TX Techniques TX FM1 User manual

TXFM1/S/R/A
VHFFMBroadcastExciter/Transmitter
withIntegral5-bandAudioProcessor,StereoEncoder,RDSEncoder
andRemoteRDSTA(TrafficAnnouncement)DTMFController
TechnicalManual
Nopartofthisdocumentmaybereproducedortransmitted,inanyform,byanymeans,
withoutpriorwrittenconsentofthedirectorsofTXTechniquesLimited,Harrogate,UK.
TXTechniquesLtdshallnotbeliableforanydirect,indirect,incidental,consequentialor
otherdamageorlossallegedinconnectionwiththesupplyoruseofthisproduct.
Theinformationcontainedinthisdocumentissubjecttochangewithoutnotice.TX
TechniquesLtdmakesnowarrantyofanykindwithrespecttothisinformation.TX
TechniquesLtdspecificallydisclaimstheimpliedwarrantyofmerchantabilityandfitness
foraparticularpurpose.
TXTechniquesLtdshallnotbeliableforanydirect,indirect,incidental,consequentialor
otherdamageorlossallegedinconnectionwiththefurnishingoruseofthisinformation.

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Index
1. Introduction
1.1 General 1
1.2 RDS Encoder 1
1.3 Stereo Encoder 1
1.4 Audio Processor 2
2. Safety information
2.1 General Safety Information 3
2.2 Before Operating this unit 4
3. Controls and Connectors
3.1 Front Panel Controls and Connectors 6
3.2 Back Panel Connectors 6
4. Installation
4.1 Connections 7
4.2 Switching On 7
4.3 Settings 7
4.4 Audio Processor 8
4.5 Remote TA Controller 9
5. Internal Adjustments
5.1 Carrier Frequency Setting 10
5.2 Carrier Frequency Fine Tuning 10
5.3 RF Output Power 10
5.4 Audio Processor Input Level 10
5.5 Audio Processor Individual Band Adjustment 10
5.6 Audio Processor Overall Density Level 10
5.7 19 KHz, 38 KHz, & 57 KHz Frequency Trim 11
5.8 RDS Injection Level 11
5.9 RDS Subcarrier Level 11
5.10 MPX Output Level 11
5.11 MPX Pilot Injection Level 11
5.12 MPX Subcarrier Null 11
5.13 MPX Separation Level 11
5.14 Composite Clipping Level 11
6. Circuitry
6.1 Switch Mode PSU 12
6.2 ALC 12
6.3 Audio Processor 12
6.4 Stereo Encoder & RDS Encoder 13
6.5 Modulator 13
6.6 LCD Control Panel 13
6.7 RF Amplifier 14
6.8 RF Measurement 14

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7. Block Diagrams
7.1 Switch Mode PSU 15
7.2 FM1-EXCI PCB 15
7.3 FM1-ALC PCB 16
7.4 FM1-AP PCB 16
7.5 FM1-RDS PCB 17
7.6 FM1-PA PCB 17
7.7 FM1-RF PCB 18
7.8 FM1-LCD PCB 18
8. Wiring
8.1 Wiring Diagram 19
9. Circuit Diagrams
9.1 FM1-EXCI PCB 20
9.2 FM1-ALC (L) PCB 21
9.3 FM1-ALC (R) PCB 22
9.4 FM1-AP PCB 23
9.5 FM1-RDS PCB 24
9.6 FM1-PA PCB 25
9.7 FM1-RF PCB 26
9.8 FM1-LCD PCB 27
10. Declaration of Conformity 28
11. Technical Specification 29

