Tait TB8100 Guide

TB8100 base station
Using a TB8100 with
an External Modem
Application Note TN-1114-AN
21 December 2005

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© Tait Electronics Limited 21 December 2005
Contents
1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4 Functional Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5 Connecting the TB8100 to an External Modem . . . . . . . . . . . . . . . . . . . . . . . 5
6 Configuring the Base Station. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7 Performance Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8 Frequency Response Graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Associated Documentation
TB8100 Installation and Operation Manual (MBA-00005-xx).
TB8100 Specifications Manual (MBA-00001-xx).
TB8100 Service Kit User’s Manual (MBA-00010-xx) and online Help.
Your modem User’s Guide.

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© Tait Electronics Limited 21 December 2005
1 Overview
This Application Note provides information on how to configure a TB8100
base station to use an external modem by means of the extended bypass
audio functionality. The extended bypass functionality provides an analogue
signal path with a higher than standard frequency response, but only via the
unbalanced line. This allows the connection of external modems that have
information content above 3kHz. Included in this Application Note are
instructions on how to configure the TB8100, and some performance
indications for various types of modem.
This Application Note is intended for use by system integrators.
2 Applicability
The extended bypass audio functionality is only available on reciters with
hardware version 0.04 or later. The performance has been tested using only
the unbalanced line interface on an isolated system interface board
(TBA10B0).
Note The TB8100 does not check the extended bypass functionality
against the hardware version number. It is up to the user to ensure
that the correct hardware version is used.
3Disclaimer
Tait Electronics Limited has not applied for, and will not apply for,
regulatory compliance when using the bypass or extended bypass path. This
is because Tait does not want to specify any particular external modem to be
used. The choice of external modem is therefore up to the user.
The bypass and extended bypass path has been provided to facilitate only a
data path through the TB8100. Note that normal voice-type compliances
cannot be achieved when this has been selected in the channel profile. This
is primarily due to the lack of a limiter in the transmit modulation path.
It is therefore the responsibility of the user to apply to their local regulatory
authorities for the appropriate data compliances.

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© Tait Electronics Limited 21 December 2005
4 Functional Description
Exciter Bypass The bypass path connects the audio sampled at 25kHz from the analogue
unbalanced line input directly into the frequency control loop (FCL), which
then modulates the RF hardware. The only digital filtering in the bypass
path is an optional AC filter, which has a cut-off frequency of 0.8Hz. Note
that there is no limiter present in this path.
Receiver Bypass On the receive audio bypass path, received audio from the FM demodulator
is decimated from 50kHz to 25kHz. It is then directly converted to the
analogue unbalanced line output with no further filtering.
Extended Bypass There is anti-alias analogue filtering performed on the unbalanced lines, and
also on the FCL outputs. This filtering on the bypass path causes a roll-off
in amplitude as the audio frequency is increased. When extended bypass is
enabled, the analogue filtering on the unbalanced lines is disabled by
hardware, and a digital finite impulse response filter with a constant group
delay is used in both the transmit and receive bypass paths to extend the
frequency response.

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© Tait Electronics Limited 21 December 2005
5 Connecting the TB8100 to an External Modem
Connection Connect the TB8100 to the external modem using the unbalanced line
input (Tx audio) and output (Rx audio) on the system interface connector.
If required, you can also use the Rx Gate and Tx Key lines, as described
below.
Details of the pin allocations for the different types of system interface
connector are provided in the “Connection” chapter in the TB8100
Installation and Operation Manual, and also in the TB8100 Service Kit
(Configure > Base Station > System Interface).
AC Coupled
Unbalanced Audio
Input
The modem selected may have a DC offset present for both the transmitter
and the receiver. On the standard system interface board, the transmit path
is DC coupled (with a bias of 2.5V), and the receive path is AC coupled.
We recommend that you enable the high pass filter in the Signal Path tab of
the channel profile dialogue box in the Service Kit (refer to “Configuring
the Base Station” on page 7). This will ensure that the transmitted signal is
exactly on the configured carrier frequency. If you do not enable the high
pass filter, ensure that you set the transmit carrier frequency exactly to the
required frequency by adjusting the DC bias of the external modem.
Note The CODECs used to sample the unbalanced line input are DC
capable. However, because of component variations, the DC
offset incorrectly reported by the CODEC can lead to a small
offset in the transmit carrier frequency, which is removed by
selecting the high pass filter.
Figure 1 Typical connection
Tx Key
Rx Gate
Rx Audio
Tx Audio
Ground
TB8100
System
Interface
Modem

