Letters to the Editor - SDR-Kits

and 40 MHz LO respectively. But they alsoproduce alias frequencies; for example thefirst alias of the RF DDS is 180 MHz – 20MHz = 160 MHz. The second alias of theLO DDS is 170 MHz + 40 MHz = 210 MHz.Comparing the aliases in Table 1, there isobviously only one pairing that yields a 20MHz IF when mixed. Note that the table doesnot show all possible mixing products of theshown aliases, but only those in the vicinityof the desired IF.Usually a DDS is used to produce lessthan 50% of its clock frequency. Table 2 displaysexample 2 where this is not the case.The DDS tuning words can still becalculated with the above formula. Thefundamental DDS frequencies are not thedesired output frequencies, however. But thefirst aliases turn out to be exactly the wantedsignals. Also here, only the desired signalpair mixes to 20 MHz IF.Examples 3 and 4 in Table 3 demonstratewhat happens when the DDS tuning wordscalculated with the above formula exceed theword lengths of 32 bits. In this case, the leadingbits beyond bit number 32 are simply ignored.Thus, one arrives at 32 bit tuning words again.By doing this, it turns out that the DDS sourcesare set such that aliases produce exactly thedesired frequencies. Again, only the desiredpairs produce the correct IF of 20 MHz.Finally, I would like to point to a very nicearticle by Sam Wetterlin, who has illustratedhow, by this idea and by making the clock frequenciesvariable, it is possible to constructa VNWA that covers a wide frequency rangewithout frequency gaps. 2A very small VNWA kit, which I hopewill cover a frequency range up to 1 GHz iscurrently under development. For informationon availability and pricing please visit the Website listed in Note 1.Thanks to Mike Alferman, WA2NAS, forstraightening out my manuscript.Notes1Professor Dr Thomas C. Baier, DG8SAQ, “ALow Budget Vector Network Analyzer for AFto UHF,” QEX, Mar/Apr 2007, ARRL, see alsowww.mydarc.de/DG8SAQ/VNWA/index.shtml2Sam Wetterlin, “Using DDS Aliases to Extendthe Frequency Range,” www.wetterlin.org/sam/AD9952/MultipleClockAliases.pdfHigh Speed Multi-Media WorkingGroupDear Doug,Thank you most kindly, sir, for your warmcomments about the accomplishments of theLeague’s HSMM Working Group (WG) inyour editorial in the May/June 2007 issue ofQEX. The WG’s dedication and efforts areongoing, as reflected in many radio experimentersprojects.We are particularly proud of the excellentarticle in your same issue of QEX by WGmember Roderick D. Mitchell, KL1Y, “TheIntegration of Amateur Radio and 802.11”and your excellent presentation thereof.Thank you so very much for all your outstandingefforts to keep Amateur radio on thecutting edge of modern radio and electronicstechnology.— Very 73, John Champa, NCE (K8OCL),Wireless Systems Engineer, Rockwell Collins,Former Chairman, HSMM WG, 17850Sunmeadow Dr, Apt 2303, Dallas, TX 75252;k8ocl@hotmail.comHi John,Thanks also for all the work you and theall the members of the Working Group havedone, and continue to do. I agree that RodMitchell’s article is an interesting report onsome of that work.What else do you have to report? QEXstands ready to report on the WG’s efforts.If you or anyone else on your group would liketo write for QEX, please submit your articles.— 73, Larry Wolfgang, WR1B, QEX ManagingEditor; lwolfgang@arrl.orgAutomatic Noise Figure Meter (May/June 2007)The subject article is a good example ofhow a measurement problem can be solvedby a little thought, without needing to buyor borrow expensive commercial equipment.Twenty or so years ago, I was faced with howto make NF measurements on some VHFand UHF very low noise amplifiers, but myuniversity research lab was not equipped witha noise figure meter.The solution was a calibrated noisesource, a broadband amplifier, a tunable bandpass filter and a power meter to measure thenoise with the noise source off and then on.All these items were available in my lab. TheNF was calculated from the Y factor givenby the power meter. I never thought my ideawas new, but I was surprised to find thatthe vendor for the low noise amplifiers hadnever thought of using my procedure. He wassurprised to see that my measurements werequite accurate also.Good article!— 73, John H. Bordelon, PhD, K4JIU, 1690Hampton Oaks Bend, Marietta, GA 30066-4451; k4jiu@arrl.netHi John,Thanks for the compliments. We’re gladyou liked this article.— 73, Larry, WR1B.Components Cross ReferenceDear Sir,I have some data to help fill a hole in theHP cross reference to JEDEC or 300-hpxref.pdf. HP part number 1858-0054 is an exactmatch for an Intersil (RCA) CA3096 transistorarray.We found this part in an Agilent servicemanual. By “we” I refer to a group to which Ibelong that promotes classic computer stuff.I’m also a proud ARRL member. Thanksfor helping us pass along this information!— 73, Steven Canning, PO Box 1682, LaMirada, CA 90638; cannings@earthlink.netOn the Crossed-Field AntennaPerformance, Parts 1 and 2(Jan/Feb and Mar/Apr 2007)Dear Doug,I feel the article on the Crossed-FieldAntenna was far below the technical levelset up to this point by QEX.I know there is a great deal of discussionabout the theory, but most knowledgeableantenna experts do not give this antenna anyreal credibility. In fact, the credit seems togo to the feed line.I always look forward to reading QEX andenjoy the level of articles presented. This onejust seemed out of place. It would have beenbetter placed in the April issue.— Doug Millar, K6JEY, 2791 Cedar Ave, LongBeach, CA 90806; dougnhelen@moonlink.netHello, Doug.I’m dismayed to see another IEEE articlereprint that takes eight editorial pages inQEX. I have written previously to expressmy disappointment at reprinting verbatimarticles that appeared elsewhere. Post sucha reprinted article on the ARRL Web site, ifyou wish, but I pay for a QEX subscription tolearn about new things of interest to communicationexperimenters. I have no interest inwading through dense mathematics presentedpreviously in a professional-society publication.I doubt most other “experimenters” willeither. To make matters worse, this article’ssecond part will take more editorial pages in afuture QEX. This isn’t the type of informationI signed up to receive in the pages of QEX, soI intend to let my QEX subscription lapse.— Jon Titus, KZ1G, 5526 West 13400South, Suite 105, Herriman, UT 84096;jontitus@comcast.netGentlemen,We are sorry that you did not enjoy thecrossed field antenna article. Direct reprintsof previously published, readily availablearticles have been a rare occurrence in thepages of QEX. This research seemed to beworth sharing with QEX readers.I hope you will consider sharing someof your own work with our readers. QEX isdedicated to sharing experimental work andideas about a wide array of communicationstopics.Jul/Aug 2007 3

