7
AM/FM Stereo Tuner
Regrettably, few AM radio stations broadcast music programming today, let alone with a
focus on high fidelity. Nonetheless, the tuner is built to be true to the design of the time and
provide the wide AM frequency response capability that the original designers intended. The fact
that stations do not take advantage of high fidelity capabilities is just a sidebar.
Some metropolitan locations have many stations located nearby, some at frequencies close
together and almost equal signal strength. In these cases, sideband interference is likely to be
troublesome. This problem can be overcome by antenna orientation, since the external loop
antenna is sharply directional on the null.
A wide-band AM tuner has an entirely different tuning feel than a conventional tuner, the
response and noise quieting remaining much the same over a wide tuning range. Perfect tuning is
accomplished by setting the dial midway between the band edges; an improvement in high
frequency response will probably be noted at this point.
The FM section also builds upon a classic 1960s Heathkit design. Most commercial high-
fidelity FM tuners of the 1960s conformed to a particular circuit configuration and therefore
offered similar performance figures. This tuner is a bit different in that a ratio detector is used.
The ratio detector is capable of excellent performance and has the advantage of being self-AM-
limiting, eliminating the need for multiple limiter stages. An advantage is gained by not using
limiters, since the “limiting threshold” is eliminated. Weak FM stations cannot overcome this
threshold and so are heard highly distorted and/or with high background noise. Performance of
the ratio detector is quite good on weak stations that are received at a signal strength roughly
equivalent to the specified sensitivity of the tuner. Of course, noise levels will rise with very poor
signals, but the accompanying distortion does not occur until the signal becomes too weak to be
useful. The major disadvantage of the ratio detector is that the audio output is dependent on signal
strength to some extent. This is a trade-off the original designers felt was worthwhile.
High-gain tubes are used in the IF and mixer stages of the FM tuner to give high sensitivity
and to aid the detector in its function. A high gain, low noise cascade type RF amplifier is used for
the same reason, as well as to isolate the local oscillator from the antenna. Loading and pulling of
the oscillator by the antenna circuit is minimized in this manner and external radiation from the
oscillator is substantially reduced.
