VK6WIA News Broadcast Transceiver

VK6WIA Broadcast Transceiver

1. INTRODUCTION

The unit is based on a PHILIPS FM880 link transceiver which was originally supplied to a Telecom Australia specification for telephone applications in remote areas of Australia. The FM880 in turn is part of the family of equipment including the PHILIPS FM828/FM814 series of fixed and mobile transceivers. The main receiver and transmitter exciter boards are identical in most cases and are surrounded with specialized boards and circuitry for the particular application.

The VK6WIA modifications are therefore simply another variant of the versatile PHILIPS design, although a fundamental change has been made in that the system of modulation has been changed to direct FM of the exciter reference crystal oscillator. The inbuilt phase modulator on the original FM880 exciter board is unused.

Other audio, power supply and control facilities have been added to provide the WIA Broadcast Officer with a transceiver designed to provide the regular news broadcast service.


2. FM880 RECEIVER and EXCITER/PA

Extracts from the FM880 handbook, detailing receiver and transmitter circuit description, alignment instructions, circuit diagrams and board layouts are attached at Appendix I.

All Telecom telephony related boards are discarded, although some components have been recovered and re-used; e.g. antenna changeover relay.

3. VK6WIA BROADCAST FACILITIES

A number of new modules have been constructed and added to the FM880 chassis.

These are:

(a) AGC/Limiting Amplifier.
(b) 200-3800 Hz Filter and Microphone Amplifier
(c) CTCSS Sub-tone generator.
(d) Vu and Peak level meter drive.
(e) Audio power amplifier.
(f) -12V supply.
(g) 1 KHz tone generator.
(h) Auxiliary audio amplifier
(i) Deviation audio amplifier
(j) Twin FM crystal oscillator
(k) Tone keying relay
(l) Single control relay
(m) Dual control relay

3.1 Functions

The following panel controls have been provided:-

12 Volt power switch
Microphone gain
High Level (tape) input gain
Channel switch
Microphone/High Level switch
1 KHz Audio Osci11ator switch
Audio Level Meter switch
CTCSS Sub-tone switch and LED indicator
Transmitter Monitor switch
Audio Out (Receiver) or Audio
Receiver Squelch
Receiver Audio gain
Transmit Key

On the front panel are located :-

Phone jack (Stereo wired as mono)
Audio out jack (Mono >5OKohm load)
Microphone/PTT socket
VU/Peak level meter
Transmit RF level meter

On the rear of the chassis are:-

Extension speaker jack (8 Ohm)
High level input (Two RCA sockets in parallel)
Audio osci11ator key jack
13.8 Volt input (Barrier strip  terminals  or  2  pin  Utilux)
Antenna connector (S0239)
DIN connector for External Auxiliary Unit

4. OPERATING INSTRUCTIONS:-

4.1. Apply 13.8 Volts to rear connector, connect antenna and attach microphone. Set all controls to max anticlockwise and toqqle switches to OFF (up). The DTMF / HL / Tone Switch should be on "HL".

4.2. Turn on POWER. The two meters should be illuminated.

4.3. Advance Receiver AF gain by a small amount. Rotate Receiver SQL control until noise is heard. Set AF gain for a convenient level and adjust SQL for muting threshold.

4.4. Signals on CH2 Repeater should now be received if present.

4.5. Advance the MIC gain and speak into the microphone without activating the PTT or TRANSMIT key. The microphone audio level should indicate on the VU meter. Set the MIC gain so that at the speaking level and distance to be used the VU meter on frequent peaks deflects to reference level (0 VU)

4.6. Apply test audio on the HL input. Select HL on the MIC/HL switch. The HL audio should indicate on the VU meter as the HL gain is advanced. Set the HL level for reference deflection on the VU meter.

4.7. If it is desired to listen to the audio applied so far, without transmitting, insert PHONES into the front panel jack, or use the internal or extension speaker. Select AF IN. The audio from MIC or HL can now be heard depending on which is selected. (However the MIC audio is muted if the internal or external SPEAKER is in use).