Jon, I hope you will also reconsider yourdecision to allow your subscription to lapsebased on this one article. I hope you havefound many other articles of interest andthat you would look forward to reading moreexcellent articles in the future!— 73, Larry, WR1B.ARES Communications ProtocolEditor,I am interested in investigating an improvedcommunications and message traffichandling system for use in ARES deploymentsituations. I am both an Amateur Radio operatorand a computer/information scientistand my experiences with existing UHF basedcommunications systems has shown the difficultiesinherent in voice-based, simplexcommunications and message handling.The situation I am attempting to address ishow to avoid overlapping transmissions whenoperating in a simplex mode (we are trainingfor a worst-case situation with no repeateravailability). The difficulty occurs when, insimplex mode, you have a network controlstation and remote stations, and the remotestations cannot hear each other but can communicatewith the control station. The realityis that too often we have “collisions” whenremote stations transmit simultaneously, andin emergencies this delay and or loss of successfulcommunication could be fatal. Onesolution is for remote stations to call andwait for permission to transmit; another is aroll-call protocol. For standard voice communications,the call and wait protocol makesgood sense and works well with disciplinedoperators. As we move to digital communications,however, there are better ways to avoidmessage collisions and the automated roll-call(poll-response) initiated by the control stationis the one I am investigating. Packet or evencharacter format communications wouldappear to be readily available. The missingpieces in this system relate to the networklevel protocol to control communications.The AGW suite of software and theAX25 Connected mode protocol appear tohave almost all of the features I am seeking,although not exactly in the way I want (pollresponse).Any information that your readership couldprovide on others who are also looking into, orworking on this same or related communicationsprotocol would be greatly appreciated.— J.J. Hayden, KN4SH, 45 Cory Ct, Covington,GA 30016; jjhaydeniii@earthlink.netAntenna Options — NVIS Antennas forSpecial Needs (May/June 2007)LB’s article on discussing wide band NVISantennas shows so clearly the obsession manyhave with antenna matching. Here is an antenna(terminated V) that has at least four dBmore loss than an unterminated one, favoredby some purely because it matches the sourcebetter. Take the unterminated version and put a3 dB pad in front of it and the amplifier will behappy, and more RF will reach the destination!Obviously there is more to life than SWR.I would call into question one statement,though. “Since the … transmitter [scans] sorapidly, it does not have time for the delaysassociated with changing matching networks…”. There may be situations where this is true,but in general, it is very possible to design tunablematching networks using PIN diodes thatcan respond in microsecond time frames. ForHF, it is more challenging because PIN diodesthat can work down to 3 MHz are harder tocome by, and will take longer to change state,but it is not impossible. High speed PIN diodedevices are not likely to dominate Amateurpractices anytime soon because of cost andother factors. For military applications, however,such devices provide an alternative towideband NVIS antennas.One of my clients manufactures tunableaircraft antennas that cover 30 to 500 MHz.This is a blade less than a foot in any dimension!It is used with military hopping radiosand can settle on a new operating frequency in10 µs. The radio sends the new frequency tothe antenna as a digital word and the microprocessorinside selects the proper combinationof PIN diodes to resonate this little blade tothat frequency.— Wilton Helm, WT6C, Embedded SystemResources, 320 Old Y Rd, Golden, CO 80401;whelm@compuserve.comHi Wilton,I agree that in principle it is possible toreduce the transition time within the matchingnetworks to the very short interval thatyou indicate.Unfortunately, such tuning units are notpresently available — to my knowledge— at the frequencies indicated for amateurand related NVIS services at the powerlevels typically used and at a price that mostamateurs can pay. That is the context withinwhich my notes were applicable.I also agree that in principle SWR iscertainly not everything, but with typicalamateur transceivers the output impedance isfixed within a very small range with foldbackcircuits to limit final amplifier power whenthe SWR indication is greater than a certainthreshold. Hence, the amateur need to matchis a practical matter, not a theoretical one.An alternative to high-speed matching is,of course, slower speed ALE processing, sothat the speed of channel scanning matchesthe speed of available automatic tuner switchingfor ATUs that already exist or that canbe developed to fit amateur budgets. Militaryand other services that need burst transmissionsto keep channels and data secure mayrequire the very high speed of the latestALE processors, but the emergency servicesrendered by radio amateurs and many of theagencies involved in similar services do notneed such security — only efficiency.— 73, LB Cebik, W4RNL, 1434 High MesaDr, Knoxville, TN 37938; Cebik@cebik.com4 Jul/Aug 2007