MX800 Base Station/Repeater - System Applications

Overview

The MX800 comprehensive standard features along with a wide range of options and accessories available to it means, that the MX800 is capable of being used in many different applications. The most common of which are mentioned below together with notes where applicable on the relevant aspect of the MX800.


Figure 1.1 MX800’s Setup in system rack

 

Conventional Base Station/Repeater

Upper tier RF performance figures make the MX800 an excellent choice for a conventional base station or repeater. The PA is continuously rated and receiver figure of merit parameters such as sensitivity, intermodulation and selectivity are all of a high standard. Independently adjustable PTT and CTCSS tails mean that the mute crash can be eliminated in mobiles when base transmit CTCSS is used. A 40ms RX audio delay option (T08) similarly facilitates the elimination of the mute crash in the base station side.

As standard feature when the CTCSS encode/decode option is selected is multiple tone groups. In excess of 25 tones can be selected as valid CTCSS tones on a shared RF channel. Provision is made in the programming software to enter tone pairs for each group without restriction on how the tones are selected. This provides a community repeater,which allows multiple talk groups to use the same repeater but
only hear calls that belong to their own talk group.

Many other attributes of the MX800 and its options are useful in conventional base/repeater systems.
For example:-

  • Programmable channel bandwidth
  • DCS / Digital Private Line
  • Range of front panel functions available
  • Optional two wire four wire operation
  • Tone PTT with programmable PTT tone
  • Optional balanced and isolated VF I/O and signaling lines

The base station does not need a system interface card to make it act as a repeater, as the audio path from receiver to exciter occurs within the main controller board. This can be enable via software and or hardware control.

If the repeater can be provided with a telephone line, a dialup modem or Ethernet option can be used for remote configuration and monitoring.


 

Link Transceiver

In the UHF bands (400 to 520 MHz) the MX800 is available in 1W and 50W. The 1W version being intended for link transceiver application. Transmit and receive audio is conveniently brought out to the D15 connector on the rear of the radio along with the mute and PTT signals. In addition, provision is made to inject an Analog CTCSS tone into the transmit leg of the link and a 300Hz low pass filtered CTCSS signal is available at the RX leg of the link to recover the tone. Hence the Analog CTCSS tone can be transferred (or cross banded) from one MX800 to another. On an RF path with good signal to noise ratio, this enables CTCSS synchronization across the link in the situation where multiple tone group operate at two different sites. Spectra Engineering plans to upgrade this function with a digital 4 bit interface such that the operating CTCSS group is decoded at the repeater and re-encoded or cross encoded across the link, and the process repeated in reverse at the remote end.


 

Data transceiver

The MX800 transmit audio path is user configurable to a very large degree. TX modulation signals can be injected with or without processing depending upon the individual application. The transmitter low pass filter (the deviation limiter which sets the modulation bandwidth and ensures compliance with regulatory requirements) is a Bessel filter with linear phase characteristics. Fast transmitter key-up time and mute action mean reduced signaling turn around overheads for data messages and better throughput especially in a Simplex system with predominantly short messages.


 

Paging Transmitter

Due to the two point modulation method employed in the MX800, the transmit modulation frequency response can go down to DC as correctly required for POCSAG or other FSK based modulation systems. In addition it has a DC coupled FM modulator input biased to a voltage of 2.5 volts for carrier frequency Fo. A square wave input signal of 0 to 5V injected on this point drives the modulator to the positive and negative extremes of the deviation limiter. In this case set to +/- 4.5kHz. This means that it can transmit typical POCSAG paging signals. The modulation rise time is controlled by the frequency response of the transmit audio path low pass filter and is suitable for data rates of up to 2400 baud.

A typical system arrangement is shown in drawing SC004-1A.

Option T50 provides a POCSAG paging \ universal data input with programmable data time delay in 1uS steps. Can be used with T36 Simulcast option.

Paging Transmitter System Alignment
The FSK link deviation should be set to 60% of maximum deviation.
The paging transmitter alignment is similar to the standard alignment in paragraphs 5.1.9 and 5.1.10 in the MX800 technical manual. In 5.1.9 “Procedure” item 3 set the peak deviation to +/-4.5kHz (instead of +/-4.8kHz). In item 5 set the TX modulation frequency to half the data rate of the POCSAG data e.g. 600Hz for 1200-baud data etc.
Other than these changes the TX alignment is as per the technical manual.


 

Trunking Base Station

The MX800 is widely used as a trunking base station. A one hundred percent continuously rated transmitter is vital in the high duty cycle environment of a trunking systems. MPT 1327 control channels are permanently keyed up. Optionally the FSK signalling of MPT systems can be injected flat into an non pre-emphasized input and received on an non de-emphasized output which allows the signalling to go flat to air. As a compact two RU height enclosure the MX800 permits a high channel density for a given rack height.
LTR trunking systems make use of a digital sub audible signalling scheme. Once again the low frequency modulation capabilities (down to DC) of MX800 are vital in ensuring that the signalling takes place and a low bit error rate. A marginal system will result if the Transmitter modulation response can not go below 10Hz.