4.8. Restore the Audio select to AF OUT.

4.9. Select PK DEV on the VU/PK DEV meter switch. The relative peak deviation level of any received signal will be displayed on the level meter.          +/-5KHz deviation corresponds approximately to 100% on the level meter. (This indicated level is after de-emphasis so is not a true deviation measurement on speech, but is calibrated for a single tone of 1000Hz). Reselect VU.

4.10. To transmit, either press the locking TRANSMIT key or use the microphone PTT. The RF meter will deflect to about 70% of full scale, providing a relative indication of transmitter RF output. (No Receive RF level indication is provided in the prototype units). Actual transmitter power output is about 15 watts, depending to some extent on the regulation of the power supply. Total power drain on transmit is about 5 Amps. The transmit frequency is 146.100 MHz, with CH 2 selected. With AUX selected, transmit and receive frequencies will depend on chosen crystals.

4.11. With TRANSMIT activated, select HL on the MIC/HL switch and TONE on. The VU meter should deflect to reference level, providing a relatively low distortion 1000 Hz continuous tone. The tone can be keyed by inserting a key or keying contact in the rear panel jack. The key operates a reed relay and carries a current of about 10mA from 13.8 volts. Simple contact closure or an open collector NPN transistor switch is required to key.

4.12. Still with TRANSMIT and TONE selected, select MON on. The receiver will be activated on the TRANSMIT frequency and the transmitted tone will be heard in the phones or speaker. That is, “OFF-AIR” monitoring is activated. Switching the VU/PK DEV switch to PK DEV will now show also the transmitted deviation level which should be about 70-80%. Select TONE off, and with audio sourced from the HL input (e.g. tape), this audio will be heard in the speaker or phones and its transmitted deviation level displayed. On audio peaks the modulation level should reach 100%. and occasionally beyond. Select MIC and speak into the microphone. The microphone audio can be heard in the PHONES but is muted from the loudspeaker outputs, to prevent on-air feedback. Deviation should also reach 100% or so. Advancing the MIC or HL gain, perhaps unintentionally at this stage will increase compression, but will not increase Peak deviation significantly: the peak level being held by the AGC/limiting amplifier.

4.13. Release TRANSMIT. Signals on the repeater output will be heard as normal and traffic can be exchanged with deviation levels being displayed on the level meter for receive and transmit as the contact proceeds.

4.14. If a WIA Broadcast is being initiated, select SUB to insert the crystal controlled CTCSS tone. This is required to inhibit repeater time-out and set up repeater links. The sub tone level is pre-set by an internal trim-pot and should not require re-adjustment. Its level is not affected by other controls.

4.15. The AUDIO jack on the front panel provides a sample of the audio fed to the audio PA but is of a fixed level independent of the AF gain. The output can be used for example for:-

(a) Access to the 1000 Hz tone.
(b) For relay of the received signal
(c) For recording of one or both sides of contacts.

This output requires a load of >50,000 Ohms.  Output level is about 50-100mV.

If the output is being used for relay purposes, MIC announcements can be inserted  as required by switching from AF OUT to AF IN with MIC also selected.

The PTT should NOT of course be activated for this application. The MIC announcements will appear at the AUDIO output and on the HF relay. Restore AF IN at the conclusion of the ID.

4.16. The RX MODE switch is usually set to NORMAL. When set to STBY the receiver local oscillator is disabled. When set to SX, transmit AND reception occur on the repeater INPUT, providing an emergency simplex system in the event of repeater failure, or an alternative "unrepeated" source for relay purposes.

4.17. The CHANNEL switch is usually set to CH.2. Selecting AUX (if equipped) will provide either operation on a selected simplex with full facilities, or an alternative emergency operation WITHOUT the MON and transmitter deviation capacity. A pair of crystals for receive and transmit frequencies can be plugged in to vacant sockets to provide this backup facility when required.

4.18. Tone Oscillator

Either line-up or tuning tone is provided by selecting HL and TONE, activating the 1 KHz low distortion audio oscillator in the FM880.

