6K V1.02 03-16-91
13
3.
REPEATER TRANSMITTER PTT
Your transmitter requires a "PTT" (Push-to-Talk) signal from the controller. When this signal
becomes active, the transmitter will key (transmit). At this point, it is unimportant whether your
transmitter's PTT input is "low active" or "high active", since the controller has sense-reversal
dipswitches (discussed later). Your transmitter
’s PTT input should be connected to pin 10 of J2.
To get a better idea of how to interface your particular PTT input to the controller, we'll examine the
controller's PTT output circuit. This circuit consists of a power MOSFET connected as an open-
drain driver, plus a transient suppressor connected between the output and ground. The
suppressor removes transients above about 30 V, thereby protecting the MOSFET. The MOSFET
is a large (8 A) device in a TO-220 package, but it was not selected for its high current capability.
Rather, it was used because of its low ON resistance (less than 1 ohm), making it a nearly perfect
switch.
The MOSFET's drain is connected to the PTT output, and its source is grounded. When turned on,
the MOSFET appears to be a very low resistance between the output and ground. When turned
off, the MOSFET appears to be a very high impedance, isolating the output from ground. Because
of these characteristics, the PTT circuit can control transmitters with a variety of PTT inputs, from
TTL-logic-compatible to large DC relay coils.
Some transmitters, including models made by Hamtronics, MELCO (Maggiore Electronic
Laboratory), and RCA (500- and 700-series), use a powered PTT input arrangement, and cannot
be keyed by an open-drain driver. These transmitters require a positive voltage to be sourced into
their PTT inputs to key; removing the voltage unkeys the transmitter. The current requirement can
be substantial.
A simple outboard circuit can be placed between the controller's PTT output and the transmitter's
PTT input to satisfy these requirements. Connect a large PNP transistor so that its emitter goes to
the transmitter's +12 V supply, its collector goes to the transmitter's PTT input, and its base goes to
the controller's PTT output through a 2k resistor (important). Connect a 4.7k resistor across the
PNP transistor's base and emitter. If the transmitter draws little current through its PTT (500 mA or
less), you can use a 2N2904. If the transmitter draws 1 A or less, use a TIP30. When using this
outboard circuit, place dipswitch #5 (part of S2) in the ON (closed) position. Skip the next
paragraphs detailing dipswitch #5.
If you are building the outboard circuit described previously, you will need to place the PTT
dipswitch in the ON position.
If you are driving your transmitter PTT directly from the controller, you must determine the "sense"
of your transmitter's PTT input circuit. Does the transmitter key when the PTT input is driven "low,"
or does it key when the PTT input is driven "high?" (The majority of transmitters key when the PTT
input is driven low).
If your transmitter keys when the PTT input is low, and unkeys when the PTT input is high (open
circuit), then place dipswitch #5 (labeled "PTT", it's part of S2) in the ON (closed) position.
If, however, your transmitter keys when the PTT input is high (open circuit), and unkeys when the
PTT input is low, then place dipswitch #5 in the OFF (open) position.
