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Treetop Circuits

SSB Adapters Explained

The Treetop Circuits SSB adapters are accessories specifically designed for the Collins R-388/51J and 75A-2 and A-3 series of radio receivers. These are classic receivers used by amateur radio operators, shortwave listeners, and other radio enthusiasts.

Prior to the late 1950's, voice signals were transmitted using amplitude modulation (AM) or frequency modulation (FM). On the broadcast and shortwave bands, AM was used almost exclusively. Morse code was transmitted using Continuous Wave (CW).

Receivers designed for those bands were equipped to deal with these signals, and sometimes narrow-band FM as well.

In the late 1950's, a newer technique known as single-sideband (SSB) was introduced, and became the de facto standard for amateur radio, military, and commercial service, although AM continues to be used worldwide for broadcast services.

Some receivers from the pre-SSB era, including the Collins models, have become collectors' items. Their stability, reliability, and overall performance are considered excellent even by modern standards. They will work with SSB signals, but their performance in this mode falls far short of that attained by receivers designed for SSB service.

The main problems lie in the detector and automatic gain control (AGC) circuits.

The detector is the part of the receiver in which the voice is extracted from the radio signal; the voice signal is then amplified and used to drive a speaker or headphones.

AM receivers normally use a simple "envelope detector". To receive a CW or SSB signal, they inject a small signal from a beat-frequency oscillator, or BFO, into the detector. This works reasonably well on CW, but not on SSB. In the presence of interfering signals, it creates audible intermodulation products, which worsen the interference. An envelope detector also responds poorly to varying signal levels.

A product detector circuit overcomes these problems. Various circuits have been employed in product detectors; the PD3 uses a single-balanced high-level diode mixer for SSB and CW, and a pre-biased diode detector for AM.

The AGC system (sometimes referred to as automatic volume control, or AVC) senses the strength of the signal, and adjusts the amplification to to reduce "blasting" and fading as signal strength varies.

SSB presents special problems in the design of an AGC system, because the signal disappears completely between words. A "fast- attack slow-return" circuit is needed to avoid momentary overload. This function is incorporated in our adapters.

Our adapters plug directly into the socket normally occupied by the detector/AGC tube. Some additional changes are required, as described in the Owner's Manuals. The panel controls work the same as before, and AM performance is unaffected.