Transceivers, Transverters and Repeaters 15.15

tening at Field Day, 1994, where membersof the Zuni Loop Expeditionary Force usedit on 80, 40, 20 and 15 m. There, Sierracompared favorably to the Heath HW-9and several older Ten-Tec rigs, having asgood or better sensitivity and selectivity —and in most cases better-sounding sidetoneand break-in keying. While the other rigshad higher output power, they couldn’ttouch the Sierra’s small size, light weightand low power consumption. The Sierra hasconsistently received high marks from sta-tions worked too, with reports of excellentkeying and stability.

CONCLUSIONAt the time this article was written, over

100 Sierras had been built. Many have beenused extensively in the field, where the rig’sunique features are an asset. For somebuilders, the Sierra has become the primaryhome station rig.

The success of the Sierra is due, in large

part, to the energy and enthusiasm of themembers of NorCal, who helped test andrefine early prototypes, procured parts forthe field-test units and suggested futuremodifications.8 This project should serveas a model for other clubs who see a needfor an entirely new kind of equipment,perhaps something that is not availablecommercially.

Notes1One of N6KR’s previous designs, the Safari-4,

is a good example of how complex a band-switched rig can get. See “The Safari-4....”Oct through Dec 1990 QEX.

2Band modules for 160, 12 and 10 m have alsobeen built and are available for the kit (seenote 6). PC board patterns, construction hints,alignment and troubleshooting tips, and otherinformation about the Sierra is included in theTemplate Packages section of the CD-ROMbundled with this Handbook.

3For information about NorCal, visit www.norcalqrp.org.

4Most multiband rigs draw from 150 to 500 mA

on receive, necessitating the use of a largerbattery. A discussion of battery life consider-ations can be found in “A Solar-PoweredField Day,” May 1995 QST.

5Solid-State Design, p 87. This book is out ofprint but may be available used.

6Full and partial kits are available. The full kitcomes with all components, controls, con-nectors, and a detailed assembly manual.Complete band modules kits are availablefor 80, 40, 30, 20, 17 and 15 m. For informa-tion, contact Wilderness Radio, PO Box3422, Joplin, MO 64803, tel 417-782-1397;www.f ix .net /~ jparker /wi lderness/sierra.htm.

7The alignment procedure given here is neces-sarily brief. More complete instructions areprovided with the ARRL Template Packageon the accompanying Handbook CD and thekit instructions.

8The author would like to acknowledge thecontributions of several NorCal members:Doug Hendricks, KI6DS; Jim Cates,WA6GER; Bob Dyer, KD6VIO; DaveMeacham, W6EMD; Eric Swartz, WA6HHQ,Bob Warmke, W6CYX; Stan Cooper, K4DRD;Vic Black, AB6SO; and Bob Korte, KD6KYT.

Dave Benson, K1SWL, first described thesimple 1/2 W RockMite CW transceiver for40 or 20 meters in April 2003 QST. TheRockMite — named for its crystal controland its small size — has attracted an enthu-siastic following, with thousands of thetransceivers in circulation. This projectbuilds on the original and adds versions for80 and 30 meters. Changing the RockMitefrequency is a matter of replacing the two(identical) crystals with frequencies of yourchoosing. If you change bands, however, theoutput harmonic filter and several capacitorvalues must be scaled accordingly, and thevalue of several Zener diodes may change.Details are shown in Table 15.3.

OverviewThe RockMite printed-circuit board mea-

sures 2.0 × 2.5 inches and fits in the Altoidstin that is beloved by the QRP community asan enclosure. Kits are available.1 A custommade aluminum enclosure is available fromwww.americanmorse.com.

The RockMite uses the familiar directconversion (D-C) receiver scheme shown inFig 15.12. There isn’t much to it –– an oscil-lator and a mixer convert received signalsdirectly to audio and an amplifier boosts thataudio to usable levels. On transmit, the sameoscillator serves as the transmitter frequencysource, and only gain and keying stages areneeded to bring the oscillator signal up tolevels usable for making CW contacts.

