QRZ! Ham Radio 5

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Text File | 1994-11-27 | 20.7 KB | 476 lines

r 36538 From : N8DEU @ N8DEU.#HSV.AL.USA.NA To : N4HHE Date : 940720/2240 Msgid : PY 6122@N8DEU, 36538@K4BFT $6122_N8DEU Subject : IC-820 - The Story Path : N8DEU From: N8DEU@N8DEU.#HSV.AL.USA.NA To : N4HHE@K4BFT.#HSV.AL.USA.NA <start> These notes are herewith placed in the public domain. They represent the observations of the author, and are as accurate as I can possibly make them. They are not to be taken as either endorsement or damnation of the product in any way. I disclaim all responsibility for any loss or damage, direct or indirect arising from use or abuse of this material. Caveat Emptor. -------------------------------------------------------------------------- ICOM's IC-820H - A Satellite User's Perspective ----------------------------------------------- by James Miller G3RUH 1994 July 18 Summary ------- The ICOM IC-820H will, "straight out of the box": - Work terrestrial UHF or VHF all-modes simplex/duplex very sweetly. - Work SSB/CW transponding satellites OK, albeit with a somewhat clumsy human interface. - Work satellite 1200 bps Pacsats OK, if you can accept doppler tracking with 100 Hz steps. - Work terrestrial 9600 bps packet * perfectly * The ICOM IC-820H will NOT: - Work 9600 bps satellites "outa the box". A mod is needed ... The ICOM IC-820H has a number of positive features; beautiful construction, small size, excellent documentation, good frequency management - see text. The ICOM IC-820H has a number of negative features to do with pre-amplifier and linear amplifier control, power output control, internal switches and coarse doppler tracking - see text for details. Introduction ------------ A while ago (June 1994) Icom-UK kindly loaned me an Icom-820H transceiver to evaluate. Like you, loads of questions came to mind. I wanted to know "what's the Icom-820H like? Is it really 9600 baud packet radio "data ready" outa the box? What about 1200 bps Pacsat use? Would PSK be decodeable? Would the VFOs track sensibly? Doppler tuning facilities? Satellite ready?". And many more. This is not a full blooded QST review plastered with microvolts and decibels. That'll come from the ARRL Lab. But I have measured what I needed to know. I also assume the reader knows how to use a satellite, has a reasonable idea of what to expect from this sort of radio, and doesn't (say) need "reverse tracking" explained. I'm recording the interesting differences between established practice as popularised by the Yaesu FT736R, and typical user expectation - mine. First Impressions ----------------- The manual is a model of clarity. Outstanding. Yaesu please copy. The schematics are not quite up to Kenwood standard; need more annotation, frequencies, signal names and highlighted principal paths. This would help understanding a lot. Anyway, I located the discriminator and varactor stuff OK, various internal switches etc, and things looked promising. Then I connected up two dummy loads, switched on the radio, and played with all the knobs and buttons until I'd hacked the lot. The Product ----------- The IC-820H is a dual band 144/430 MHz full-duplex all-mode transceiver. It has 100 tunable memories, 10 tunable satellite memory pairs and six assorted others. Output 30-45 watts. "Mine" also had an extended receive range: 136 to 174 MHz which was very useful. This feature is not documented in the manual. The radio is small. It's 2/3rds the height, width and depth of a Yaesu FT-736R. But then it has no internal mains power supply, and neither will it accommodate two extra band modules like the Yaesu. It requires a 13.8 volt supply, about 16 amps, and is suitable for mobile operation. A microphone is not supplied and neither is a carrying handle. In England the combined cost of the radio and a PS-55 supply (GBP 245) is about GBP 1940; an FT736R from the same dealer is GBP 1700. GBP = Great British pounds sterling. The Hidden Features ------------------- Being small there are only 32 buttons and knobs on the front panel compared with 67 on the Yaesu, so the lesser used functions, about 20 of them, such as dial dim, pre-amp feed control, RIT rate, CAT baud rate and so on are pre-stored via a configuration process. To invoke this requires the radio to be switched off and on again up to four times, which seemed unnecessarily clumsy to me. Some other functions are relegated to slide switches inside the radio, and adjustment requires you to remove the covers. This takes a few minutes. See later. Normal Tuning Management ------------------------ The radio is organised around a Main Band and a Sub Band, each assigned to UHF or VHF or vice versa. You transmit on the Main band only. You receive on both Main and Sub simultaneously. There are duplicated volume and squelch controls; Main appears in the left headphone, Sub in the right; an internal switch allows these sounds to be mixed or separated. There is an internal speaker and there are two external loudspeaker sockets; smart wiring selects the expected sum/separated combinations. Storing, retrieving