Lowe_HF-250_user.pdf

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1、 HF-250 General Coverage Receiver. Users Manual. Contents Introduction. 2 Getting started. 3 Controls and Connections 12 Operatingthe HF-250. 17 Care of your Receiver. 30 General notes. 32 Optional Units. 33 Circuit description. 34 Receiver specification. 36 Circuit Diagrams. 44 (C) 1995 Lowe Produc

2、tion Ltd. Bentley Bridge, HF-250 User Manual HF250 User Manual Introduction. The term Communications Receiver was originally used in its quite literal sense to describe a radio receiver which was part of a point to point Communications link. These radio links were normally manned by trained operator

3、s, and Morse Code was the usual method of transmitting information. Because of the specialist nature of the system and the fact that the operators were technically trained the Communications receiver itself was quite often a complex piece of equipment. Over the past twenty or so years, a marked chan

4、ge has taken place in HF spectrum occupancy and there has been a considerable increase in the use of short wave broadcasting, air traffic control, news agency transmissions and so on. The interest in listening generated by this spread of activity has lead to a demand amongst the general public for r

5、eceivers which will enable them to keep in touch with world affairs by short wave radio. Clearly, these users of receivers are relatively unskilled in the handling of complicated equipment and this in turn has resulted in the introduction of simpler receivers. However, simplicity of operation has of

6、ten been accomplished by a compromise in actual performance, and the results obtained from some of these simple receivers have been quite disappointing. The design and development of the HF-250 was based on straightforward objectives :- To obtain sufficint RF performance for the receiver to operate

7、without problem in crowded bands with many strong signals To combine complete control of a necessarily complex piece of equipment with easy operation for the user. To achieve both the previous objectives within a reasonable price range. We believe that these stated goals have been reached, and that

8、the HF-250 receiver represents a truly new approach to meeting the demands of the educated short wave listener. Gettinq Started The Power Supplv and other Connections. The HF-250 requires an external DC supply of between 10 and 15 volts. The absolute maximum supply is 16 volts, and if this is exceed

9、ed damage may occur to the receiver. The supply polarity is negative ground only, and although reverse polarity protection is built in it is wise to ensure that any supply is correctly connected. Be sure that the receiver power switch is OFF before plugging in or unplugging the power connectot. In m

10、ost countries, the HF-250 will be supplied with a small 12 volt regulated power unit which is designed to operate from the local mains power. Remember that this supply will be operating all the time that it is connected HF-250 User Manual HF250 User Manual to the mains outlet, and it is a wise safet

11、y precaution to disconnect it when the receiver or clock are not in use. In the United Kingdom the power supply is fitted with a three-core mains lead, and the earth connection (yellow/green) is connected to the negative (Earth Connection ground) terminal of the receiver. This provides a reasonable

12、earth connection for the receiver, but in some cases, where mains-borne interference is prominent, it will be necessary to provide the receiver with a good RF earth, either in addition to the mains earth or in some cases instead of it. If the receiver is used with any other mains power supply it sho

13、uld be able to comply with BSI standards relating to Class 2 insulation. External Loudspeaker. A small internal loudspeaker is provided in the HF-250 so that it is self contained, but although it can provide reasonable all round audio quality, clearly in the limited space available compromise has to

14、 be made. You will find that if the volume control is set to a high level there may be some audio feedback caused by vibration induced by the internal loudspeaker. If it is necessary to operate the receiver for long periods at high audio levels the use of an external loudspeaker is recommended. Beca

15、use the HF-250 is capable of giving a high quality audio signal, we suggest you use a good external loudspeaker, a small bookshelf type Hi-Fi unit is satisfactory. We can provide a suitable unit as an optional accessory with the correct connecting lead for the HF-250. Any external loudspeaker should

16、 have an impedance of 4 to 8Q. Record Output. Many keen listeners like to tape record any interesting stations they hear, and a low level audio output has been provided, also a pair of relay contacts for switching on the tape recorder in connection with the built in timer. The record out socket acce

17、pts a 3.5mm mono jack plug and provides a level suitable for feeding into the line input of most tape recorders or amplifier systems. An attenuating resistor should be added in the lead if feeding directly into the microphone input of a cassette recorder. The output level at this socket is not affec

