 may be omitted in some circumstances. It will be appreciated that transmitter 11 may form part of an intermediate microwave repeater along the transmission route or it may comprise one terminal of the microwave system.

In operation, we will assume that the incoming fdm signal at point A comprises a multiple frequency sepctrum of the form:

which signal represents either a message payload or a noise load.

During the linearity measurement, the level of the incoming signal will be modulated by modulator 10, causing the level at point B to vary periodically between

Using a suitable slot in the frequency spectrum, generator 12 inserts a test-tone:

into the input signal to transmitter 11 during the linearity measurement.

The input to the transmitter at point B will then be given by the equation:

Transmitter 11 amplifies this signal and introduces distortion. Considering only third order distortion and neglecting transmitter delay, the output of transmitter 11 at point C at the test frequency .omega. will be given by the equation:

where G is the linear (amplitude) gain of the transmitter,

.alpha. is the in-phase distortion coefficient, and

.beta. is the quadrature phase distortion coefficient.

The desired test frequency output is selected by passing the transmitter output through bandpass filter 19 which is centered at .omega.. The output of this filter is now multiplied by two quadrature samples of the test-tone, R sin(.omega.t+.phi.) and R cos(.omega.t+.phi.).

More specifically, in synchronous detector 17, V.sub.o (.omega.) is multiplied by R sin(.omega.+.phi.) producing a difference frequency output:

As discussed above, the value of k alternates between 1 and k.sub.1 resulting in a corresponding modulation of V.sub.k. The peak-to-peak amplitude of this rectangular modulation is:

Use of a new variable .theta. = tg.sup.-1 .beta./.alpha. will simplify equation (4) to:

Similarly, we will find that detector 14, wherein V.sub.o (.omega.) is multiplied by R cos(.phi.t+.phi.), will produce a modulated output signal given by the equation:

The difference frequency output from synchronous detectors 14 and 17 also includes the down-shifted noise passed by the test-tone filter 19. To reduce this noise, the detector outputs are passed through narrow band filters 21 and 23, each centered at the modulation frequency .omega..sub.m.

The parameters R, k.sub.1, a, .alpha., .beta., and .theta. in Equations (5) and (6) are constant. Since the output power, P.sub.o of transmitter 11 equals G.sup.2 .SIGMA.A.sub.n.sup.2 and the third order IM coefficient m.sub.3 is proportional to (.alpha..sup.2 +.beta..sup.2 /G.sup.6), we can write:

By taking the sum of the squares of Equations (7) and (8), we could obtain

and thereby eliminate the unknown phase angle .phi. of the reference signal, thus, making it possible to determine P.sub.o .sqroot.m.sub.3. However, this squaring operation would reduce the already low signal-to-noise ratio at the output of the detectors, since it would rectify the background noise, which now would dominate in the squared output.

I have found that the sensitivity of our measurement technique is increased significantly if, instead of taking the sum of the squares of Equations (7) and (8), we take the larger of the two. This selection of the larger signal will not affect the signal-to-noise ratio. The uncertainty we introduce into the I.M. measurement by doing this is the ratio between the maximum value of either cos (.phi.-.theta.) or sin (.phi.-.theta.) and their crossover point; i.e., the ratio of 1 and 1/.sqroot.2. Thus, by taking the maximum M of Equations (7) and (8), we can determine P.sub.o .sqroot.m.sub.3 between the limits:

thus, in the worst case m.sub.3 will have a value 3 dB higher than the value indicated by the measurement.

When the message power, P.sub.o, is known, M can be translated directly into values of the intermodulation coefficient m.sub.3. When P.sub.o is not known, and can be expected to fluctuate with the message load, we can determine whether m.sub.3 is larger or smaller than a critical value L by comparing M with a second signal P.sub.o .sqroot.L in comparator 27. The signal P.sub.o .sqroot.L can be obtained from square law detector 26 operating on the message spectrum, via coupler 24.

Because the processing of the test-tone from generator 12 between the output of transmitter 11 and input to comparator 22 is linear, the signal-to-noise ratio is not affected. The signal-to-noise ratio at the input to comparator 22 is determined as follows.

Assuming a 25 dB predistorter improvement applied to a TWT M.sub.a-b+c = - 92 dB; a message load of +27 dBm; and, a test-tone level of +20 dBm (both at the transmitter output), then the test-tone distortion, if taken alone, represents a signal level:

If modulator 10 modulates the message level by 0.5 dB during the test, the resulting rectangular test-tone modulation has a peak-to-peak level which is 19 dB lower; and the first harmonic at the modulation frequency is 23 dB lower, or -66 dBm.

When the system noise is 41 dBrncO, the noise power in a 3 kHz slot is -47.8 dBmO. Since the TL at the transmitter output is +5.6 dB, the noise in a 3 kHz slot at that point is -42.2 dBm.

Assuming a message level modulation frequency of 50 Hz (.omega..sub.m) and the use of modulation filters with a noise bandwith of 1 Hz, the noise level at the test-tone output becomes -77 dBm.

The signal-to-noise ratio available for the measurement is, therefor, 77 - 66 = 11 dB, and the presence of the noise increases the signal level out of the modulation filter by 0.3 dB.

I stated above that the equivalent test signal level at the transmitter output is -66 dBm. To obtain this signal level using a conventional two-tone IM test would require test-tone levels P.sub.x, where

or

Thus, to obtain the same signal-to-noise ratio in a two-tone linearity test would call for tone levels of +19 dBm at the TWT output leading, as previously mentioned, to the distinct possibility of interference in adjacent FM systems.

Of course, the invention is not limited to use with single-sideband systems, but may be used with other forms of modulation. Nor, is the invention limited to microwave systems as it may also be used at lower frequencies with equal success. One skilled in the art may make various changes or substitutions to the layout of parts shown without departing from the spirit and scope of the invention.