Fullscreen HTML5Med Res (??MB)HTML4

AMATEUR RADIO BY GARRY CRATT, VK2YBX A general coverage shortwave receiver from England Ever wanted a first class communications receiver but without all the complexity that seems inevitable with sets from Japan? If so, consider the Lowe Electronics HF-225 general coverage receiver. It is made in Derbyshire, England. With well over one million active amateur operators in such a small country, it is not surprising that most amateur equipment manufactured today originates from Japan. Their huge domestic market represents sufficient commercial incentive for manufacturers to design new models on a regular basis. However, it should not be taken for granted that Japan is the only source of amateur radio equipment. A Brit- ish company, Lowe Electronics, has achieved notable success in recent years with its range of shortwave receivers. Their initial model, the HF125, has now been superseded by the HF-225 and the professional version , the HF-235. The design and development was, according to Lowe, based on straightforward objectives: (1) to obtain sufficient RF performance for the receiver to operate without problems in crowded bands with many strong signals; (2) to combine complete control of a necessarily complex piece of equipment with easy operation for the user; and (3) to achieve these objectives within a reasonable price range. The result is a compact desktop receiver offering multimode coverage from 30kHz to 30MHz, microprocessor controlled PLL tuning, dual conversion, three selectable IF filter bandwidths, a backlit 5-digit liquid crystal display, and an analog signal strength meter. The receiver also has 30 memories for storage of oft-used frequencies, an inbuilt lithium battery for memory backup, and can be operated from a 12V car battery or the supplied AC adaptor. Circuit features The interior of the Lowe HF 225 shortwave receiver is very clean & easy to access. The microprocessor which controls it operates in "static idle" mode for most of the time so that it does not produce any unwanted signals. The, HF-225 is a dual conversion superhet receiver, using up-conversion to a high first intermediate frequency (IF) of 45MHz, and a second IF of 455kHz for the selective filters. This design gives good image rejection at all tuned frequencies in the HF band (any image frequencies will fall outside the coverage of the receiver) and good filter shape factors for the 455kHz IF. MARCH 1993 81 The Lowe HF-225 shortwave receiver covers the range from 30kHz to 30MHz & eschews the complex styling & controls of most Japanese receivers. Despite its straightforward front-panel layout, it features microprocessor controlled PLL tuning & has 30 memories for storage of oft-used frequencies. control which can be set for treble cut or bass cut. Receiver tuning is achieved by varying both the local oscillator and the heterodyne oscillator (used for IF conversion). The local oscillator ranges from .45.030MHz to 74.999MHz in lkHz steps, while fine tuning is provided by the heterodyne oscillator covering 44.544MHz to 44 .545MHz, in 128 steps. The final carrier insertion frequency is determined by the mode selected, so that the appropriate IF passband filter is in the correct position relative to the carrier for sideband reception, either USB or LSB. The local oscillator signal is produced by a PLL synthesiser but all frequencies affecting the on-channel tuning of the receiver are crystal derived to ensure good accuracy and low drift. All switching and tuning functions of the receiver are controlled by a dedicated microprocessor which receives commands from the front panel controls and sends information to the receiver control register and the PLL system on a serial data bus. The single-chip microprocessor is supported by a controller driving the LCD and a frequency mem.ory with battery backup. No spurious signals All these components are mounted away from the main RF PC board on a separate board behind the front panel. The rear panel carries sockets for an external keypad, 12V DC power, record The control system uses the "static out & an external loudspeaker. There's also an FM squelch level control, inputs idle" principle, where there are no for both 50Q & 600Q antennas & a switch to select the antenna type. signals other than a basic clock oscilThe most gain in the receiver oc- lator running until the operator reSignals from the antenna pass curs at 455kHz, where several stages quires a change in the receiver conthrough one of six band-selecting filusing multi-element ceramic filters figuration. The system then reacts to ters before being fed to the first mixer. These filters attenuate strong signals are configured to give bandwidth po- commands from the controls, before that are well removed from the de- sitions of 2.ZkHz, 4kHz, 7kHz and · returning to the "idle" condition. lOkHz. This method of operation virtually sired frequency and help to reduce eliminates spurious signals from the interference from even order control system from being picked up intermodulation products. Particular Signal detection