and swapping frequencies and modes is a doddle. Everything you could possibly expect is provided. Tuning rates are very sensible, and can be quickly adjusted from 1 MHz right down to 1 Hz resolution on SSB/CW, 100 Hz on FM. There are two VFOs each for VHF and UHF plus 100 tunable memories initially apportioned 50/50 between VHF and UHF, but you can alter this up to 80/20 either way. The idiom is a little different from the FT736R's, and took practice to acquire, but within a couple of hours became second nature. I liked having both frequencies in view. The amber LCD display has very crisp characters and delights the eye. Because you transmit on Main and receive on Sub, and can control each quite independently, you can (and do) operate satellites when in "normal" mode. Unfortunately RIT, passband Shift and optional CW-Narrow do not work on the Sub band, i.e. on the full-duplex receive frequency. Neither do manual AGC fast/slow select nor the mechanical S-meter. Instead on Sub band, CW or SSB mode automatically chooses AGC fast or slow for you, and there's an LCD bargraph S-meter. Satellite Tuning Management --------------------------- When you enter "Satellite" mode either the satellite VFO pair is used, or one of the 10 satellite memory pairs, or you can transfer the "Normal" frequency pair across (and back again when you leave). When in satellite mode, Main and Sub band frequencies track together, either normal or reverse - as for example with Oscar-13 mode-B. Alas, there is no proper "untrack" facility; to alter one frequency independently of the other you have to hold in one of two alternative front panel buttons whilst turning the main tuning knob. With practice I managed sort-of using my thumb and forefinger for the main knob, and third or fourth finger for the buttons; left handers would find it nearly impossible. It's really a two handed job, and is excruciatingly fiddly. I discovered an undocumented kludge that partially obviated this; using the microphone Up/Down buttons, only one frequency changed. However the smallest step size from the mic buttons is 100 Hz. (See later). In fact, to tune TX and RX independently you need to adopt a change of attitude to the radio. Just forget all about a so-called "satellite" mode! Do your satellite operating in "normal", and only engage "satellite" when you want to do a quick bit of ganged tracking, or retrieve a frequency pair from the 10 satellite memories. I don't think this is how the designers envisaged things. Intriguingly there is a blank button position actually marked Satellite, sandwiched between Normal and Reverse. I wonder why it's not fitted and called "Untrack"? Perhaps once it was. As before, since satellite mode receive is on Sub band, passband Shift, CW-Narrow, manual AGC fast/slow select and mechanical S-meter are inoperable, but a RIT has been provided. (Does anyone ever use RIT?) It would have been far, far better if there were a genuine "satellite" mode with Main receive and Sub for transmit. Main has the larger digits which are square in the middle of the radio and it's clearly the object of your visual, mental and operating focus. And of course Main has all the RIT/SHIFT/CW-N/AGC/METER controls working for it. I really wish I didn't have to say this, but "satellite" mode looks at best to be an afterthought grafted on because the control microprocessor makes it easy. "It's only software". The Yaesu FT726R first appeared ten years ago, the FT736R six, so the operational needs of satellite operators are well established. Why have Icom made such heavy weather of it? Preamplifiers & Linear Amps --------------------------- You can send +10v up either, both or neither of the VHF/UHF antenna sockets. This supply is removed from the relevant socket on transmit. The manual does not specify a maximum loading; I tried 100 ma and 200 ma and the voltage remained steady; at 330 ma it began to droop. The limit is set by dissipation in a PQ20VZ51 regulator on the display unit. Other than this 10v supply, there is no provision for hard switching of pre-amplifiers or linear amplifiers unless you confine yourself to one band, when you can of course use the PTT line via the accessory socket. Serious operators will regret this omission. In contrast, the Yaesu FT736R has four control lines, one for each band. Internal TX/RX changeover is by PIN diodes, so it's fast and silent. RF Power Control ---------------- The front panel sports a high/low power control button. Low power is 4-5 watts. If you have an external linear amplifier or a transverter you need to be able to vary the output power continuously. There is no knob provided for this. There is instead an ALC facility on the accessory socket. The control voltage is -4 to 0 volts into "more than 10K", but you must provide a supply and a pot to do this. And a box, and a place to put it. Tacky. There's some free space on the rear panel though ... There are several ways to key this transmitter; there's the Tone button, mixed in with other frequently used buttons and easily hit. Then there's the bigger Transmit button, and the