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1. Introduction
1.1 General
The TX-FM1/S/R/A is a 30W VHF FM Broadcast Exciter Transmitter with integral
stereo encoder, RDS encoder and multiband audio processor (with ALC and
clipping).
At the heart of the TX-FM1/S/R/A is a high quality modulator, which employs
proprietary 'Complimentary Varicap Compensation' for linear characteristics and
constant modulation levels across the entire FM band. Carrier frequency is selectable
via internal rotary dial switches, with a trimmer provided for output power adjustment
from 10 to 30W. The efficient MOSFET RF power amplifier includes a 7-pole
Chebyshev output filter and uses high power rated metal clad mica capacitors and
large air-wound coils for exceptional ruggedness.
Traditional slow acting needle meters have been replaced by the latest analogue to
digital measurement technology, with menu driven LCD screens for monitoring and
adjustments.
SWR cut back, and both audible and visual alarms activate in the event of
mismatches in the antenna system.
Careful design and layout of internal circuitry results in low board space, no wire
jumper or 'zero ohm' links. Every connection to every PCB is pluggable, allowing fast
serving. The use of 'through-hole' components means components can be replaced
easily by engineers using traditional methods.
The TX-FM1/S/R/A is designed to operate continually for many years without
maintenance. There are no cooling fans to replace and the carefully specified, quality
components ensure ultimate reliability.
1.2 RDS Encoder
The RDS Encoder broadcasts most RDS data types (excluding EON information),
and includes dynamic switching of the TA flag using DTMF signalling via programme
audio.
1.3 Stereo Encoder
The Stereo Encoder is part of the above RDS option board but is available without
RDS by means of reduced component population. The pilot tone and subcarrier are
generated from a single clock source for perfect phase correlation, and a high quality
DSSC modulation technique is employed for high quality results. Stereo operation
can be defeated using the LCD menu control. Composite clipping is applied to Coder
output.
1.4 Audio Processor
The 5-band audio processor provides optimum loudness, within tight deviation limits
and, together with the ALC section, ensures a constant level during both short term
and long term fluctuations. A ‘noise gate’ prevents noise on landline feeds or STL's
rising during long pauses, by returning the ALC section to 0dB gain.

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The five independent compressors on each channel feature wide filter bands for
natural sound and a non-linear compression curve characteristic, which prevents the
unnatural amplification of very low level sounds and noise. Multiple peak limiting
stages provide the control needed to ensure that final clipping circuitry operates with
negligible distortion or audible effect, across all kinds of programme material. This
analogue multi-band audio processor is the result of over three years of development
and achieves a spectral density and loudness similar to digital products costing many
thousands of pounds. Two boards are fitted for stereo operation.

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2. Safety Information
2.1 General Safety Information
Use of this device into a radiating antenna requires a valid Wireless and Telegraphy
licence.
Never operate this device without a suitable 50 ohm load connected to the RF
OUTPUT socket, or without a suitable antenna.
Use of this device as part of a transmission system, or combined transmission
system not specified by the manufacturer, may require further testing to ensure that it
remains compliant with the essential requirements and other relevant provisions of
Directive 19999/5/EC. Approval from a 'Notified Body' and clearance from the
Spectrum Management Authority may also be required.
Never allow audio, or audio peaks, in excess of +8dB enter the AUDIO IN LEFT
and/or AUDIO IN RIGHT input sockets, as this may lead to overmodulation of the
carrier. Always use an external audio limiter in cases where audio levels may exceed
+6dBu. FAILURE TO COMPLY MAY INVALIDATE ANY WARRANTIES OR
APPROVALS GIVEN TO THIS PRODUCT.
RISK OF FIRE! High power RF (Radio Frequency) energy could cause ignition of
combustible surfaces during fault conditions. Installation should be left to qualified
personnel. RF can cause severe burns to skin. Ensure antenna systems and feeder
cables are not situated near – or could fall onto – any combustible surface.
Installation must adhere to safety regulations and the requirements of the relevant
authorities. We recommend that at least two people are present during installation.
Keep a file containing installation instructions and plans, including details of the
transmission system (antennae, feeders, filters, etc) and operating instructions for all
equipment at the transmission site at all times. Display posters detailing first aid
treatment and treatment for electrical shock, along with telephone numbers for
contacting the emergency services in the event of personal injury.
BERYLLIUM OXIDE IS LETHAL. Some components inside this appliance contain the
highly dangerous TOXIC substance Beryllium Oxide. In case of damage contact the
manufacturer or its Agent immediately. Seek advice from the local authority when
discarding this appliance.
To reduce the risk of electrical shock, do not remove the cover, or any screws. NO
user serviceable parts inside; refer servicing to qualified personnel.
To reduce the risk of fire or electrical shock, do not expose this appliance to rain or
moisture.
To reduce the risk of fire, always replace fuses with the same type and rating.
Do not operate this appliance without a suitable 50 ohm dummy load or a suitable,
matched antenna connected to the RF OUTPUT socket on this apparatus. Although
the output devices used in this appliance are intended to be open and short circuit
tolerant, MIS-OPERATION MAY RESULT IN DAMAGE NOT COVERED BY ANY
WARRANTY.
The heatsink at the rear of this appliance may become very warm under normal
operating conditions.

