Harman/Kardon TU920 Tuner
If you have a long memory, you may recall the Harman Kardon Citation 14 and 15 tuners, introduced in the early '70s. These were, the first FM tuners to use phase-locked-loop (PLL) stereo demodulators. This innovation, combined with the growing use of integrated circuits (ICs), changed the face of tuner design to the extent that present-day tuners in just about any price category will outperform the very best you could buy 20 years ago.
Since then, there have been only two significant innovations. First came true digital tuning, which is mainly a convenience feature. Second were circuits designed to enhance effective stereo sensitivity by means of ingenious noise-reduction schemes. Now comes a third, from Harman Kardon, which the company calls Active Tracking tuning.
First introduced (appropriately) in the Citation 23 tuner, the Active Tracking system greatly enhances adjacent-channel selectivity without the traditional penalties of high distortion and poor channel separation. Its effectiveness is immediately apparent in our data column. The selectivity figures in the "wide" mode (Active Tracking off) are already quite respectable, but the effect of switching to "narrow" (Active Tracking on) is astounding. While alternate-channel selectivity improves by about 40 percent, adjacent-channel selectivity jumps by a factor of almost six to more than 40 dB-easily the highest reading we have ever seen for this measurement and as good as many tuners can do for plain old alternate-channel numbers. Channel separation does go down and distortion increases moderately, but neither change is dramatic enough to constitute audible degradation. The worst we can say about the Active Tracking circuit is that it almost doubles the capture ratio, which could make multipath more problematic in some situations. Otherwise, the system is almost purely beneficial.
At this point, you may be wondering why anyone would want 40-plus dB of adjacent-channel selectivity when most of us have been getting by on 4 or 5 dB- fairly typical figures for even very good tuners. The reason has to do with the way the Federal Communications Commission (FCC) allocates the FM spectrum. Each FM channel is 0.2 MHz wide, within which a station is allowed to modulate its carrier ±75 kHz around its assigned center frequency (the one you tune to-91.1 MHz, for example). That leaves a 50-kHz guard band between stations that are adjacent on the dial-not enough space to prevent interference on a conventional tuner. Recognizing this technological limitation, the FCC assigns channels so that no two stations within a given reception area are less than two channels (0.4 MHz) apart. This is designed to ensure clean reception on any tuner with decent alternate-channel selectivity, which is easily achievable.
But times have changed. Tuners have become more sensitive than they were in the early days of FM broadcasting, enabling them to pull in signals from farther away. At the same time, the galloping success of FM radio has crowded the band to such a degree that, in major cities, it is absolutely jammed. There are virtually no open channels in New York City, for example: You'll find a station almost every 400 kHz along the dial. The big rub comes when you live a moderate distance outside such an area or, in the worst case, between two of them. Then you may find stations on adjacent channels fighting for your tuner's attention. Less obvious, though perhaps more intriguing, is the sin of omission. Your tuner might successfully suppress a distant, weak station occupying a channel adjacent to a nearby, strong station. This is better than interference, since you can pick up at least one station clearly, but if your tuner has superb adjacent-channel selectivity (as the TU-920 does), you might be able to get good reception of both with an antenna capable of coaxing adequate signal strength from the farther transmitter.
Previous approaches to obtaining high adjacent-channel selectivity have relied on extremely sharp bandpass filters in the IF (intermediate frequency) stage to strip away signals outside the desired channel. Unfortunately, the steeper the slope of a filter, the greater the phase shift it creates. And in FM, phase shift at this stage translates directly into distortion. The TU-920 uses less aggressive IF filters, which yield the perfectly reasonable selectivity figures shown in our data column for the wide IF mode. Switching the Active Tracking system on actually reduces the filter slopes and engages what Harman Kardon describes as a sophisticated PLL circuit that homes in on the desired channel's carrier frequency. The output from this circuit drives another that mimics the signal from the tuned station, except that it has a maximum deviation of ±85 kHz. It is the output from this second circuit that feeds the TU-920's FM detector. As a result, the tuner becomes essentially insensitive to out-of-channel signals, yielding high adjacent-channel selectivity without large amounts of distortion-inducing phase shift.
Apart from the selectivity, performance is about what you would expect from a tuner in this price range. Sensitivity is very good, and noise and distortion are adequately low despite the latter's deterioration at high frequencies. Response is perhaps a shade less even than we're used to seeing these days, with a tiny bump in the midtreble, a slight rolloff at the very top, and an even milder drop at the bottom. Separation, on the other hand, is quite good with the Active Tracking system on and outstanding with it off. We would prefer somewhat better pilot and subcarrier suppression, but in neither case is there cause for serious concern.
Features are also fairly typical, with one twist. Although the tuning is entirely electronic, you control it with a knob. A clockwise turn scans up the dial, a counterclockwise turn scans down. Going from one end of the dial to the other takes about 17 seconds. You can select automatic tuning (in which the TU-920 will seek the nearest station that it considers strong enough for decent stereo reception) or manual tuning, which steps in half-channel (0.1-MHz) increments. Switching to manual also turns the muting off, but it does not affect the reception mode. Instead, there is a separate mono/stereo switch.
The TU-920 provides 16 station presets on eight buttons plus a shift key. Each preset will hold one frequency on either the AM or FM band. A display window in the middle of the front panel indicates frequency, band, whether a station is tuned, whether it is in stereo, and the signal strength (on a five-LED readout with thresholds ranging from 18-1/2 to 54-1/2 dBf). The increments are more tightly spaced in the middle of the range, between 27-1/2 and 38-1-/2 dBf, where the information is most needed for antenna orientation. We were pleased to find that the back panel sports an F connector for 75-ohm FM antenna connections as well as the usual screw terminals for AM and 300-ohm FM antennas. There's even an unswitched AC outlet.
Operation of the tuner is absolutely straightforward, and performance is, as the numbers suggest, first-rate. The Active Tracking circuit did help us pick up some stations that otherwise would have been garbled beyond recognition or entirely missing; with a high-gain directional antenna, we no doubt could have found even more such stations. It's a terrific idea, and we hope to see much more of it in Harman Kardon products. Interestingly, we found that sensitivity seemed to increase a little with the Active Tracking turned off, so there is a good reason for the system's front-panel switch. In short, if you just need a basic tuner, the TU-920 will do the job; if you need something more sophisticated to handle difficult reception conditions, you're still covered. And either way, it won't cost you an arm and a leg.