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© Tait Electronics Limited 21 December 2005
Receiver Gating If the receiver is not gated, the received audio will be continuously presented
to the external modem. This may lead to the occasional false packet
detection, due to noise falsing the decoder. One option to prevent this is to
gate the received audio and/or use the Rx Gate signal to indicate to the
modem when a valid RF signal is present.
Note In this case, you will probably have to configure the external
modem to cope with the timing sequence of audio presented from
the unbalanced line. Refer to the TB8100 Specifications Manual
for gating response times.
Tx Key If you do not require continuous transmission, you can use the Tx Key input
on the system interface board to key up the TB8100, and to key it down
when transmission is not required.
Note In this case, you will probably have to configure the external
modem to cope with the timing sequence of audio presented to
the unbalanced line for transmission. We also recommend that
you refer to the Tx Timers tab of the signalling profile dialogue
box in the Service Kit, and select suitable values or disable timers
as appropriate for your system.

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© Tait Electronics Limited 21 December 2005
6 Configuring the Base Station
Extended bypass is configured in the channel profile assigned to the
operating channel. If a different operating channel is selected, the signal
path configured for that channel will take effect.
Important Selecting this function bypasses the limiter. This may allow
overdeviation and cause the equipment to become non-
compliant. It should be selected only when the TB8100 is
connected to approved level-controlled devices (modems,
trunking controllers, etc.).
Important With extended bypass you cannot use the balanced line or
internal talk-through repeater connections from receiver to
transmitter.
To configure the signal path for extended bypass.
1. Run the Service Kit and open the Add or Edit Channel Profile dia-
logue box for the appropriate profile.
2. Select the Signal Path tab.
3. Select the Bypass check boxes for Path B (both transmit and receive).
Extended bypass check boxes appear alongside the Bypass check
boxes.

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4. Select one of the Extended bypass check boxes. This selects the
other Extended bypass check box as well. (You cannot configure
extended bypass separately for the transmit and receive paths.)
7 Performance Specifications
Extended bypass has been tested with external modems as follows:
The performance figures given in these specifications are typical, and are
supplied only as a guide to the expected performance. The actual
performance achieved by your system will depend on the specific external
modem used, and the interface levels into the TB8100.
narrow bandwidth C4FM 9600bps
external modem based on a CML FX919 chipset
narrow bandwidth GMSK BT=0.5, 4800bps
external modem based on a CML FX489 chipset
wide bandwidth GMSK BT=0.5, 9600bps
external modem based on a CML FX489 chipset
NB C4FM NB GMSK WB GMSK
Minimum Receiver RF Level for a Robust Link ––a–109dBm
(0.79µV)
–109dBm
(0.79µV)
Strong Signal Packet Error Rate (PER)*
*for RF levels of 0dBm
<10-4 <10-4 <10-4
Signal Displacement Bandwidth >1kHz >1kHz >2kHz
Selectivity ––a71dB 79dB
Modulation Fidelity*
*measured as Error Vector Magnitude, through combined
receive and transmit paths
3.5% 6% 6%
a. Unavailable at time of publication.

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© Tait Electronics Limited 21 December 2005
Publication Information
Issuing Authority This TN was issued by: Kurt Ebrecht
Technical Publications Manager
Distribution Level Associate, Associate System Integrator, Accredited Service Centre,
Accredited Service Centre System Integrator, or Tait Only.
Publication History
Amendment Record
Tait Contact Information
Corporate Head
Office
New Zealand
Tait Electronics Limited, P.O. Box 1645, Christchurch, New Zealand
E-mail (Marketing): [e-mail]
E-mail (Sales): [e-mail]
Technical Support Technical Support Manager
Tait Electronics Ltd, P.O. Box 1645, Christchurch, New Zealand
E-mail: [e-mail]
Internet http://www.taitworld.com
Publication Date Author
21 December 2005 D Reynolds
Publication Date Page Amendment
21 December 2005 First release

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© Tait Electronics Limited 21 December 2005