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2.2 Before Operating This Unit
These instructions should be read in full before the transmitter is operated.
The safety and operating instructions should be retained for future reference.
All warnings on the transmitter and in the operating instructions should be adhered
to.
All operation and user instructions should be followed.
The transmitter should not be used near water.
The transmitter should be mounted into a well ventilated standard 19 inch equipment
rack. It should be situated so that its location or position does not interfere with its
proper ventilation. Particular attention should be paid to the bottom ventilation holes,
and convection of the rear heatsink, which can be impeded if the unit is placed on a
flat surface.
The transmitter should be situated away from heat sources.
The transmitter should be connected to a power supply only of the type described in
the operating instructions or as marked on the unit.
Precautions should be taken so that the grounding or polarisation means of an
appliance is not defeated.
Power supply cords should be routed so that they are not likely to be walked on or
pinched by items placed upon or against them, paying particular attention to cords
and plugs and the point where they exit from the transmitter.
The unit should be cleaned only as recommended by the manufacturer.
The power cord of the transmitter should be unplugged from the outlet when left
unused for a long period of time.
Care should be taken so that objects do not fall and liquids are not spilled into the
enclosure through openings.

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The transmitter should be serviced by qualified service personnel when: - The power
supply cord or the plug has been damaged; or - Objects have fallen, or liquid has
been spilled into it; or - The transmitter has been exposed to rain; or - The transmitter
does not appear to operate normally or exhibits a marked change in performance; or
- The transmitter has been dropped, or the enclosure damaged.
The user should not attempt to service the transmitter beyond that which is described
in the Operating Instructions. All other servicing should be referred to qualified
service personnel.

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3. Controls & Connections
3.1 Front Panel Controls and Connectors
AF (Audio Frequency) Monitor BNC. For monitoring of the input to the modulator (AF
spectrum, including MPX and RDS subcarriers). 1kΩ1V p-p (max).
RF (Radio Frequency) Monitor BNC. Allows monitoring of the RF output. Output level
will be the output power attenuated by approximately 40dB. 50Ω.
SoftKeys. These keys correspond to selections indicated on the adjacent LCD.
Menu 'Shuttle' Controller. Turn this control clockwise or clockwise to navigate forward
or backward through the LCD menu screens.
The initial "SELECT" menu allows the "SETTINGS" or "METER" screens to be
displayed. Under the settings menu, RF OUPUT, MPX, DTMF and PIN settings can
be changed. Under the meter menu, selecting "RF" provides monitoring of forward
("RF FWD") and reflected ("RF REF") power measurements. "AF" provides a
graphical PPM meter showing the sum of the LEFT and RIGHT audio inputs. Please
note that this is NOT a deviation meter. Because the internal limiting always ensures
that deviation is +/- 75KHz, or less, a deviation meter is unnecessary.
Please note: Pressing the Softkey corresponding to "EXIT" will always take the TX-
FM1/S/R/A to the "ENTER PIN" screen, as an anti-tamper feature.
3.2 Rear Panel Connectors
AC POWER. Connect a power lead with an IEC connector (and a 5 Amp fuse in the
plug or at the distribution panel) to this socket.
AUDIO IN (LEFT) XLR. Electronically balanced audio input. Apply a +4dB balanced
audio feed to this socket.
AUDIO IN (RIGHT) XLR. Electronically balanced audio input. Apply a +4dB (max)
balanced audio feed to this socket.
RF OUTPUT 'N' Socket. Connect a matched, pre-tested antenna system (using
suitably rated feeder cable), with a return loss of <=20dB, to this socket.