Several crucial details are missing fromthis oversimplified picture, however. Theoperator who calls “CQ” with a crystal-con-trolled D-C rig will most likely get replies onzero-beat with his signal and without somemeans of shifting frequency (offset) betweentransmit and receive, will copy only low-frequency thumps. Additionally, the joy ofsending CW will be somewhat tempered bythe lack of a sidetone circuit to monitor yourown sending.

By using an 8-pin PIC microcontroller, itbecomes possible to add an iambic keyer

along with other functions. This can be donewith minimum cost and with little printedcircuit board acreage. Having made the de-cision to use a controller chip, a spare pin onthat IC was dedicated to providing a 700 Hzsidetone during key-down conditions. Thecontroller also supplies a TR control signaland a shift signal. This shift signal merelyprovides a dc voltage level to a varicap (tun-ing) diode to pull the crystal oscillator fre-quency between transmit and receive. TheTR offset is reversible, as described later, sothat the RockMite offers two possible oper-

THE ROCKMITE — A SIMPLE SINGLE-BAND CW TRANSCEIVER

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15.16 Chapter 15

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Transceivers, Transverters and Repeaters 15.17

ating frequencies. This function has tradi-tionally been done with a double-pole switch,but it’s easier and cheaper to perform thatfunction in firmware.

There is one other noteworthy trick em-ployed in the RockMite. Builders of simplereceivers for 40 meters have all experiencedthe joys of listening to shortwave broadcastsmixed in with their CW. For most simplegear, the high levels of broadcast RF causeintermodulation distortion (IMD). This canbe mitigated by the use of more robust(higher-current consumption and complex-ity) receiver front ends. Another approach isto ensure that the broadcast signal levelsreaching the receiver mixer are attenuatedenough to avoid their IMD effects. If you’re

interested in only a small segment of anamateur band, a sharply tuned (narrow)band-pass filter may be used to good effectto accomplish that. The RockMite uses thisapproach by utilizing a second crystal at theoperating frequency at the receiver front end.The performance improvement with theadded crystal is significant.

Circuit DescriptionThe RockMite schematic is shown in Fig

15.12. Local oscillator Q4 is a crystal- con-trolled Colpitts oscillator and runs continu-ously. Its operating frequency is determinedby crystal Y2 and the surrounding compo-nents. Diode D6 is a varicap (tuning) diodeand it furnishes a voltage-dependent capaci-tance. This effect is used to pull the crystaloscillator frequency about 700 Hz betweentransmit and receive to provide a beat-fre-quency offset. The voltage applied to D6through resistor R10 is 0 V with Q2 turnedon (conducting) or it is the rated Zener volt-age of D5 with Q2 off.

A sample of the local oscillator signal iscoupled to the base of Q5. Q5 provides novoltage gain but instead serves to improvekey-up isolation between the local oscillatorand the antenna. This ensures that the key-up energy to the antenna (back-wave) is neg-ligible. Equally important, the lowered sig-nal level at the antenna terminal preventsblocking effects from desensitizing the re-ceiver.

The output of the buffer stage is coupledvia C13 to the power amplifier stage, Q6.Diode D8 provides a clamp function, mak-ing it easier to drive the base of Q6. Transis-tor Q6 runs Class C, is driven hard and, intheory, has only conducting and nonconduct-ing states for high efficiency. The waveformat Q6’s collector would ideally be a squarewave. In practice, there’s considerablewaveform distortion at that signal point and,

Table 15.3RockMite Component Values by BandBand 80 m 40 m 30 m 20 mFreq (MHz) 3.560 7.030 10.106 14.060C1 47 pF 47 pF 47 pF 47 pFC2 33 pF 47 pF 47 pF 47 pFC10, C11 68 pF 68 pF 33 pF 39 pFC12 47 pF 47 pF 33 pF 39 pFC15, C17 560 pF 470 pF 330 pF 220 pFC16 1200 pF 1000 pF 680 pF 470 pFC18 330 pF 150 pF 82 pF 82 pFD3 1N5231B 1N5231B 1N5231B 1N5233B

(5.1 V Zener) (5.1 V Zener) (5.1 V Zener) (6.0 V Zener)D4 1N5231B 1N5231B 1N5231B 1N5230B

(5.1 V Zener) (5.1 V Zener) (5.1 V Zener) (4.7 V Zener)D5 Omitted 1N5236B 1N5231B 1N5230B

