A Tiny "Thinking" Tuner
Mitsubishi Model M-F01 stereo FM tuner, in metal case. Dimensions: 10 3/8 by 2 3/4 inches (front), 8 3/4 inches deep plus clearance for controls and connections. AC convenience outlets: 2 unswitched (max. 400 watts total). Price: $340. Warranty: "limited," three years parts and labor. Manufacturer: Mitsubishi Electric Corp., Japan; U.S. distributor: Melco Sales, Inc., 3030 E. Victoria St., Compton, Calif. 90221.
Without the heat-dissipation problems of a power amp, the control complexity of a preamp, or the size restrictions imposed by the cassette, it would seem that the FM tuner is the easiest component to micro-miniaturize. And, indeed, the Mitsubishi M-F01 affords the same circuit sophistication that we have come to expect in full-sized quality tuners.
Its quartz-crystal-controlled phase-locked-loop synthesized tuning system grabs the station, wresting control from the fumble-fingered even on relatively weak stations. A green LED-flanked by two red ones that direct you toward the desired goal-indicates the point of optimum tuning. Once lock is achieved, the tuning dial changes color from light to dark green.
Five LEDs display signal strength, and they do so in a more useful manner than many other such arrays, which are something of a fad. The first comes on at a 20 1/2-dBf input level, corresponding to somewhat better than 50 dB of mono quieting. The second, at 32 dBf, assures 65 dB of mono quieting and a marginal 40 dB of stereo S/N. By the time the third illuminates (at 40 1/2 dBf), mono reception is virtually ideal (70 dB of S/N) and stereo quieting approaches the magic 50-dB figure. The fourth and fifth LEDs (56 1/2 and 64 dBf) denote progressively quieter stereo reception. Considering the nondefeatable quartz lock, the tuning aids thus are appropriate to the M-F01 in both threshold points and physical compactness.
As added encouragement to orient your antenna for best reception, a pair of multipath output jacks on the rear panel may be connected to an oscilloscope display or monitored aurally for minimum multipath distortion. (One of the jacks, wired directly to the FM-detector output, could serve to drive an external discrete-quad demodulator, should such a system appear.) Both fixed and variable stereo output jacks are provided, the latter controlled by a rear-panel adjustment.
The size of micro components does take its toll in the tuning dial, which is necessarily short (just under 5 inches) and sparsely calibrated (every 1/4 MHz). The tuning mechanism lacks the smooth feel of the massive flywheels we're accustomed to on standard-size tuners, and it is not quite so free of backlash. But these foibles are largely counteracted by Mitsubishi's quartz-lock tuning and relatively accurate dial.
In both stereo and mono, the M-F01 requires a few dB more signal strength to achieve 50 dB of quieting than some other tuners; with stronger signals, however, the quieting is quite acceptable. Sensitivity is relatively uniform across the band, with a bit more "reach" for distant stations at the low end of the band than at the upper extremity.
At 65 dBf, THD+N is acceptably low. Though it exceeds what we have measured on some other quality tuners, we suspect this may be due to a slight misalignment of the detector circuit vis-รก-vis the quartz-lock system in our sample and the undefeatable lock prevents tuning manually for the best figure. (The superb distortion figures that can be achieved on the bench with some tuners require extremely precise tuning and so are less likely to' be obtained in practice than under controlled test conditions.) Spectral analysis of the distortion shows it to be mostly second harmonic (with the exception of some pilot intermodulation on the high-frequency stereo measurement), so it will be relatively un-annoying.
The muting threshold seems well chosen: When the tuner comes to life, mono quieting is 50 dB. The stereo threshold is at almost the same input level, which assures that no stereo signals will be lost-though the mono mode delivers more acceptable listening on such weak stations.
For a single-bandwidth tuner, the four-pole, linear-phase IF filter strikes an adroit compromise between selectivity and conflicting parameters such as capture ratio and stereo separation. Alternate-channel selectivity should be adequate for all but the most crowded listening areas, and the capture ratio is very good. On the bench, in fact, we found that, when presented with a strong alternate-channel signal, the M-F01 chose to go into mute rather than contaminate the station to which it was tuned. AM suppression is fairly good.
Because the tuner eliminates the 19-kHz stereo pilot via a canceling circuit rather than a notch filter, it achieves superb bandwidth in both mono and stereo. (It actually-and very atypically-is flat within 1 dB to 20 kHz in mono.) The 19-kHz pilot is suppressed adequately in the left channel and almost totally in the right; the 38-kHz subcarrier lies below our ability to measure. Separation is more than adequate throughout the important midband region.
The level control lets you match the output of the M-F01 to that of other program sources. The maximum output voltage (greater than 1 volt) should assure an adequate adjustment range for almost any system. The output impedance of the variable circuit changes with the level-control setting; since it can be fairly high (greater than 5,000 ohms), interconnecting cables should be kept to reasonable length-say, 10 feet or less. The voltage at the fixed-output terminals is about 40% of the maximum, and the impedance is higher (13,000 ohms), so shorter cables-say, no greater than 3 to 4 feet-are in order. Depressing RECORDING LEVEL CHECK introduces an output tone whose level equals approximately 60% modulation.
The quartz-lock circuitry certainly simplifies tuning, especially for the neophyte. Once you're within lock-range of the station and the signal strength is sufficient to bring the M-F01 out of mute, it takes over and makes the final adjustment.
The experienced audiophile may find its "mind" too opinionated; on relatively strong stations, however, we find no grounds on which to quibble materially with this tiny but sophisticated tuner's decisions.
FM SENSITIVITY & QUIETING CHARACTERISTICS
Mitsubishi M-F01 FM Tuner
Capture ratio
Alternate channel selectivity THD + N 1'/: dB 56% dB stereo mono at 100 Hz 0.44% 0.22% at 1 kHz 0.53% 0.23% at 6 kHz 0.50% 0.52% IM distortion (mono) 0.092%
Stereo pilot inter-modulation 0.13%
AM suppression 51 dB Pilot (19 kHz) suppression 57 1/2 dB
Subcarrier (38 kHz) suppression >85 dB, S/N ratios (at 65 dBf)
stereo 67% dB, mono 75 3/. dB
Muting threshold 18% dB
Also see:
Mitsubishi Model M-T01 cassette deck (review, Jun. 1979)
Mitsubishi micro-components (ad, Jun. 1979)