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4. Installation
4.1 Connections
Connect a matched, pre-tested antenna system (which complies with the regulations
of the license) to the RF OUPUT socket on the rear of the unit.
Connect a balanced +4dB (max) audio feed to the AUDIO IN (LEFT) and AUDIO IN
(RIGHT) XLR sockets.
Connect the transmitter to a suitable power source using the supplied IEC connector
power lead. Ensure that the appliance is protected by a 13A fuse either at the supply
plug or at the distribution panel.
4.2 Switching On
Ensure a suitable load is connected to the RF OUPUT socket before connecting the
unit to mains power.
Upon power-up, the display will show the unit's firmware release number. After a few
seconds, it will prompt for the PIN to be entered (or may go straight to the “SELECT”
menu if the PIN function is not enabled).
Enter the correct PIN.
Please note: If an incorrect frequency (eg. Below 87.5MHz or above 108.0MHz) has
been selected on the internal frequency setting switches, the unit will not initialise.
Instead it will display "ALARM: SYSTEM ERROR" after a short while. Disconnect the
mains supply, select a valid frequency internally, and then power-up the unit once
again.
4.3 Settings
The following options can be accessed via the “SETTINGS” menu:
RF OUTPUT. Selecting ‘Disable’ on this menu function mutes the RF output, putting
the unit into a ‘hot standby’ mode.
MPX. Disabling this setting will defeat the MPX Encoder, resulting in a monophonic
signal being transmitted .
DTMF. Disabling this option prevents DTMF tones from activating the TA flag.
PIN. Disabling this option prevents the transmitter from requesting a PIN code, upon
power-up before output. Please note that if the PIN setting is set to ON, the RF
output will not resume following a power outage, without the intervention of an
operator (entering the PIN code), in order to protect the unit against theft. If continual
supply cannot be guaranteed and transmission is required to automatically re-
commence following a power outage, the PIN setting must be disabled (note that this
will not, however, afford any anti-theft protection).

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4.4 Audio Processor
Audio Processing parameters are set at board level during manufacture, using
carefully selected component values.
When listening off-air, slow variations in loudness may not be apparent due to the
action of the internal ALC, which amplifies or attenuates a wide range of audio levels,
to a single, optimum level.
A +4dB (maximum audio level) test tone will result in roughly 50% modulation level,
due to the nature of the internal Audio Processor, when monitoring off-air using a
Modulation Meter or Analyser. Normal program (music and/or speech) will show full
±75kHz deviation.
PPMs – including the TX-FM1/S/R/A's graphical one – exhibit a fast response
and slow decay, and may not appear to represent music or speech in a predictable
manner. Also remember that meters that indicate the output of a radio station may be
connected to an off-air monitor receiver and will not respond in the same way as the
one on the transmitter (which is ahead of any processing). The graphical PPM is the
sum of both Left and Right channels and is for indication only. It is not intended to be
an accurate measurement of the audio level entering the unit.
By nature, multi-band audio processors greatly amplify any distortion present on
certain elements of source material (a particular instrument, for example), even if it is
not evident when previewed without audio processing. Also, CD recordings mastered
at very high levels, often drive many CD players reproduction circuitry into 'clipping'
(distortion). If, after ensuring that audio levels are correctly set, objectionable
distortion is heard, try alternative programme material and/or source equipment,
before suspecting that the transmitter or its audio processor is at fault. Audio
processing may also accentuate the distortion effect of microphone Voice
Processors.
The addition of an audio limiter in the audio feed, before the TX-FM1/S/R/A,
with the onset of limiting set at slightly above +4dB, is recommended.
Additional (external) audio processing normally has little effect on the sound,
because the internal processing counteracts any external processing. However,
experimentation is encouraged, and ‘mix enhancement’ products with ‘stereo
widening’ often yield excellent results in conjunction with the TX-FM1/S/R/A’s internal
processor.
It is strongly advised that the audio applied to the TX-FM1/S/R/A has little or no 'EQ'
applied. The integral 5-band Audio Processor, acts like a 'dynamic graphic equaliser',
optimising the tonal quality of the programme source. It is likely that this will
counteract any external tone equalisation. The point at which any additional EQ
would become noticeable on the received transmission, would be under extreme
levels of applied EQ. This would cause unpleasant sound, degradation of the audio
and possibly even overmodulation if DC or square waves (ie. distorted or clipped)
audio is applied.