18、ted by the Volume or Tone controls, so that the loudspeaker can be used to monitor whilst recording. The record output can also be used for driving most types of receiver ancillary equipment such as RTTY, Facsimile or Morse Code decoders. The output level is about 350 mV RMS from a source impedance

19、of 5k. Types of Signal. The HF-250 is equipped to receive most types of transmission likely to be encountered within its tuning range, and although most users will be familiar with these, here is a short section on this topic that may be useful to the beginner. AM (Amplitude Modulation). This was th

20、e earliest method used of audio modulation of an RF carrier wave, and is still almost universally used for long, medium and short wave broadcasting. An AM signal is fairly HF-250 User Manual HF250 User Manual 7 easy to tune in, and given a reasonable signal strength, the receiver may not need to be

21、spot on in frequency. However when conditions are poor, AM can be difficult to resolve -one particular problem is frequency selective fading and this is discussed later. AM Selectivity. A radio signal occupies a certain portion of the radio spectrum which is known as its bandwidth. The bandwidth of

22、an AM signal is twice its highest modulation frequency, and because of this broadcasters are restricted to transmitting audio frequencies below 5 kHz so that they do not occupy too much spectrum. In the long and medium wave broadcast bands, station frequencies are separated by 9 kHz (10 kHz in the U

23、SA) so there is little or no overlap of adjacent signal bandwidths. In the short wave bands however, the stations use a nominal 5 kHz spacing, and some broadcasters do not abide by any rules at all, so there is considerable signal overlap. The HF-250 is provided with four different filter bandwidths

24、 because of this very problem. If you are receiving a strong signal in a clear part of the radio spectrum then you can use the 10 kHz filter and obtain the best fidelity. The stronger and closer adjacent stations are, the narrower the filter you will need, and the more muffled the sound will be beca

25、use high frequencies are removed. The 7 kHz filter provides a good compromise for most medium wave conditions, and the 4 kHz filter for short wave. The 2.2 kHz filter can be used under severe conditions, but it is really only suitable for speech reproduction. When AM mode is selected on the HF-250,

26、the 7 kHz filter is initially switched in. If you want to change to a different filter you can use the filter select function. You may find that reception of a station is improved by tuning the receiver slightly above or below its carrier frequency. This is quite a useful technique if there is a str

27、ong adjacent signal that you dont want. As long as the carrier signal is within the receivers filter then all will be well, but if you tune too far or select a narrower filter then the signal will become distorted. SSB (Single Sideband). An AM signal can be considered as a carrier wave combined with

28、 two identical sidebands which contain the modulating audio signal. It is possible to remove one of the sidebands without losing any vital information, and immediately halve the bandwidth occupied by the signal. In practice the carrier wave is also removed (or partially suppressed) to improve transm

29、ission efficiency, and the result is a single sideband transmission. SSB transmissions are used extensively for voice communication, particularly to aircraft and shipping, and also by radio amateurs. It is possible to use either of the two initial sidebands of a signal, so there are two distinct typ

30、es of SSB transmission; Upper Sideband (USB) where the sideband frequency is above the carrier frequency, and Lower Sideband (LSB) 8 HF-250 User Manual HF250 User Manual where it is below. Nearly all commercial transmissions are USB, as are amateur transmissions at frequencies above 10 MHz. At frequ

31、encies below 10 MHz radio amateurs use LSB by convention. To receive an SSB transmission, the receiver must re-insert the missing carrier signal. If this is not done the signal will sound just like Donald Duck, - try listening to an SSB signal in AM mode for this effect. For correct reception the re

32、ceiver should be tuned exactly to the carrier frequency. The HF-250 has a very slow tune rate on its SSB mode to facilitate accurate tuning, but you will need a steady hand. The pitch of the received voice will change as you tune through the signal, but only at one tuning position will it sound like

33、 a natural voice. A 2.2 kHz bandwidth filter will just accommodate the audio frequencies used for voice transmission, and this is the filter most commonly used for SSB reception. The HF-250 will automatically select this filter for LSB or USB modes, but under good signal conditions selecting the 4 k

