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VK6WIA Broadcast Transceiver Don Graham VK6HK 廣播 無線電 電路 儀表
The unit is based on a 為基礎PHILIPS FM880 link transceiver 發射機
which was originally supplied to a Telecom Australia specification 澳洲規範 for telephone 遠距電話 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.
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
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
7.2. Microphone Preamp.麥克風前置放大器
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.高功率強力麥克風
7.3. 1KHz Oscillator.信號產生
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.視聽電子訊號
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.濾波
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.限制 放大
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.
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.高功率強力麥克風
7.3. 1KHz Oscillator.信號產生
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.視聽電子訊號
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.濾波
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.限制 放大
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.
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.
電話撥號
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:-
dtmf fsk protected Dual Tone Multi Frequency frequency shift keying
http://www.epanorama.net/circuits/teleinterface.html#general
dtmf fsk protected Dual Tone Multi Frequency frequency shift keying
http://www.epanorama.net/circuits/teleinterface.html#general
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