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4.5 Remote 'TA' Controller
The DTMF remote controller may be used to dynamically switch the RDS TA (Traffic
Announcement) 'flag'. The short burst of 'RDS ON' tones are used to activate the
RDS TA 'flag'. Ensure that the tones are clearly audible.
Music and/or voices played at the same time could prevent the tones from working.
If, after the TA mode has been activated, the 'RDS OFF' tones are not received
within 4 minutes, the TX-FM1/S/R/A will automatically cancel the TA mode. Abuse of
this system may incur penalties from the broadcasting Authorities.
It is important that the level of audio entering the TX-FM1/S/R/A, and therefore the
level of the RDS tones is around +4dB. Whilst the DTMF detector has its own ALC
circuitry, it is 'ahead' the ALC and Audio Processor circuitry. If the signal is too low, it
may not activate the DTMF detector. Similarly, if the level is too high, it may be
distorted, and will not be correctly decoded by the DTMF circuitry.

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5. Internal Adjustments
5.1 Carrier Frequency Setting
Select the desired carrier frequency on SW1,2,3 & 4 on “FM1-EXCI” PCB. For
example, to set the unit to operate on 98.4, turn SW1 to “0”, SW2 to “9”, SW3 to “8”
and SW4 to “4”. The new frequency will take effect when the unit is next powered-up.
5.2 Carrier Frequency (Fine Tuning)
Connect a frequency meter connected via the front panel “RF” BNC socket. Adjust
VC1 on “FM1-EXCI” PCB.
5.3 RF Output Power
With the front panel LCD set to display forward RF power, turn VR1 on “FM1-EXCI”
PCB to achieve the desired output power. To set the power accurately, use an
external calibrated RF power meter (with a suitable RF test load) connected to the
“ANTENNA” socket on the rear panel.
5.4 Audio Processor - Input Level
This board-level setting is factory set using a specific component value. R31 (right
audio channel) and R32 (left audio channel) on the “FM1-ALC” board determines the
overall level entering the multiband audio processor.
Decrease Normal Increase
1K5 (min) 2K2 4K7 (max)
Use of values other than the default values fitted during manufacture may result in a
deterioration of the broadcast sound quality.
5.5 Audio Processor - Individual Band Adjustments
These board-level settings are factory set, using specific component values. Values
below 47K or above 100K are not recommended. Use of values other than the
default values fitted during manufacture may result in a deterioration of the broadcast
sound quality.
80Hz LPF 200Hz BPF 650Hz BPF 2100Hz BPF 5000Hz HPF
R4 R6 R19 R20 R32
Increase Normal Decrease
Decrease value (47K min) 68K Increase value (100K max)
5.6 Audio Processor Overall Density Level
This board-level setting is factory set, during manufacture, using a specific
component value. R42 sets the overall density level. Use of values other than the
default value fitted during manufacture may result in a deterioration of the broadcast
sound quality.

