At 12:30 Saturday afternoon the head of the U.S. Strategic Arms Limitation Talks delegation. Ambassador Ralph T. Earle met for 90 minutes with Deputy Foreign Minister Valdimir Semenov, chief of Soviet Union's SALT delegation, at the U.S. mission.

It was the ninth session of the week for Earle, Semenov or their delegation members, a clear sign of the stepped-up pace of the SALT treaty negotiations. There have been weeks in the past with only one or two sessions, and even in those past positions were merely restated.

A new SALT agreement would pick up where the 1972 SALT I treaty began and build on the principles established at the 1974 Vladivostok meeting between former President Ford and Soviet President Leonid Brezhnev.

While most public attention generally focuses on statements of the national leaders, it is the complex but seldom publicized work of the negotiating teams in Geneva that actually bring diplomatic efforts such as the SALT treaties into being.

Focus of the Geneva activities is the joint SALT draft text. As of today it is 52 pages long. It consists of the proposed treaty, a protocol itemizing areas where final treaty action is delayed, and a joint statement outlining principles for future SALT III negotiations.

Where the two parties disagree, both sides' versions are printed. In the English-language version, the U.S. position is printed first; in the Russian version, the Soviet view comes first.

The task before the delegations these days is to simplify the language so that all that remains are purely political disagreements. There would be settled under present plans at one final summit meeting between President Carter and Brezhnev.

Within the delegation the conservative estimate is that up to two more months may be needed to reach a point where the joint test is ready for a possible summit. Other sources say discussions here could be concluded in four weeks.

It is a long arduous task where each word is debated not just in one language, but in two. Because the only public annoncement abot the negotiations here is the single-sentence prss release when a formal session is held, little is known of actual negotiating procedure.

Last week, to give some hint of the process, there were two formal sesions where Earle and Semenov as delegation leaders, made formal 30-minute presentations on a particular issue. The delegations then broke for coffee or tea and split into informal one-on-one discussion groups where no records are kept.

The later sessions, called "bilaterals," give each side the opportunity to ask questions about what has been presented or to pursue other possible courses for negotiation.

These sessions, U.S. delegation members say, provide the best chance to see where the Soviets may be willing to change a position or are seeking clarification of something the United States may be offering.

Also last week, there were three sessions of the drafting committee - the U.S. Soviet group that puts together, the actual language of the joint text.

Last Friday, a session of the drafting committee tried to find a way to express an idea about missiles as exactly in Russian as it was continued in the English version of the draft treaty.

Wednesday the heads of delegation met for three hours and on Thursday night, and Semenov dined together with wives and conducted a hit of between the social conversation.

On Friday, the scientitic members of the two delegations met together for an informal question and answer session and an unexpected request by Semenov to meet Earle on Saturday ended the week.

The process follows diplomatic protocol that has been followed for years. The site of each meeting rotates between the U.S. and the Sovie missions.

Although both sides are under formal instructions and the meetings are highly structured the character and personalities of the delegation members themselves play a key roll.

Semenov, 70, has run the Soviet delegation since SALT first began. Schooled in the formalities of old diplomacy, he rarely makes a statement at formal or informal sessions without first consuiting his prepared notes.

Earle, at 50, exhibits a more relaxed style. At Saturday's surprise session, he met Semenov in sportshirt and jecket, while the Soviet was carefully dressed in a dark business suit.

Semenov rarely will reply to a statement made in a formal session, particularly if it deals with weapons. Earle, a former Defense Department official, understands the subject matter and occasionally will break the formality of the session with an impromptu response.

The other members of the U.S. delegation often have their own approaches and in some cases their own principals back in Washington. They include: Lt. Gen. Edward T. Rowny, an Army man, who represents the Joint Chiefs of Staff. An outspoken critic of haste in negotiations with the Soviets, he fears the United States too often trades away its backup positions, allowing the Soviets to outwait the American negotiators.

Gerald Johnson represets Defense Secretary Harold Brown and is the scientist on the delegation. A former associate director of Livermore Laboratories, his career was spent directing tests of nuclear weapons. Johnson seldom consults Brown who, he says, named him as a "representative of the U.S. government" without specific instructions to press for a Defense Department position.

Frank Perez, a career minister in the Foreign serves as the delegation's chairman for the drafting Service, represents the State Department and Service, represents the State Department and serves committed.

There is also a team of experts and special advisors, some from the CIA. They keep the delegation abreast of the latest information in Soviet, weaponry so that the proposed treaty language covers all the weapons and detection possibilities known to the United States.

They also supply the delegation with information on political events within the Soviet Union that could have impact on negotiating positions.