Insertion of a key into the rear panel jack allows keying of the oscillator for identification or Morse practice, either on air or if AF IN is selected, for local purposes. For example a practice Morse recording could be prepared by recording from the AUDIO socket without activating PTT or TRANSMIT.

See also the following section for Automatic Morse messages (and DTMF control tones).

5. EXTERNAL AUXILIARY  UNIT

5.1 DTMF tone generation for control of remote facilities has been provided for in the External Auxiliary Unit.

5.1.1 To send DTMF tones proceed as follows:

With the Auxiliary box plugged in to the FM880 rear panel , on the FM880 Front Panel select “HL” on the “MIC/HL” audio switch. Select “DTMF” on the adjacent three position toggle switch. While transmitting, key in the desired DTMF code on the keypad.

5.1.2 To Change the input frequency of VK6RAP proceed as follows:

The repeater normally is on 146.100 MHz. To change the receiver frequency the appropriate DTMF code must be keyed in manually while transmitting  on the frequency of 146.100 MHz.

Then all operation and control of VK6RAP must be on the new frequency. To change the repeater input back to 146.100 MHz transmit the correct code on the alternate frequency. The repeater is now back on its normal frequency of 146.100 MHz.

Note that all DTMF tones on VK6RAP are not re-transmitted through the repeater. Every DTMF tone is detected at the repeater and within 50 milliseconds the repeater audio is muted for one second. Hence a string of DTMF tones are not heard on the repeater's output except for the first 50 milliseconds of the first tone.

5.2 Automatic Morse code generation has also been provided for in the External Auxiliary Unit.

5.2.1 The Morse groups are as follows:

(1) CT (Commence transmission)
(2) 73 DE VK6WIA AR  (Concluding signoff)
(3) BT  (Interval or break signal)
(4) E E E E E   (15 Dots - use at commencement to cue relays)
(5) TEST DE VK6WIA   (Include in test transmission)
(6) VK6WIA
(7) BT      VK6WIA     BT   (Relay ident pause)
(8) AR   (End of transmission, short version)

5.2.2 Operation is as follows;

(a) On the FM880 Front panel set the MIC/HL switch to "HL". Set the adjacent three way-toggle switch to "TONE".

(b) On the external keypad box, select the required message number on the 8 position switch.

(c) Briefly press the red button on the Auxiliary Unit and the message will be sent once by keying the tone oscillator in Morse.

(d) Reset the three way toggle to the centre "HL" position when finished.

6. ALIGNMENT

6.1 Receiver and Exciter/PA:-

Before commissioning the facilities external to the FM880 boards, both the receiver and exciter/PA boards should be aligned to 146.7/146.1 MHz in accordance with the original Philips handbook. Sufficient extracts from the handbook to do this have been provided at Appendix I.

Note that the RF Level meter trimpot mounted on the meter should be set to minimum sensitivity during the recommended alignment for the exciter/PA.

6.2 Additional Faci1ities:-

6.2.1  -12 Volt Supply.

Apply +13.8 volts. Adjust TP1 for maximum -ve volts at the input to the -12 volt regulator. A voltage >15 volts should be available with a load of about 40 mA.

6.2.2. Limiting Amplifier.

Set all trimpots to maximum. Apply a tone to the input of about 50mV r.m.s. Adjust TP2 until V out is about 60mV r.m.s. Vout can be increased by adjusting TP3 until Vout is approx. 4 times Vin. The  limiter will now compress voltages in excess of 50mV in. TP1 and TP4 now become  Limiter  In  and Limiter Out gain controls for final system alignment.

6.2.3. Peak Meter adjustment.

Apply a signal modulated with a 1 KHz tone to 5KHz deviation to the receiver. (Alternatively use average speech modulated traffic). Switch meter to PK DEV. Adjust TP2 on the metering board so that the panel meter reads 100%.

The meter amplifier balance trimpot TP3 may be adjusted at this time, however it appears to be very uncritical in its operation.

6.2.4. Input and VU Meter Alignment.

Set MIC gain to about 50%. Switch to MIC and VU. Speak into microphone and adjust VU metering TP1 so that voice peaks reach OVU on the VU meter scale.

