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madę with a 300-ohm antenna sińce most antennas have this impedance. This insures that the same yardstick is used on all tuners. It may not be gen-erally reaiized but measurements per-formed at 75 ohms will give results, in microvolts, which are half those ob-tained with 300-ohms impedance al-though performance of the tuner is not one hair better. The reason is that the same power is reąuired by the tuner for a given performance and this power is simply the square of the volt-age at the antenna terminals dividcd by the antenna impedance.

Second, a 300-ohm dummy antenna wili have to be used and the input volt-age to the tuner measured by replacing the tuner with a voltmeter having an input impedance of 300 ohms. This input vo!tage is the same as the "soft microvolts" used before except that here the 300-ohm impedance is speci-fied. This will insure that tuners are designed for best possible performance with a standard 300-ohm antenna rath-er than for the lowest microvolt figurę, irrespective of impedance.

Third, if a tuner should have morę than 3% distortion it would be im-possibie to obtain a usable sensitivity figurę no matter how high the input to the tuner might be. In this way, the customer is assured of getting a tuner with no morę than 3% distortion at any input voltage equal to. or higher than, the usable sensitivity test input. Generally, this distortion figurę will run considerably lower. Distortion is measured at 75 kc. deviation at a 400-cps ratę which is standard test modu-lation in the IHFM Standard as well as 100% modulation for an FM broadcast station. In this type of measurement care must be taken to insure that the nuli filter has a high-impedance input to avoid attenuating the frequencies far removed from 400 cps. A low-im-pedance filter may cause distortion in the audio circuits of the tuner and a filter which attenuates the second and higher harmonics of 400 cps will give a distortion figurę which is much lower than the actual distortion.

Vo!ume Sensitivity

Closely linked to the measurement of usable sensitivity is the measurement of "volume sensitivity," as cov-ered in Section 6.03.03. This covers the r.f. input rcquired to produce an audio output 20 db lower than that produced by a 100,000 #iv. signal. High-quality, high-gain tuners will measure zero microvolts in this test while tuners with insuflTicient amplification or limit-ing may show a microvolt figurę which is greater than that obtained in the usable sensitivity test. For this pur-pose, Section 6.03.03 States:

"The ruted scnsitwity of a tuner shall be eąuul to the highest number of microvolts obtuined in all testu of Sections 6.<>3.02 and 6.03.03. . . /’ and these tests to be madę at 90. 98, and 108 mc. All this should insure that tuners with rated scnsitivities in Iow microvolt figlires should have good sensitivity, Iow distortion. and adequate

October, 1959 audio output irrespective of signal strength.

As users and designers of FM tuners know, sensitivity is not the only im-portant tuner characteristic although it is oftcn given as the major specifi-cation. Equally important is the tun-eFs ability to reject interference.

Capture Rafio Test

The capture ratio test is a measure of the tuner’s ability to reject un-wanted signals occurring at the same frequency to which the unit is tuned. This test shows the inherent effective-ness of the detector, limiter, and auto-matic volume control circuits. For this test, two signal generators are re-quired, one of which is modulated 100% and the other unmodulated. A dummy antenna, which pcrmits two signal generators to be connected to a single tuner, is used in this test.

To measure the rated capture ratio of the tuner the modulated generator is set to produce a 1000 /w. input to the tuner while the output of the unmodulated generator is advanced until the audio output of the tuner falls 1 decibel. The output of the generator is then recorded. Next the output control of the unmodulated signal generator is advanced until the audio output of the tuner falls a total of 30 decibels. This figurę is again recorded.

The ratio of the two values thus recorded is converted to decibels and this figurę is divided by 2. This result is defined as the "db capture ratio” for 100% modulation. This is the ratio of desired or undesired signal required for 30 db suppression of the undesired signal. This test is extremely important sińce there are a number of stations in the northeastern section of the United States which are assigned the same irequency out are physically separated by less than 100 miles. If a tuner has a Iow capture ratio. then one or the other of these stations can be received in some areas without noticeable interference by simply rotating the antenna to pick up the signal from the desired station.

Selectivify Test

Equally important is the tuner’s ability to reject interference and signals which differ in frequency from that of the desired signal. Part of this measurement is performcd in the se-lectivity test.

Test conditions are the same as those described in the capture ratio test except that the interfering signal generator is separated in frequency from the desired signal by two standard channels (400 kc.). The signal generator delivering the desired signal is set to produce an input of 100 mv. to the tuner and the audio output voltage with 100% modulation is recorded. Then the modulation of the signal generator is switched off. The interfering signal produced by the other signal generator is 400 kc. removed from that of the desired signal. This interfering signal is modulated with 400 cps. 75 kc. deviation and the output of this signal generator is advanced until the audio output of the tuner has risen to a value 30 db below the previously determined audio output. The two signal generator output voltages are recorded and converted to a ratio which is then con-verted to decibels. This decibel figurę gives the selectivity of the tuner.

The same procedurę is followed in the case of AM tuners except that 30% amplitudę modulation is used.

It should be noted that all of these (Continued on p<*ge 117)

		I50A			AftaT
STANDARD i FM SIGNAL l< GENERATOR |		_	STANDARO 300 0MM I_ BALANCEO DUMMY antenna _AAAAAAA__		„ FM RECEIV€R “O**^0 WITH BALANCEO ANTENNA INPUT _OaiiT
0N0		ison

Fig. 1. Standard 300-ohm balanced dummy antenna and the method oi connectlon.

Fig. 2. Standard 300-ohm unbalanced dummy antenna and the method oi connectlon.

Fig. 3. Method oi measurement oi input signal intensltles and connection method.

Fig. 4. The standard 200-^/iL dummy antenna is between generator and »eceiver.

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