normal PTT (mic or TNC). More than once I accidentally hit the Tone button, sending a minimum of 4 watts skywards. That would have wiped out my S-band converter had it been connected. RF Attenuators -------------- You can attenuate the RF input of either or both receivers, by 15db, from a front panel button. This is in lieu of a (big) RF Gain knob, and an is excellent feature. Many preamplifiers have far too much gain; an S-band pre-amp plus converter most certainly does. Being able to cut the signal down to size prevents cross modulation and overloading. Digital Satellite/Terrestrial Operation --------------------------------------- Before describing this, it's necessary to tell you how the data input and output audio is routed, because it is not unconditional. You might like to draw yourself a little sketch. Both signals are presented to the 8-pin DIN Accessory socket on the rear panel. The incoming transmit audio (TXAudio) passes though a slider switch marked PACT/AMOD : - In the AMOD position the TXAudio passes to the Main subsystem where it meets up with the regular pre-amplified microphone sound for use in FM or SSB modes, and then through some audio processing. - In the PACT position the audio goes directly to the varactor diode of Main's FM section. The receive audio (RXAudio) also passes through the PACT/AMOD slider switch: - In the AMOD position, RXAudio is picked up from either the Main or Sub receiver, according to the setting of another internal slider switch marked MAAF/SAAF, and is squelched. - In the PACT position the audio is collected directly from the discriminator of the Main FM circuit, via a 4k7 resistor and 100nf coupling capacitor. It's unsquelched of course. These switches are not accessible without removing the bottom cover. An access hole could surely have been placed next to the accessory socket, which would allow these switches to be tickled with a small screwdriver. Users will probably drill a couple of holes in the bottom, or cut away some of the underside ventilation grille. But why should you need to touch them at all? Read on ... 1200 bps PSK Satellites ----------------------- FO-20, PacSat, Lusat and Weber require an FM uplink, to which is applied audio PSK. The downlink is conventional carrier PSK, and the system is full duplex. Therefore the internal switches must be set to AMOD and SAAF (see above). The uplink "eye" as received at the satellite is OK; it's pinched about 4 db. Remember the TXAudio has been through the regular FM modulator circuits. It's a good idea not to yell into the microphone at this time, unless the Mic gain pot is at minimum, since both signals are added. If you flip the MOD switch to PACT the uplink modulation is text-book perfect, but then you lose your Sub band PSK receive audio! You get Main FM. Grrr! Downlink 1200 bps PSK reception from the Sub band receiver is excellent, as too is Oscar-13's 400 bps telemetry signal. The only snag with these PacSats is doppler tracking the PSK signal. (See later). 9600 bps DFM Satellites ----------------------- Uosat-22, KitSat-23 etc require the TXAudio to be applied direct to the transmit FM varactor. RXAudio must be picked off directly from the FM discriminator, and the system is full duplex. Thus the internal MOD slider switch needs to be set to PACT. But that immediately picks up the wrong RX audio - from FM Main's discriminator. For satellites we need FM Sub's discriminator output. Consequently you CANNOT operate the 9600 bps satellites with an Icom IC-820H "straight out of the box". There is a solution, but we're back to modifications I'm afraid. What you do is locate the Sub receiver discriminator IC20, pin 9 and fly that signal out on your own lead. This requires you to remove the big PCB called Main Unit, turn it over and do some fine re-work among the Sea of SMD. Alternatively you can pick up a downstream version of the signal without removing the PCB at the optional tone-squelch unit-B socket J20, third pin from the "J". Incidentally this signal is DC coupled to the discriminator chip, so you can implement closed-loop AFC externally using one of the many published circuits. The source impedance is 47K (R329). I checked Uosat-22 and KitSat-23 on Sub-band using this modification. The UO-22 "eye" is poor when it leaves the satellite, with a lot of LF flutter which has always made decoding difficult. But the 9600 bps performance of the Sub band receiver is so good it adds little extra aberration and data decoding was quite satisfactory. The KitSat-23 "eye" was wide open, and data detection perfect. Since there is no AFC indication for Sub band, tracking the changing doppler shift unaided requires either very good judgement, an external system as above, or computer control. Oh, and once again set the mic gain to zero, or microphone sounds will be added to your transmission. 9600 bps DFM Terrestrial ------------------------ Terrestrial 9600 bps packet works perfectly, "right outa the box". Over the last six years I've tested innumerable radios for 9600 bps operation. The Icom IC-820H now shares top place with Kantronics' D4-10. (The latter is however 2-channel, crystal controlled and UHF only). 