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5.7 19kHz, 38kHz and 57kHz Frequency Trim
Using a frequency meter, monitor Pin 7 of IC6 and adjust VC1 for 19,000Hz. This will
also, in turn, adjust the 38kHz and 57kHz subcarrier frequencies.
5.8 RDS Injection Level
The RDS injection level can be adjusted using VR5 on the “FM1-RDS” PCB.
5.9 RDS Subcarrier Null
The DSSC generation of the RDS signal requires the 57kHz carrier to be nulled, for
correct operation. VR1 on “FM1-RDS” PCB controls this setting, and requires the use
of a spectrum analyser connected to the front panel “AF” monitor BNC socket
5.10 MPX Output Level
The overall MPX level (ie. audio, pilot and subcarrier) can be altered using VR6 on
the “FM1-RDS” PCB. This setting requires the use of a modulation monitor
connected to the front panel “AF” monitor BNC socket
5.11 MPX Pilot Injection Level
The level of pilot present in the stereo broadcast can be varied, independently of the
overall MPX setting, by adjusting VR4 on the “FM1-RDS” PCB. This setting requires
the use of a modulation monitor connected to the front panel “AF” monitor BNC
socket
5.12 MPX Subcarrier Null
The DSSC generation of the stereo multiplex signal requires that the 38kHz carrier is
nulled, for correct operation. VR2 on “FM1-RDS” PCB controls this setting. This
setting requires the use of a spectrum analyser connected to the “AF” monitor BNC
socket.
5.13 MPX Separation Level
VR1 on “FM1-RDS” PCB controls stereo separation, which should be set using an
oscilloscope connected to the front panel “AF” monitor BNC socket and a 1kHz tone
connected to either the left or right audio input only. Setting the RDS injection and
MPX pilots levels to minimum will improve clarity of the displayed waveform.
Maximum separation is achieved when the waveform exhibits a flat baseline.
5.14 Composite Clipping Level
VR7 and VR8 on the “FM1-RDS” PCB adjust the amount of clipping applied to each
polarity of the MPX waveform. A digital storage oscilloscope is required for this
adjustment. Both trimmers should be adjusted for exact symmetry, and so that
overshoots (above and below the average limited/clipped audio levels) are removed.

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6. Circuitry
6.1 Switch Mode PSU
The Switch Mode PSU provide a single +28VDC 6.5A output from a wide range of
input voltage supplies (90 – 264VAC 47-63Hz, or 120 – 370VDC). It features power
factor correction and compliance to generic EMC standards. A version capable of
Level 2 surge immunity (as defined by ETS 300 447 standard) can be supplied if
specifically requested at the time of ordering. The SMPSU is over voltage and short
circuit protected, and is a self-contained unit for fast, easy replacement.
6.2 ALC
The left and right circuits are identical. The following component references relate to
the left audio channel only.
This board also incorporates a switching regulator, which provides a +/- 12V supply
for this, and other boards, via PL4 (which also includes a +24V supply rail).
XLR balanced audio passes through an RF filter (FB1, FB2 and C1,C2) and is
converted to unbalanced audio by IC1 (Balanced Line Receiver).
IC2 buffers the Left and Right audio channels which are then bridged and presented
to the LCD meter and the DTMF TA controller as a mono source, via socket PL5.
The audio passes through IC3(A), the gain of which is controlled by OR1 in its
feedback path. The output of IC4 (precision rectifier) and IC5 (mute control) feeds
OR1. High audio levels are gradually reduced, or long term low levels boosted.
Extended periods of silence result in the ALC slowly returning to an overall 0dB gain
level, thus reducing noise on landlines or STL feeds.
Op-amp IC6 is configured as an amplifier enabling the ALC output level to be
adjusted (by changing the value of R31) . This will increase/decrease the input level
for the audio processor.
6.3 Audio Processor
Dual op-amps IC1, IC3 and IC5 dual op-amps form the band filtering. IC2, IC4 and
IC6 are dual VCAs set to perform a limiting/leveling action on each band. Their
outputs are combined by IC5b. LED3 and 4 provide ‘brick wall’ limiting. IC6b
performs an overall ‘sliding pre-emphasis’ limiting action. LED1 and LED2 further clip
the waveform ahead of FILTER 1, which is a sharp 15kHz low pass filter. Since the
input and output impedance have to be carefully matched for this filter to work
correctly, R47, R48, R49, R44 are used to ensure the correct drive and load
impedances.

