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M15-OM-11/11
INTRODUCTION
The Mentor Model M15 receives and transmits on p to six discrete channels in the vhf aviation
band between 118 and 137 MHz (25 KHz channel spacing). It operates from a 14 v dc s pply, and is
intended for both vehic lar and airborne application as well, and can also be sed as a base
station. Its advanced design feat res and s rface mo nt constr ction provide compactness, light
weight and high reliability. The only external components req ired are a s itable antenna with
coaxial cable, an aviation-type microphone and a 4- or 8-ohm speaker.
CIRCUIT DESCRI TION
The receiver is a single-conversion s perheterodyne with fo r varactor-tracked RF t ned circ its
and d al-gate MOSFET transistors in the RF amplifier and mixer stages. The local oscillator ses 3rd
overtone crystals, and is followed by a MOSFET freq ency tripler stage. Receiver selectivity is
primarily determined by a six-pole 10.7 MHz crystal filter connected between the mixer and I.F.
amplifier. The latter consists of two cascode integrated circ it amplifier stages. A tomatic gain
control is applied to the RF amplifier and the first I.F. amplifier stages.
A conventional diode detector is followed by a noise limiter, a dio amplifier, sq elch “gate” and
an integrated circ it a dio power amplifier capable of delivering 4.5 watts of a dio power into a 4-
ohm speaker. The transmitter oscillator also ses 3rd overtone crystals, followed by varactor
tracked MOSFET tripler and b ffer amplifiers. Three nt ned broadband stages boost the
transmitter carrier power to approximately 5 watts. Amplit de mod lation is applied to the last
two stages, the "driver" and “final” amplifiers.
The transmitter signal passes thro gh a 7-element Tchebychef low-pass harmonic filter which also
contains two PIN diodes f nctioning as a T-R switch. The transmitter mod lator is separate from
the receiver a dio amplifier, and incl des an a dio AGC amplifier that a tomatically adj sts for
variations in operator “microphone techniq e”. This circ it also prevents overmod lation.
The mod lator power amplifier consists of an integrated circ it amplifier followed by
complementary bipolar transistors. This arrangement eliminates the need for an a dio transformer,
res lting in less low freq ency distortion while minimizing the size and weight of the radio.
The 25 watt RF transmitter power amplifier contains a single mosfet power transistor, operating
class AB, on a microstrip type printed circ it board. Two internal relays operate when the p sh-to-
talk microphone switch is pressed. These relays switch the amplifier’s inp t and o tp t
connections, so that in the receive mode the signal from the antenna is passed directly back to the
M15’s receiver, while in the transmit mode the M15’s transmitter o tp t is applied to the 25 watt
amplifier, whose o tp t is in t rn connected to the antenna. The power amplifier also contains a 5
section low-pass filter that greatly atten ates transmitter harmonics so that the system meets FCC
req irements for signal p rity.