9600 bps packet requires the TXAudio to be applied direct to the transmit FM varactor. RXAudio must be picked off directly from the FM discriminator, and the system is simplex. Thus the internal MOD slider switch needs to be set to PACT, which also selects RXAudio from FM Main's discriminator. The transmitter circuit's frequency response is from about 15 Hz to well beyond 6 kHz, so the outgoing signal has superb fidelity. If the drive signal exceeds 1.6 volts pk-pk, corresponding to about +/- 5 kHz deviation, modulation is switched off abruptly and stays off until you reduce the drive. A nice touch. The correct drive level is 1 volt pk-pk for +/- 3 kHz deviation and I confirmed this by measurement. The FM Main and Sub receive circuits are similar. Main uses a pair of Icom part no. FL-211 crystal roofing filters (no spec) and a muRata SFH455E ceramic final filter; Sub uses a pair of FL-212 and the muRata CFW455E. The "E" suffix means 15 kHz bandwidth. The SFH types have particularly flat delay characteristics, and are pin compatible with the more common general purpose CFW series. Main's fidelity is outstanding, with a flat frequency and delay response to over 6 kHz. The "eye" was essentially perfect. You can be mistuned by up to +/- 4 kHz before the "eye" starts to look mangled, and +/- 5 kHz if the packets are short. The Sub receiver is almost equally good, but you can't get at it without modifying the radio as described earlier. No quibble with Icom's claims here; 9600 bps simplex works 101%. PSK Satellite Doppler Tracking ------------------------------ When using a 1200 bps PSK digital satellite such as FO-20, PacSat, Lusat or Weber it is essential that the PSK modem can control the radio receive frequency in a closed loop fashion, preferably in small steps. The universal means of doing this is via the Up/Down buttons of the microphone socket. The smallest step from the mic Up/Down buttons of the IC-820H is 100 Hz. This really is too big, as the sudden lurch from one frequency to the next will invariably cause momentary loss of demodulator lock, with attendant corrupted characters. The Up/Down line is also accessible from the accessory socket, but it shares a pin with the ALC control. You select which from an internal slide switch. There seemed to be no way of changing the mic button step size to smaller than 100 Hz. An oversight? RS-232 Control -------------- Icom's system is called CI-V (Communication Interface 5), and is accessed by a rear panel jack with a bi-directional service. The voltage is TTL ish. You're supposed to buy the CI-V interface which converts to RS232 levels, and also, I assume, de-multiplexes the input and output. The manual only provides a limited description of the control codes required; I guess a full treatise comes with the interface. In particular I couldn't determine what frequency resolution is available via RS-232 control. Other Observations ------------------ 1. There is a miniature 40mm fan inside the PA section which comes on when the radio is too hot, and the TX is keyed. I spotted the fan on the schematic, but I couldn't find it inside the radio until I provoked it into action during a megabyte file transfer at full power and a 75% duty cycle. The noise is less than a typical computer's. 2. If you want to use speech you must switch your TNC off or disconnect it from the accessory socket, otherwise your speech will be obliterated by data. This is not mentioned in the manual! 3. There is no VOX system. Witticisms ---------- The manual is beautifully laid out, with explanations crystal clear, quite devoid of Janglish. Fortunately two useful tips escaped the proof-readers: IF Shift Control, page 24: "Especially in CW mode, a mechanical noise may sound when rotating the [SHIFT] control, however, it is not a transceiver malfunction." I'm still decoding that one. Satellite Notes 1., page 35: "NEVER set the output power too high. Too much power will shorten the satellite's life." Ah so. Conclusion ---------- The IC-820H wasn't really designed with satellite operation is its primary application. It's unlikely to win the hearts of serious satellite users, in the same way as Yaesu's FT736R, mainly because of its lack of flexibility. But an average user who wants to try out transponding satellites such as Oscar-13 will find it a satisfactory starting point. With the Pacsats, 1200 bps doppler tracking is awkward, and 9600 bps full duplex operation requires you to modify the radio. The IC-820H is a nice radio if your needs are normal VHF/UHF operating, although serious VHF/UHF users will bemoan the lack of control over external equipment. It is also fine for low speed data transmission, and is among the first general purpose radios that provides 9600 bps packet radio simplex capability straight out of the box, at which it excels. Acknowledgement --------------- My sincere thanks to Dennis Goodwin at Icom-UK for the IC-820H loan. (C)1994 James Miller G3RUH <end> ----- End of message 36538 from N8DEU @ N8DEU.#HSV.AL.USA.NA ----- HSV ==>