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6.4 Stereo Encoder/RDS Encoder
IC6 contains outputs a 19kHz, 38kHz and 57kHz clock from a single crystal, XTAL 4.
RC filtering converts the digital pulses into sine waves.
IC7 creates the MPX signal with a suppressed carrier, the output of which is switched
by IC5. The 19kHz pilot signal is also switched by IC5 and then mixed to the
multiplex signal by IC8, a quad op-amp.
IC4 monitors the bridged (mono) audio from the ALC board for DTMF tones and
outputs serial data representing the decoded tones. IC3 is the main RDS processor;
a powerful 8-bit microcontroller capable of executing 5 million instructions every
second. It not only calculates and outputs the RDS data stream and the associated
Cyclic Redundancy Code in real time, but also contains an algorithm to decipher the
DTMF code validity, and sets theTA flag accordingly. The serial RDS data stream is
clocked out of IC3 by IC2 which digitally synthesises the RDS pulses. IC1 creates the
RDS signal with a suppressed carrier.
6.5 Modulator
The MPX signal modulates VCD1. The RF carrier frequency is tuned by a PLL voltage
across VCD3 and VCD4. The amount of capacitance effect the modulation varicap
has on the tuned circuit (VCD3, VCD4 and L1) is controlled by VCD2 in order to
introduce more modulation at one end of the tuning range. This has the effect of
maintaining a constant modulation level across the entire FM band. TR2 and TR3
form a ‘Cascode’ oscillator, which is buffered by TR4, and switched by TR5. This
allows the exciter output to be muted via the font panel LCD menu settings, during
tuning, and under output mismatch conditions.
An RF transformer T1 matches the buffer to the modulator output transistor TR6
which provides an efficient 2W (nominal) output.
IC2 provides the PLL tuning function, and is controlled by serial data from the host
microcontroller, IC3, which also continually monitors the PLL for a locked condition,
creating a logic output signal accordingly. An op-amp PLL filter is used based on
designs in the PLL IC manufacturer’s datasheet and application notes.
A voltage sample, proportional to the RF output power, is connected to PL3. This
controls the base of TR1. The more positive its base, the more OR1 reduces the
resistance on the VADJ pin of regulator VREG1. This lowers the voltage to TR4 and
TR6, lowering the modulator board’s output, thus forming an ALC function, the
threshold of which is controlled by VR1.
6.6 LCD Control Panel
IC1 is a microcontroller containing proprietary firmware to perform all metering,
system monitoring and set-up functions. D1, D2, R1 and C15 form a simple passive
rectifier with a time constant which creates a voltage proportional to the peak of the
bridged (mono) audio feed at PL3. This voltage is sampled by an on-chip D to A
converter in IC1.
5V logic control signals for TA function and MPX settings (on or off) are provided on
PL1.