Following the second IF, a full-wave by the receiver. Fig.1 shows a block attention has been paid to separating diagram of the receiver. strong medium-wave signals from the envelope detector serves as a low disThe performance of the receiver is rest of the HF spectrum and to attenu- tortion AM detector and as a source of indeed impressive. At frequencies ating signals above 30MHz which AGC (automatic gain control) voltage. When noise spikes are detected, these above 2MHz, its sensitivity to SSB might be received as IF images. signals was measured at 0.3µV for Surprisingly, there is no RF stage pulses fire a monosfable which mutes l0dB signal to noise ratio, and 0.6µV before the mixer and this, coupled the audio stage for the duration of the pulse. The second IF also feeds a prod- for AM signals modulated to a depth with the use of a high-performance uct detector, used for SSB and CW of 70%. The dynamic range is better transistor mixer, gives high dynamic range and good overload immunity. A reception, when the IF is mixed with than 90dB at 50kHz spacing and the 4-pole 45MHz crystal filter with a a carrier signal. Audio filtering is ob- reciprocal mixing is also better than 15kHz bandwidth limits the signals tained through the use of a high Q 90dB at l0kHz in the SSB mode. All peaked response filter centred around image and spurious responses have fed to the second mixer and removes 800Hz. The receiver also has a tone greater than 80dB rejection. image responses from the second IF. 82 SILICON CHIP Fig.1: this block diagram of the HF-225 receiver shows its complex IF filter set-up. It is a dual conversion superhet receiver, with a first IF of 45MHz & a second IF of 455kHz for the selective filters. OPTJ)NA,L lw"".Slf/ri\ DETECTOR ,r-,f1: '"'" .,.,., VOL TONE EXTLS -q:J NT LS Rfte:IVER COHTROL DISPLAV DRIVER SER l~L 88.888 eus FRfQUEMCY >'EMORV LCD FREQUENC Y DISPLAY Operating the receiver is quite simple. The unit is switched on using the combined ON/OFF-Volume control and displays the last frequency used on the 5-digit display. Two buttons, UP and DOWN, select the required frequency in megahertz and the exact frequency is then tuned using a conventional rotary control. Tuning on SSB and CW is in 8Hz steps. This allows accurate resolution of SSB and ECSS reception in the AM mode. Those unfamiliar with the term ECSS (exalted carrier, selectable sideband) may be interested in the basic principle. 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 an SSB filter which is wide enough to allow only one sideband. This filter must attenuate the carrier by a minimum of 20dB for the technique to work with any degree of success. The receiver can be used in the SSB mode, with the incoming carrier tuned to zero beat, and the accompanying sideband treated as a true SSB signal. Either the upper or lower sideband can be selected using either LSB or USB modes, so interfering stations can often be eliminated. This can improve MOO< SWITCH T'UN.,.G intelligibility of the signal dramatically and the ability to select an IF filter having a 2.2kHz bandwidth is a most desirable feature. Mode selection S-l"ETER SHAF'T ENC00£1t OPTIONAL l(EYPAO gation and fading, particularly after dark when the ionosphere reflects most HF signals. The FM detector is designed to decode signals having a bandwidth of 12kHz or less and the receiver has a squelch control which can be turned on manually via a rear panel control, or via the filter select control located on the front panel. Mode selection is achieved by a rotary switch and initial filter selection is automatic and dependent on the mode selected. AM switches in the ?kHz filter, while selecting SSB (USB or LSB) or CW selects the 2.2kHz Conclusion With a recommended retail price of filter. Checking the filter in use is easy; a $1425, the HF-225 may not fit every momentary press of the "Filter Se- · budget but for those shoitwave enlect" button causes the LCD to dis- thusiasts seeking close to the ultimate play the current filter bandwidth. A shortwave receiver, this model is cersubsequent press of the same button tainly worth serious consideration. A causes the next filter to be selected number of accessories for the HF-225 and displayed. After three seconds, are available, including the K-225 exthe display returns to indicate the ternal keypad, B-225 nicad battery receiver operating frequency. pack, W-225 whip antenna and the SThe synchronous AM detector uses 225 external loudspeaker. The unit is a narrow deviation phase locked os- supplied fitted with the D-225 "decillator to replace the incoming AM tector unit" , a PC board containing a carrier. When phase locked, this osnarrow band FM detector and a syncillator has exactly the same frequency chronous AM detector. as the incoming signal but incoming Our sample receiver came from the level changes have no effect on the Australian distributor, Emona Elecdetector, provided that there is some tronics Pty Ltd, 92-94 Wentworth Ave, signal on which to lock. This helps Sydney, NSW 2000. Phone (02) 211 reduce the effect of multipath propa- 0988. SC MARCH 1993 83