Transceiving RTTY (RadioTeleTYpe) with the Small Wonder Labs Rock-Mite by William K. Mabry, N4QA, updated 22 February 2004 WARNING ! Due to 100 % duty cycle during transmit intervals, an adequate heatsink must be used on the Rock-Mite's final amplifier transistor, Q6. Long transmissions should be avoided. Q6 may be destroyed if it is not sufficiently protected against overheating. For this project, I am using a Rock-Mite-20, a simple homebrew hardware interface circuit, and an IBM ThinkPad 600E PC running 'MMTTY' ver. 1.65 software by Makoto Mori, JE3HHT. For Rock-Mite-RTTY to use a serial port on your PC, you must do the following on the MMTTY screen: Click 'Option', 'Setup MMTTY', 'TX' tab and select the appropriate PTT port...for example, 'COM1'. Also, ensure that the 'Invert Logic' checkbox is not checked. Next, click the 'Misc' tab and ensure that the 'COM-TxD(FSK)' button is selected. Finally, click 'OK'. Currently, my RM-20's transmitted 'Mark' frequency is ~ 14088.35 KHz and its 'Space' frequency is ~ 14088.18 KHz (170 Hz shift). Using a crystal which is cut for approximately 14088 KHz at Y2, the above 'Mark' frequency is the natural or default value for my RM-20. The above 'Space' frequency is achieved by shunting the Rock-Mite's D5 zener diode with an adjustable 'shift' resistor which is switched in and out during FSK modulation by 'MMTTY', the PC's comm port and my interface circuit. The receiver's passband filter crystal, Y1, should be cut for a frequency that is within a few KHz of that for Y2. On 23 April 2003, I called Bomar Crystals at 1-800-526-3935 and spoke with a fellow named Michael. I ordered one each HC-49/U2, fundamental mode, series resonant, 30ppm (freq tolerance and temperature stability) for: 14083 KHz 14087 KHz The reason I specified series resonant was that I didn't want to guess about the loading the xtals would see when installed in the Rock-Mite-20, but I presumed that their in-circuit oscillation frequencies would wind up somewhere in the RTTY subband...they did. In the Rock-Mite-20, the '14083 KHz' xtal oscillates at ~14088.35 KHz(mark) and... The '14087 KHz' xtal oscillates at ~14092 KHz(mark). The reason I ordered two xtals, a few KHz apart, was to give me two frequencies on which to operate within the 20-meter RTTY subband. Example: Using the 14088 xtal in the transmitter at Y2 and the 14092 xtal in the receiver bandpass circuit at Y1...allows me to transmit on 14088 and still be able to receive on 14088 just fine because of the Rock-Mite-20's wide receiver passband. Interchanging the two xtals allows me to 'transceive' on 14092... Dave's (K1SWL) Rock-Mite transceivers are, IMHO, special radios. At first I had thought that, being crystal-controlled for both transmit and receive, the Rock-Mite would be very limited in its capabilities. Soon, however, I learned that the combination of dual-frequency operation and somewhat 'wide' receiver passband constituted a very powerful 'one-two' punch, indeed. While very expensive and impressive radios abound in this high-tech age, their tighter filtering might sometimes give the operator a narrow view of the world. The information presented below presumes that RTTY communications between two stations take place at or near the Rock-Mite's 'Mark' and 'Space' frequencies as specified above. Variation from this condition may produce differing results. When receiving RTTY, the Rock-Mite's 'normal CW frequency change' switch must be toggled such that the Rock-Mite is operating on its 'second (non-powerup) frequency'. The 'Rev.' (Reverse) button on the MMTTY screen must be engaged. Also on the MMTTY screen, I set the 'Demodulator'... 'Mark' frequency at 820 Hz, (the approximate shift between xmit & rcv on my RM-20). The way to accomplish this is to click 'Option', 'Setup MMTTY', 'Demodulator' tab and enter 820 in the leftmost 'HAM Default' box. The rightmost 'HAM Default' box should have 170 (so called narrow FSK) stored there. Finally, click 'OK'. Once this is done, you can make '820' appear in MMTTY's 'Demodulator', 'Mark' box whenever the 'HAM' button is clicked on the main MMTTY screen. When transmitting RTTY, the Rock-Mite's 'normal CW frequency change' switch must be toggled such that the Rock-Mite is operating on its powerup frequency. To be successful at duplicating my 'Rock-Mite-RTTY' results, a user must also become proficient in the operation of 'MMTTY' or equivalent software which manipulates the PC's serial port. See my interface diagram at http://www.n4qa.com/rockrtty/rockrtty.jpg for Rock-Mite to PC interconnection details. To operate the Rock-Mite in CW mode, just stop pounding the keyboard and start pounding the CW key/paddles :) . You might add a switching arrangement for selecting the RTTY/CW subband xtal sets, as desired. Here at N4QA, I just tack-solder... whichever xtal set I want to use...onto the Rock-Mite-20 board. Of course, the Rock-Mite is capable of communications using other FSK modes as well as its native CW. RTTY is but one extant FSK mode. To a certain degree, it matters not how information is encoded to (and subsequently decoded from) the FSK envelope. Emission rules and regulations apply as usual. As of the date which appears near the top of this document, I have made Rock-Mite-RTTY contacts on the above-specified frequencies with the following geographical locations from my QTH in Pulaski County, Virginia: Lake Vermilion, Minnesota, USA (My first-ever RTTY contact, which occurred on 06 May 2003)...(now twice) Winnipeg, Manitoba, Canada (twice) Pahrump, Nevada, USA Austin, Texas, USA Hot Springs, Arkansas, USA Dallas, Texas, USA (twice) Decatur, Indiana, USA Deerfield Beach, Florida, USA Woodmere, New York, USA Loxahatchee, Florida, USA Stroudsburg, Pennsylvania, USA Salem, Alabama, USA Breaux Bridge, Louisiana, USA St. Inigoes, Maryland, USA Blumenau, Santa Catarina, Brazil San Pedro Sula, Cortes, Honduras St. Louis, Missouri, USA Special Event Station 9A0PAX Croatia *** Celebrating Pope John Paul II's third visit to Croatia *** Mason, Ohio, USA Palm City, Florida, USA Lexington, Kentucky, USA Plano, Texas, USA Elk Grove Village, Illinois, USA Wisconsin, USA Field Day 2003 contact Iowa, USA " " " " Prince Albert, Saskatchewan, Canada " " " " VE5CCH, club station of Carlton Comprehensive High School All above contacts made using a slightly modified Small Wonder Labs Rock-Mite-20 running 300 milliwatts to an end-fed, single strand 17-gauge aluminum wire which is approximately four-wavelengths at 14088 KHz and whose height above ground is approximately fifteen feet. I have no connection with any above-mentioned vendor.