QEX

Fig 2—The frequency synthesizer (Part a) is connected to four BCD encoders, to the LED PLL indicator, and to the Colpitts VCO (Part b).

Fig 2—The frequency synthesizer (Part a) is connected to four BCD encoders, to the LED PLL indicator, and to the Colpitts VCO (Part b).

4 BCD encoders are connected to pins 9 through 24. Sixteen 47-kfi resistors connected between these inputs and ground ensure that a logical 0 will be present on the IC's input whenever the BCD encoders' contacts are open. The VCO (Part b) signal enters the IC on pin 1. An error signal is available on pin 4. This signal is fed back to the VCO to correct its oscillating frequency. A PLL signal will be present on pin 28 and will be amplified by means of a PNP transistor to drive the LED.

In principle, any kind of VCO can be considered. In order to achieve very-high frequency stability and minimize phase noise, the oscillator, T1, is a Colpitts type. The output of the amplifier, T2, is connected to pin 1 of the synthesizer (Part a). In turn, the error signal from the synthesizer, pin 4, is fed to a filter consisting of a 47-kfi fixed resistor, a 10-kfi pot, and a 4.7-/iF capacitor. The error signal is taken from the rotor of the pot and sent to the VCO (Part b) Varicap diode. Note: To obtain a frequency as pure as possible, it is necessary to adjust the 10-kfi pot so as to reach PLL sync (LED from pin 28 via T4) within 0.2 to 1 second. The frequency set on the BCD encoders is then present at the output of the second amplifier, T3, with an amplitude of 1-2 VP.P.

Part c—Optional VXO

The synthesizer (Part a) in conjunction with the VCO (Part b), produces a frequency programmable from 4.5 to 9.999 MHz in steps of 1 kHz. For continuous coverage in this frequency range, it is possible to vary the crystal frequency by means of a VXO (Part c). The crystal frequency may be pushed or pulled by ± 3 kHz around 4.096 MHz without affecting the stability. The emitter of T5 is connected through a 1-nF capacitor to pin 27 of the MC 145163. Frequencies in excess of 1 kHz can be set on the BCD encoders for continuous coverage of the frequency range. When using the VXO option, pin 26 of the MC 145163 is left open.

Conclusions

The natural application of such a frequency synthesizer is quick and easy construction of a small 7-MHz transceiver; a 3.5-MHz rig may also be made provided that the self L1 be adapted to this frequency by adding a few turns. Or it could be used with a mixer, a prescaler, or a PLL, to generate highly stable HF or VHF signals.

No more than 3 to 4 hours are needed to build this synthesizer on perf board, at a cost of about $20. The stability is the same as that of the crystal, and frequency drift vs supply voltage is much less than 100 Hz. Further, the system is much more shock resistant than any good VFO.

You will find the synthesizer easy to use on the air, and you will have a professional product with little time invested.

The AMTOR Mailbox Standard: A Progress Report

(continued from page 9)

by accepting the first call sign (my call) as a command word, which responds in the same way as a first sign-on.

The mailbox software is being made available to stations already operating licensed mailboxes, or to individuals who have applied for a license. As a result, six stations are active at the time of writing, four of which are fully multichannel scanning/forwarding. All these are run by mailbox operators with programming experience, and so the full source code has been made available to them. This has enhanced the ease of exchange of ideas for improvements and has speeded up the finding and fixing of bugs since solutions can be discussed and communicated over the network itself. The performance has been impressive. The forwarding protocol has proven to be completely garble-proof, and there has been no evidence of the duplicate-message problem prevalent on packet. As expected, many users have spent quite some time sending messages by both

AMTOR and packet routes where both exist, and the AMTOR route is usually an order of magnitude faster. The nonzero error rate on message text has not caused any comment from packet users, nor has the restriction to upper-case characters. AMTOR users have found the forwarding syntax easy to drive manually, and some have even written their own software for auto-polling for their messages, since the "double-command" rule has also been applied to the process of removing messages from the system. At GB7PLX, a total of 450 different stations account for 60 log entries per day, with most of the forwarded traffic coming in and out on AMTOR rather than packet.

The Future

I hope I have shown that the "AMTOR Mailbox Standard" which I described in February has grown into a useful contribution to the development of amateur communication techniques. I hope that this article may help to promote interest in extending the network further.

January 1991 15

Correspondence

Stripped-Down TNC

The multi-TNC thing (June 1990 QEX) is nice, but i feel it would be better to make up "stripped-down" TNC 2s as daughter boards and a double-sided mother board with four TNCs on the top and four TNCs on the bottom. It could be set up as a rack-mount assembly. I wouldn't even bother doing it at RS-232 levels. TTL would work, I guess. The mother board might even have ten minutes of back-up power and, at a power fail of six minutes, stop accepting incoming data, and at seven minutes feed out a "power fail" message to all ports. The way I see it, you could build up all the daughter board TNCs with no sockets, no clock, no RS-232, and come up with a box in kit form for around $300-350 for the eight TNCs and the mother board.

I do not see the need for separate cabinets, RS-232, etc, except for the fact that many TNCs are already out there. But they could be sold off to get more hams on packet. Those chips used are probably fairly expensive and at TTL/CMOS, etc, things might be far cheaper, etc.—Joseph A. Wolos, WA10CK, 1139 St James Avenue, Springfield, MA 01104-1375

Telemetry Reception with the PK-232

The PK-232 can copy satellite telemetry from AO-13 and UoSAT 11. A minor hardware modification is needed to copy UO-11. Satellite-related bulletins as well as satellite status reports are sent in plain text for part of each telemetry frame daily on AO-13 and most days on UO-11.

Reception of PSK telemetry sent by F0-20 and the Microsats requires an added external PSK modem. Reasonably priced ones are available as kits from AMSAT-UK and TAPR. TAPR sells a modem disconnect board that plugs into a 40-pin IC socket inside the PK-232 and offers an easy way to interface any PSK modem to the PK-232. The external modem connector on the PK-232 does not have the clock data, so using it requires modifications to the PK-232 circuit board. Installing the modem disconnect board is easier and leaves you with the PK-232's external modem disconnect port free so you can now have two external modems. Both of mine are used, so I now need three!

The following settings will copy AO-13 SSB Baudot RTTY telemetry which is sent on 145.812 MHz on the hour +15 and +45 minutes when operating mode B, and on 435.651 MHz on the hour +00, +15, +30 and + 45 minutes when in mode JL:

BA, RXR = OFF (mode B), RXR = ON (mode JL), Wl = OFF, RB 50

Frequencies are plus or minus Doppler shift. An oscilloscope connected to the PK-232 will simplify keeping up with the Doppler shifted tones on AO-13 as a shift in the cross pattern is seen easier than a shift of the PK-232's leads. Tones do not change with Doppler on the UO-11 as it transmits FM AFSK.

RXR does not operate above 300 bauds because of

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