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20-62 VHF & UHF TRANSMITTER
Product Manual
Sea Air and Land Communications Ltd, 10 Vanadium Place, Addington, Christchurch 8024, New Zealand
March 2020
Simulcast Modulation Delay
Overview
Simulcast systems are used to allow multiple transmitters on the same frequency to provide an
extended coverage area. In such a system there may be areas covered by more than one
transmitter, and these signals may interfere with each other. Normally the stronger signal would
overpower the weaker one, but in overlap areas which of the two signals is strongest may
change rapidly with movement and environmental changes. The interference problem is
normally causes by the receiver receiving conflicting data bits, or data edges from different
transmitters due to poor synchronization. To overcome this problem the transmitters can be
synchronized by using the same external source data (over a data link). A delay can be applied
to one of the transmitters to account for the delays in signal propagation and the data link of the
other transmitter. When correctly set-up it does not matter which transmitter is the strongest at
the receiver at any given moment, the data remains exactly the same.
The simulcast synchronizing delay can only be applied to externally generated NRZ data. This
data must be connected to the “Trigger input #1” pin, NOT the direct external modulation pin.
The same PTT input should be used.
The 20-62 can apply a delay of between 10us and 2550us (+/-10us) in 10us steps. It also supports
data inversion on the “Trigger input #1” pin. (Data inversion is not available for the direct external
modulation input).
Propagation delays cannot be removed, so the transmitter with the least total delay (data link +
RF) is the one that should have an additional delay added. Free space RF propagation delay is
3.3us/km, this includes RF data links, while cable based links may have a delay of 5us/km. There
may be additional delays in a linking system that must also be taken into consideration.
Sacoto can be used to configure Simulcast or delayed operation, under the “Modulation” tab.
Select “External modulation”
Enter a “Delay time” (delay = 0 means direct external modulation)
Select “Inverted data” if required
Apply the modulation to “Trigger input #1”
Key the transmitter by pulling the PTT input low.
The internal pull-up on trigger input 1 is designed to balance filter delays and provide symmetric
modulation when driven with an open collector output. Input filters add a delay of 250ms.
Alternatively, a version of the 20-62 is available on special request with reduced delay and the
ability to accommodate a wider range for input sources. This version can also provide a
modulation output when used as the transmitter of a cross-band link to remote simulcast
transmitters. A special version of the 12-84 is available with a received modulation output for
cross-band linking purposes