34、Hz filter may offer improved clarity. CW (Continuous Wave, ie Morse). Morse code is usually transmitted by interrupting a single carrier wave, and it occupies a very narrow bandwidth. In terms of ability to get a message through under difficult propagation conditions morse is an efficint method, alt

35、hough modern error-correcting digital data systems are also very good. CW signals are received in the same way as SSB signals, with the carrier inserted in the receiver producing a beat note with the incoming signal. In the CW mode the HF-250 provides an 800 Hz offset between the display and the int

36、emal carrier, so that a note is heard at 800 Hz when the receiver is tuned exactly to the signal. The HF-250 initially selects the 2.2 kHz filter in CW mode and this should be used for finding and tuning signals. As an alternative, a narrow 200 Hz filter is provided, and its use will greatly reduce

37、the background noise, allowing the Morse to be more easily read. Careful tuning is needed to place the signal at the peak of this filter, which is centred on 800 Hz. RTTY (Radio Teletype). The method of sending teleprinter messages by HF radio link is to use two closely spaced signals, transmitting

38、one or the other to send binary data. Each teleprinter character is encoded into a different sequence of tones which are transmitted in a bewildering combination of different speeds, tone shifts, and codes. RTTY signals are tuned in SSB mode on a receiver, but require a special terminal unit to deco

39、de and display the actual text. FAX (Facsimile). Pictorial information (often meteorological data) is transmitted over HF radio links for reception by shipping. As with RTTY, a special facsimile decoder and display or printer is required for its reception. FM (Frequency Modulation). When the DU-250

40、detector option is fitted to the HF-250, the receiver will receive FM signals. In the context of an HF receiver this means narrow band FM, which occupies a 10 HF-250 User Manual HF250 User Manual 11 bandwidth of around 12 kHz. This is not to be confused with broadcast FM transmissions which have ban

41、dwidths in excess of 150 kHz, and are normally transmitted at VHF or UHF where there is sufficint spectrum to accommodate them. FM signals in the HF spectrum are usually found either in the 27 MHz Citizens Band or in the 28 MHz amateur band. It is typical of FM receivers that they produce a large am

42、ount of noise when there is no signal at the aerial. To overcome this a squeich system is employed to automatically turn off the audio output unless a signal is detected. The HF-250 squeich system may be turned on or off manually by using the filter select control. The filter bandwidth is fixed at 1

43、2 kHz in FM mode. AM Propagation and Fading. During AM signal reception it is possible to experience severe fading problems, particularly after nightfall. This is mainly due to the signal reaching the receiver by several different paths from the transmitter, and it is most common after dark because

44、this is when the ionosphere reflects most HF radio signals. Fading occurs when the signals arrive at the receiver in antiphase (having travelled different distances) and then cancel each other out. This will only occur at a few specific frequencies at any one instant, hence the term frequency select

45、ive fading. If a selective fade reduces the carrier level of an AM signal, but leaves the sideband levels unaltered, a receiver with a conventional AM detector will not be able to correctly reproduce the signal, and the output will be distorted. There are two techniques that can be used to improve t

46、he situation; ECSS, and Synchronous (or Phase-Locked) AM detection. ECSS (Exalted Carrier, Selectable Sideband). The ECSS technique makes use of the fact that with a good, selective receiver, capable of resolving SSB, an AM signal can be passed through the SSB filter which is only wide enough to all

47、ow one sideband through. The filter must attenuate the carrier signal by at least 20dB for this technique to work with any success. The receiver can be used in the SSB mode with the incoming AM carrier tuned to zero beat, and the accompanying sideband treated as a true SSB signal. Either the upper o

48、r lower sideband can be selected using either USB or LSB mode, so interfering stations can often be eliminated. The improvement in intelligibility is often dramatic, and it is well worth trying out ECSS and developing the ability to use it. The HF-250 when used with its 2.2 kHz filter is ideally sui

49、ted to ECSS reception. AMS (Synchronous AM). The difficulty in receiving music signals with the ECSS method is that it is very difficult to match the receivers injected carrier exactly with the frequency of the incoming carrier. Any difference results in a frequency shift of the audio signal, and the consequent loss of harmonie relationships. 12 HF-250 User Manual HF250 User Manual 13 The synchronous AM detector in the DU-250 option uses a narrow deviation phase-locked oscillator to replace the incoming