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PRE–EMPHASIS SWITCHES.
Two programmable switches S1 and S2 on audio board 800–194 enable the user to select flat, 25, 50, or 75
micro–second pre–emphasis characteristics. The selection of one of these options in the transmitter requires a
corresponding selection of S1 and S2 on receiver audio board 800–205. Refer to STL–10A adjustment location dia-
gram at the end of this manual for the location of these switches. The switches, as shown in the diagrams, are
positioned for 75 micro–seconds at the factory. 75 micro–second pre–emphasis and de–emphasis produces the best
threshold noise performance in an FM system. It does however present problems in modulation control. Some
users therefore may wish to use 50 micro–second, 25 micro–second, or flat processing. AM stations will find flat
processing desirable since the STL can be treated as a pair of wires in proof of performance tests without possible
overmodulation of the link at high audio frequencies. With 75 micro–second pre–emphasis, audio input levels to the
transmitter should be reduced 20 dB below 100% modulation when running frequency response tests to prevent
overmodulation at the higher audio frequencies. With a full quieting RF signal into the receiver, flat pre–emphasis
will produce a system signal–to–noise ratio approximately 6 dB less than 75 micro–second pre–emphasis. Received
signal levels less than full quieting may produce a difference of as much as 13 dB between flat and 75 micro–second
pre–emphasis.
FREQUENCY MEASUREMENT.
The RF output frequency of this transmitter should be measured and adjusted as often as necessary to insure
on–frequency operation and to comply with regulations (refer to TUNE–UP PROCEDURE in the following text).
STL–10A SYSTEM PERFORMANCE TESTS.
The STL–10A transmitter, R–10 receiver with the associated antenna system can be tested and compared with facto-
ry test data included in this manual. The following procedures should be followed in order to obtain reliable and
accurate results.
Before audio tests or subcarrier tests are begun check the receiver "SIG. LEVEL" METER for required minimum
signal. A conversion from VU to microvolts is given under OPERATION in the R–10 receiver manual. For a 950
MHz system using 50 KHz FM deviation, typical noise levels are:
For the above system with 20% subcarrier injection, the following noise level on the subcarrier
(Marti SCG–10 – SCD–10 System) was measured: (no modulation main or sub).
With ultimate S/N ratio, main to sub crosstalk should be –40 to –45 dB (using Marti SCG–10 – SCD–10 Subcarrier
System).
1 uV for 20dB S/N ratio
3 uV or 50 dB S/N ratio
10 uV for 60 dB S/N ratio
20 uV for 70 dB S/N ratio
50+ uV for ultimate S/N ratio
10 uV for 40 dB Subcarrier S/N ratio
20 uV for 47 dB Subcarrier S/N ratio
30 uV for 50 dB Subcarrier S/N ratio
150 uV for ultimate Subcarrier S/N ratio
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