Technical Description of Maxon unit.

The 49-HX is a low power vox operated frequency synthesised transceiver operating in the 49 MHz range The synthesiser MC145166 is a 10 channel parallel addressed CMOS Dual PLL IC designed for use in cordless phones operating in the 49 and 46  MHz frequencies .This contains two parallel  synthesisers with with their own mask programmable counter ROMS for receive and transmit loops, with two independent phase detectors. They have a common reference osc and divider chain, shared by the receive and transmit  circuits. The maxon uses only 5 of the 10 available divide ratios to obtain 5 channels. The reference crystal is 10.240Mhz which also provides the receiver second mixer injection to convert the first IF at 10.7MHz down to 460KHz. It is also  being divided down to 5 kHz as the reference frequency for the synthesiser. The transmit PLL controls a single   VCO transistor and buffer stage operating on the output frequency .The buffer stage feeds an untuned  single amplifier with a tuned circuit in the collector that in turn is link coupled to a final power stage with a tuned circuit as a collector load, this stage has an output of about +10 dBm . The base loaded antenna (in the earpeice) is permanently connected to this output stage , The receiver link coupled front end tuned circuit is also directly  coupled to the antenna. The Narrow Band FM  Receiver is based on the Motorola MC3363 IC, a dual conversion receiver with a first IF of 10.7 MHz and second at 455KHz. The receiver has the capability of an input sensitivity of better than <0.3 uV for 12 dB sinad  it also contains a RSSI output pin (12) with a   signal strength range of 80 dB and  programmable squelch circuitry, this is set by a fixed or variable resistor up to 200k (typically 68-120k) from pin 12 to the rail voltage, to set the  signal mute level for the receiver when there is no signal. The mute opens reliably at 1.0 uV.

The  electrically short stainless antenna is fed by 50 Ohm coax from the transceiver ,it is a base loaded whip with a slug tuned  adjustable loading coil in the ear piece of the headset .Tune this slug with a non metallic tool via the access port   underneath  the "Maxon" name plate at the outside of the headset while the headset is being worn ,this allows for the head capacity coupling to the antenna. Tune for maximum field strength .

Transmit and receive is by solid state switching.  The microphone (not noise canceling) feeds into a CMOS Logic  74HCU04  biased as  six individual linear amplifiers , the first two stages feed into a single transistor stage with the deviation control pot in the emitter. This pot center tap connects to the transmit VCO varicap. Part of the microphone transmit audio is fed into the audio amplifier that drives the earpeice this is heard as  sidetone. The VOX control circuitry taps audio signal off after the first inverter amplifier and feeds it into another inverter amplifier.   This has the vox sensitivity pot connected across the amplifier as a variable feedback resistor, the output of this variable gain amplifier is conditioned by another inverter and drives a single  common emitter transistor stage used as a switch to drive the T/R circuitry with a small hold time period of around a second, when triggered into transmit.

Intercom Mode involves switching both microphones and the earpeice(rider and Pillion)  into parallel . This enables the rider and pillion to communicate.  However the rail voltage to the transmitter is turned off when intercom mode is selected thus the pillion cannot talk to other radios on the same channel.

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