 

Systems Base

Typical small systems environments. Once again the user interface presented at the rear of the radio and the software programmable functions through MXTOOLS give systems designers and large degree of control over the base station.
In a system, which operates in one RF band, it is particularly convenient and cost-effective if all base stations can be made and programmed identically. This reduces the number of spare base stations required to maintain the system. MX800 supports this mode of operation in as much as the operating characteristics of up to 255 channels can be pre-programmed in all of the base stations, and insertion of an on-site channel selector and configuration plug selects the particular operating parameters for that base station in that location. Spectra will introduce additional features in this area.


 

Repeater With Morse ID

A programmable built in Morse ID encoder makes it convenient to use the MX800 as a UHF CB repeater, Amateur repeater or auto identified repeater.


 

Simplex Base Station

Option T06 for the MX800 is a coaxial changeover relay. In a Simplex system with a single antenna and common transmit and receive frequencies this can be used for connection of the transmitter and or receiver to the antenna. Provision is made in the programming to introduce a 50ms delay on transmit to allow the relay to changeover prior to RF ramp up.


 

Duplicated Base Station

Spectra engineering has developed an Automatic Changeover Units (MXACU) to facilitate duplicated base station operation.
The MXACU Automatic Changeover Unit (ACU) is designed to allow connection of two identical MX800 base stations, activating only one of them at a time in active/standby configurations. It automatically monitors and tests for fault conditions and switches operation to the healthiest base station should a fault be detected.


 

Power Save Base Station

For solar powered sites and other power critical applications the MX800 is capable of a Power Save (Low Standby Current) Mode. This option can be implemented in three stages. Stage one implementation replaces the micro controller linear voltage regulators with switching regulators. Response times are unaffected. Stage two involves removing power from the exciter when the radio is in standby mode. In this case RX responses times are unaffected. In stage three the RX power is cycled on and off at a user selectable duty cycle. Essentially the choice of mode of operation involves a compromise between response time and average current consumption. Current consumption of 250mA is achievable with a typical response time in the order of 1sec.


 

Tone Remote Control

The MX800 has the ability to provide tone remote control functionality to assist with control of remote base station / repeater applications. This is based on similar protocols to that used by the Motorola Tone and Repeater Interface Control Kit (TRICK) .

The TRICK may be used in three basic configurations:

  • Simplex base station - In this configuration, the TRICK provides a Tone Remote Control (TRC) interface between a the radio and a wireline control console.

  • Semi-duplex base station / repeater - In this configuration, the TRICK provides a Tone Remote Control (TRC) interface between a radio and a wireline control console.

  • Standalone repeater - In this configuration the TRICK controls the radio in the absence of a wireline console. The TRICK has 5-tone signalling (SELCALL) capability built-in, allowing the repeater to be controlled over air.


 

Voting Base Station

The MX800 with option to T19 can provide a Variable tone encoder. This encoder is compatible with a commonly used Variable tone-voting arbitrator.
MX800 can also be used with a SINAD voting arbitrator. Mute status of the receiver can optionally be signalled to the voting arbitrator via tone or through DC key E & M signalling.
Spectra engineering has current plans to implement the base station component of a race voting system using a Central arbiter.


 

Simulcast base station

Precise control of transmitter RF frequency is essential in simulcasts systems. Option T12\T36 provides an external TX reference oscillator input for injection of highly stable oscillator. Almost any frequency may be injected.
In addition, each transmitter channel frequency can be offset individually by small amounts if the internal reference frequency is used. The 1PPM frequency Stability option is specified for a minimum of 8PPM adjustment. Take care if too much offset is used as this may effect the modulation symmetry (T12 only). The use of this feature and a low frequency carrier dithering has proven to eliminate the previous requirements of extremely high stability reference frequency inputs.
The MX800 has been widely installed for the use in simulcast systems. These DSP based systems provide automatic compensation for changes in modulation delay characteristics.
The T36 Option extends the modulation capabilities of the MX800 when an externally locked reference input signal is required. In Simulcast systems where the application does not just require just Voice modulation but also the additional capability of either Data, POCSAG, CTCSS or DCS, then a transmitter is required to have a modulation bandwidth that extends to down towards 5Hz or better still to DC (0Hz). This new option provides that capability by incorporating digital RF techniques. In addition, the carrier can be offset in each transmitter in steps of 1Hz.

 

 


       
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Last Updated: Wednesday, May 9, 2013