(N.B. VU meter gain is on the low side. It may be necessary to increase  MIC  gain  above 50%  to  achieve  reference deflection, depending on the microphone in use)

Switch to HL and TONE and set the 1KHz oscillator level trimpot so that the VU meter again deflects to 0VU.

6.2.5. Deviation alignment.

Switch to TRANSMIT (into dummy load). Switch to HL and TONE. Switch to MON and PK DEV. Adjust Limiter IN trimpot to maximum. Reduce Limiter OUT trimpot until Deviation shown on the meter is 100%. Reduce Limiter IN trimpot until indicated deviation is about 80%.. Switch to MIC and speak into microphone. Speech peaks should approximate 100%. deviation.

When finally testing with speech traffic, it may be necessary to make minor adjustments to Limiter IN and Limiter OUT, but avoid any indication of over deviation as metered or onset- of excessive distortion as monitored on AF OUT.

6.2.6. RF Level.

The RF level metering indication may be adjusted to a convenient relative indication by adjusting the trimpot mounted on the meter itself.

6.2.7. Audio monitoring balance.

Switch to HL and TONE, plus MON. Activate TRANSMIT. Measure audio level at the AUDIO jack (with CRO or use the loudspeaker and calibrated ear). Switch between AF IN and AF OUT and adjust trimpot TP1 at Auxiliary audio amp output until the level is equal on either position.

6.2.8. CTCSS Sub-tone.

The level of CTCSS sub-tone should be set to produce an FM deviation of  +/-500Hz. This ideally should be done with the aid of a suitable deviation meter or communications monitor.

Alternatively, select SUB, when the warning LED should light. Set the CTCSS level trimpot to about 25%. travel. Activate TRANSMIT and release. The subtone return signal should be heard on the repeater "tail". If not, increase subtone level and try again until just sufficient level is attained to reliably trigger the subtone return.


7. CIRCUIT DESCRIPTION

7.1. Power Supply

The transceiver is intended to operate from a regulated 13.8 Volt external supply. The FM880 power control circuitry provides +10 volt regulated for ancillary circuitry, switched +10 volt regulated for receiver and transmitter respectively, and a variable voltage  for the PA driver stage which is part of an RF power regulating system. The transceiver is also protected against  accidental power supply polarity reversal. Note that the switched +10 volt lines are NOT short circuit protected.

It was decided to provide in addition a negative 12 volt rail so that IC's requiring a dual rail could be used with the added advantage of a potentially greater voltage swing being available for direct FM for example.

The negative supply uses a 555 rectangular wave generator driving a voltage quadrupler arrangement via switches BC337/BC327 with final regulation provided by a 3 terminal 12 volt regulator. A degree of pre-regulation is provided by feedback to pin 5 of the 555, which varies the mark-space ratio of the generated waveform.

As with any switching power supply, noise spikes must be eliminated, as the switching frequency is in the audio range. The inductance L1 and associated by pass capacitors remove noise from the supply rail, while additional filtering by RFC and associated l00uF capacitor keep undesirable spikes out of the final output. With these precautions noise has not been a problem. The supply can deliver in excess of 50 mA.
 
7.2. Microphone Preamp.

The microphone preamp is a standard design, capable of lifting the output from 600 ohm or 50000 ohm microphone inserts to a compatible level with the 50-100 mV expected from the HL input.

7.3. 1KHz Oscillator.

The oscillator is based on the Wien Bridge principle, with the frequency of oscillation determined by the phase shifting components R1C1, R2C2. The level of feedback to maintain oscillation is controlled by the AGC system including the 1N414S, 7.5 Volt zener diode and the 2N3819 FET. The AGC network controls the effective resistance to ground from the inverting input to the 741 and hence maintains its gain as just sufficient to maintain oscillation. Distortion is thus held to a 1ow 1evel. Oscillation is inhibited by applying a negative bias to the 2N3819 gate. Keying is provided by a reed relay in the output line. Direct keying of the oscillator was not provided as the time constants necessary in the circuit produced an unsatisfactory keying characteristic. Inhibition of the oscillator has been used to eliminate the possibility of low level audio cross talk into the microphone or HL channels when the oscillator is not required.


