700T Frequency Synthesizing Tuner

A traditional Dial with Digital Tuning Accuracy
Our objective in designing the new 700T tuner was to create a product capable of recovering FM broadcast signals indistinguishable from those monitored in the broadcast studio itself. Digital tuning techniques and crystal-controlled frequecny synthesizing have been used in previous FM tuner designs for ultimate tuning accuracy. But previous efforts in this direction have always been accompanied by complex multiple button keyboard arrangements which ignore those human engineering concepts which are so important to the user. KENWOOD engineers have combined the traditional, easy-to-interpret linear dial scale with frequency synthesis for a tuning accuracy of better than 0.0024% - equal or superior to the station transmitter's own frequency accuracy.
The importance of accurate, center-of-channel tuning cannot be overstated. It is a primary prerequisite for lowest distrotion of reproduced signals, for best signal-to-noise performance, and for optimum stereo signal recovery with maximum separation.

Consider the graph of Figure #1 - it clearly shows that as detuning occurs, total harmonic distortion rises rapidly, reachning significant and audible values even with detuning increments measured in tens of thousands of Hertz. Translated to frequency, tuning accuracy of 0.0024% means that at a desired frequency of 100MHz, the maximum tuning error possible with Kenwood's 700T would be 2400Hz - less than a quarter of ten kHz! Although tuning is accomplished with a comfortable and familiar tuning knob/variable capacitor arrangement, it's a tuning knob with a difference because tuning increments occur every 200kHz - the exact width of a single FM channel. Such perfect tuning was accomplished in the 700T Synthesizing Tuner by incorporating one standard crystal oscillator, plus a second, variable tuned oscillator. The 2MHz frequency of the reference oscillator is divided by a factor of 80 to produce a reference signal at 25kHz. Translated to digital pulses, the signal produced by the reference oscillator is compared to the signal produced by the local, variable oscillator and the output of this comparator circuit is DC amplified and applied to the local oscillator, as illustrated in the block diagram of the 700T (figure #2).

Perfect Tuning Confirmed by Green LED
A two-step muting and LED (Light Emitting Diode) control unit receives inputs from the frequency synthesizer, as well as from a special noise-sensing circuit in the IF section of the 700T. Thus, mute release is dependent not only on signal strength, but on accuracy of tuning as well, and positive illuminated red and green LED's signal precision tuning. Any departure from perfect tuning is instantly denoted by one of the two red LEDs.

New Pulse Noise Blanking System
An entirely new noise blanking circuit has been designed for the 700T. Kenwood's exclusive Pulse Noise Blanking System actually reduces or eliminates the audible effects of ignition noise interference by means of an eight-stage comparator-filter-noise analysis circuit which in no way affects either frequency response or fidelity. This circuit alone employs 8 transistors, 2 multi-purpose integrated circuits, 10 diodes and specially designed low-pass and high-pass filter configurations. Elements of this PNBS circuit are also shown in Figure #2.

Exceptional IF System
The IF system of the 700T contains multi-element ceramic filters, two integrated circuits, two bipolar transistor stages and a highly linear ratio detector circuit, all of which combine to produce an unsurpassed alternate channel selectivity of 100dB. Combined with the performance of the frequency-synthesized, perfectly tuned front end, the tuner section proper achieves an incredible signal-to-noise ratio of 85dB in monophonic operation and 80dB in stereo mode.

Exclusive DSD Plus Phase-Lock-Loop MPX
Kenwood's renowned Double-Switching Demodulator circuit (DSD) found in many of its receiver and tuner products has established itself as the ideal decoding technique for recovering well separated left and right program signals from the composite stereo modulation signal used during two-channel transmissions. In the 700T, a complete phase-lock-loop circuit has been added to the DSD demodulation system to obtain greateset separation over the entire audio spectrum at lowest distortion levels. Stereo reception is indicated by a solid state LED [...]. Typical mid-band separation obtained with the 700T is 45dB at less than 0,3% THD and at least 35dB of separation is maintained over the entire frequency spectrum from 20Hz to 15kHz. Both sub-carrier output products and SCA residual signals are suppressed to an inaudible 65dB.

Signal Strength / Multipath Detection System
The signal meter of the 700T serves as more than a signal strength meter. It is an integral part of a sensitive multipath detection scheme which facilitates proper orientation of an FM antenna for best, interference-free reception. A pair of oscilloscope terminals augment the usefulness of this meter, enabling the listener to observe and minimize multipath interference on any externally connected oscilloscope.

Excellent AM Reception, Too
The sensitive AM section of the 700T has been designed to provide AM reception limited only by the transmitter signal itself.An IHF sensitivity of 13µV, combined with signal-to-noise capability of 50dB and THD of only 0,5% result in AM reception which can be finally be categorized as "high-fidelity".