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A 5V logic signal from “FM1-EXCI” arrives at PL2 when the PLL is locked . The RF
on/off control signal is sent back to the modulator board via the same connector.
Voltages proportional to the forward and reflected RF levels present on the “FM1-RF”
board arrive at PL4. These voltages are sampled by an on-chip D to A converter in
IC1.
R2 and ZD1 create a reference voltage for IC1 from the 5V supply rail which is
smoothed by C10.
IC1 directly drives sounder S1, and performs the decoding for the front panel
encoder ENC1 and the push button switches SW1 and SW2.
6.7 RF Amplifier
The RF amplifier design uses a Motorola RF MOSFET device. C1, L1, C2, and L2
form an input matching circuit. ZD1 rectifies the RF forming a bias for TR1 and R1, in
conjunction with the board circuit pad presents the correct input impedance to TR1.
L5,C4,L5,C5 form an output matching circuit for TR1 and C7, L7, C8, L8, C9, L9,
C10, L10 and C11 form a 7-pole Chebyshev RF output filter.
6.8 RF Measurement
C3, C4, R4 and R5 create a decoupled -40dB sample of the coaxial output line. C1,
C2, T1 together with R1, R2, D1 and D2 provide voltages proportional to the forward
and reverse RF power levels. The ‘reflected’ voltage is nulled by VR1 which is
connected to the RF output via R3. IC1 and IC2 buffer the forward and reflected
voltages, which are kept below 5V, under all conditions, by ZD1 and ZD2.

15
TX-FM1/S/R/ATechnicalManualwww.txtechniques.co.uk
7.BlockDiagrams
7.1SwitchModePowerSupply
7.2FM1-EXCIPCB
PFC
CIRCUITPOWER
SWITCHING
ACTIVE
START
CIRCUIT
RECTIFIERS
&
FILTER
LIMITING&
DETECTION
CIRCUIT
PWM&PFC
CONTROLO.V.P.
O.T.P.
O.L.P.
24Vdc
24Vdc24Vdc
5Vdc±12Vdc
VFO
PLL
uPC
BCDFREQUENCY
SELECTSWITCHES
FILTERAND
ERRORAMP
INTERMEDIATE
RFPOWER
AMPLIFIER
VARIABLEPOWER
REGULATOR
ALC
VOLTAGE
AFINRFOUTTO
SWRBOARD
LOCKED
RF
INHIBIT
FASTLOCK
CONTROL

16
TX-FM1/S/R/ATechnicalManualwww.txtechniques.co.uk
7.3FM1-ALCPCB
7.4FM1-APPCB
BALANCED
LINERECEIVER
BALANCED
LINERECEIVER
ALC&
NOISEGATE
ALC&
NOISEGATE
PRECISIONRECTIFIERS
PRECISIONRECTIFIERS
AFINAFOUT
AFOUT
AFIN
DTMFDETECTOR
FRONTPANELMETER
AUDIO
RCLOWPASS
FILTERCOMPRESSOR
LIMITER
COMPRESSOR
LIMITER
FINALCLIPPER
15kHzLOWPASS
FILTER
COMPRESSOR
LIMITER
COMPRESSOR
LIMITER
COMPRESSOR
LIMITER
COMPRESSOR
LIMITER
AUDIO
RCBANDPASS
FILTER
AUDIO
RCBANDPASS
FILTER
AUDIO
RCBANDPASS
FILTER
AUDIO
RCHIGHPASS
FILTER
AFOUT
AFIN

17
TX-FM1/S/R/ATechnicalManualwww.txtechniques.co.uk
7.5FM1-RDSPCB
7.6FM1-PAPCB
RFIN
+24VDCSUPPLY
RFOUT
MOS-FET
PASTAGE
7-POLE
RFLOW-PASS
FILTER
INPUTMATCHING
NETWORK
SELF-BIAS
OUTPUTMATCHING
NETWORK
MASTER
CLOCK
SOURCE
uPC
DIVIDE
BY8,12,24
uPC
DATASINE
SYNTHESIS
SUPPRESSED
CARRIER
MODULATOR
MATRIX
AMPLIFIER
SUPPRESSED
CARRIER
MODULATOR
SQUAREWAVE
TOSINEWAVE
FILTERS
uPCDATAEEPROM
WITHREALTIMECYCLIC
REDUNDANCYCODE
GENERATOR
DTMFDETECTOR
WITHALC&
SERIALOUT
AF(DTMF)
PROCESSED
AUDIO(L)
PROCESSED
AUDIO(R)
COMPOSITE
CLIPPING
EXCITEREXCITER