7.4. CTCSS Generator.

The requirements of this generator are stability and repeatability, so the generator has been based on a crystal controlled source, using a cheaply available 4.000 MHz crystal and a clock IC type MM5369AA.

The MM5369AA produces a high amplitude square wave which is "brute force" filtered by a two stage ladder filter to produce a somewhat distorted but useable sine wave. The level of distortion does not appear to be sufficient to provide intrusive harmonic content in the audible range, but no doubt an active filter would provide a cleaner wave form. Ample level is available to provide direct injection to the frequency modulated crystal oscillator.

7.5. Audio Filter.

An audio band pass filter has been provided to limit the bandwidth of all audio presented to the limiting amplifier and of course for transmission. The circuit is actually a low pass followed by a high pass section. Gain of the circuit is 1 with little distortion contribution at the level of operation. The 3 dB bandwidth is about 200-3800 Hz. The 200 Hz cutoff also helps to keep undesirable audio out of the sub-tone band.

7.6. Limiting Amplifier.

The limiting amplifier has been included rather than clipping to keep distortion contribution down. Clipping can produce greater "talk power" but also can produce distortion products which are magnified in the relay system on which the wide coverage of the W1A broadcast depends.

The limiter uses a specialised control 1C type MC3340 for which the manufacturer specifies low distortion in the limiting application. In the standard circuit used, a control voltage is developed in a 741 1C with half wave detection and filtering applied to a 2N3819 FET, which in turn controls the gain of the MC3340. Some extravagant claims are made for this design, which while it does the job could be improved by the inclusion of a full-wave detector, and a reduction in attack time. To avoid delay it has been used “as is”.

A number of adjustments are provided by trimpots. TP3 sets the gain of the feedback amplifier while TP2 sets the limiting threshold. TP1 and TP2 provide conventional input and output level controls for overall system alignment.

It was found necessary to operate the limiter from closely regulated power supply rails to avoid an unsatisfactory lock-up and slow release condition when switching to transmit. The effect was due to about 0.5 volt shift in the 13.8 Volt rail originally used to supply the positive feed. The present "unbalanced" power supply rails are an undesirable consequence, but don't appear to have introduced any uncontrollable penalty.

7.7 Modifications to VK6WIA FM880 - 25th March, 1994

7.7.1 Facilities have been added to generate DTMF control tones. Control is via an External Auxiliary Unit which plugs into an 8 pin DIN connector on the rear panel of the FM880.

For example, to allow VK6RAP operation on an alternate frequency to provide some degree of protection from accidental interference when running news broadcasts through Perth repeater VK6RAP, proceed as follows:-

To switch to the nonstandard input frequency:

(a) Commence transmission with the bandswitch set to "CH. 2."

(b) Select "HL" on the "MIC/HL" audio switch. Select "DTMF" on the adjacent three position toggle switch.

(c) Send the appropriate DTMF code on the external keypad to instruct VK6RAP to change its input frequency.

(d) Select "AUX" on the rotary channel select switch to change the FM880 to transmit using the alternate frequency

(e) Centre the three position toggle switch to "HL"

(f) Select "SUB" to transmit the CTCSS link control tone.

(g) Cease transmission and note if the subtone return signal is heard, signifying a successful changeover.

All other operations of the FM880 should operate as normal.

DO NOT FORGET TO RESTORE THE INPUT FREQUENCY TO 146.100 MHZ AT THE END OF THE SESSION by reversing the procedure!

7.7.2 An additional facility has been included in the External Unit, enabling the automatic morse keying of the FM880 tone oscillator. Eight different groups are stored in a ROM, each group being selected by the 8 position rotary switch.  The keyed tone can be used in the preparation of tapes, (taking audio via the "AUDIO" jack, "PHONES" jack or external speaker jack on the rear panel) or during "live" transmission.

See also under Section 5 “External Auxiliary Unit”.

Home