(7.5 V Zener) (5.1 V Zener) (4.7 V Zener)L1 15 μH 10 μH 6.8 μH 4.7 μHL2, L3 2.2 μH 1 μH 0.68 μH 0.47 μHL4 5.6 μH 3.3 μH 3.3 μH 1.5 μH

Fig 15.12 — The RockMite transceiversimplified block diagram (inset) andschematic. Most components arestocked by major distributors such asDigi-Key, Mouser and Ocean StateElectronics (see the TISFind database atwww.arrl.org/tis). Resistors are 5% 1/4 W.C1, C2, C10, C11, C12, C18 — NP0 disk

capacitor, 5%. See Table 15.3 forvalues.

C3, C13, C101, C102, C108 –– 0.01 µFdisk capacitor.

C4 –– 0.022 µF monolithic capacitor.C5, C8, C14, C104, C109, C110 –– 0.1 µF

monolithic capacitor.C6, C105-107 –– 100 pF disk capacitor.C7, C103, C111 –– 47 µF, 25 V

electrolytic capacitor.C9 –– 3.3 µF, 50 V electrolytic capacitor.C15, C17 –– Disk capacitor, 5%. See

Table 15.3 for values.C16 –– C0G monolithic capacitor 5%.

See Table 15.3 for values.D1, D2, D7, D8 –– 1N4148 diode.D3, D4, D5 –– Zener diode, 0.5 W. See

Table 15.3 for values.D6 –– MV1662 varicap diode.L1, L2, L3, L4 –– Molded RF choke, 10%

tolerance. See Table 15.3 for values.Q1, Q2, Q3 –– 2N7000 FET.Q4, Q5 –– 2N4401 transistor.Q6 –– 2N2222A transistor.U1 –– SA612AD mixer/oscillator IC.

(Surface mount part is used on the kitPC board. If building from scratch,consider the SA612AN in a DIPpackage.)

U2 –– LM1458N dual op-amp IC.U3 –– 12C508A-04/P microcontroller.

(Must be programmed before use. SeeNote 1.)

Y1, Y2 –– HC49/U crystal (20 pF load) foroperating frequency of interest.Crystals for popular QRP frequencies,including those shown in Table 15.3,are available fromexpandedspectrumsystems.com orAF4K.com.

in any case, it’s nothing you’d want to applydirectly to an antenna.

Capacitor C14 couples this waveform tothe output harmonic filter, which comprisesL2 and L3 and C15, C16 and C17. Since theoriginal RockMite article was published,FCC requirements for spectral puritychanged from –30 dBc to –43 dBc. A seriesL-C circuit (L4 and C18) between the out-put of the low pass filter and the antennaprovides the needed additional harmonic at-tenuation. In an effort to save space and re-duce construction complexity, subminiatureepoxy-molded RF chokes were used insteadof the traditional toroids. For the frequen-cies and power levels encountered in theRockMite, performance appears adequate –– loss and self-heating were not significant.Power output is about 500 mW with a 13 Vdc supply and it will work at lower supplyvoltages.

The receiver is continuously connectedto the antenna through coupling capacitorC1. Diodes D1 and D2 limit the key-downvoltage swing appearing at the receiverfront-end to safe values. The presence of Y1at the receiver front-end may seem some-what startling, but it serves as a narrowband-pass filter to keep RF energy from fre-quencies far removed from the operatingfrequency to a minimum. The SA612 mixer,which does the conversion from RF to au-dio, needs all the help it can get.

Readers may recognize the circuit as anadaptation of a Roy Lewallen, W7EL, cir-cuit — a widely used series-LC TR switch.The inductance in this circuit is being fur-nished by crystal Y1 at a frequency slightlyoff its series-resonant point. Perhaps lessobviously, capacitor C2 forms an L networkin combination with a portion of the crystalmotional inductance. It’s impedance step-up; there’s about 10 dB of voltage gain priorto the mixer input (U1, pin 2). The values of

Chapter 15.pmd 7/29/2008, 2:02 PM17

15.18 Chapter 15

C1 and C2 were twiddled empirically toyield a 6 dB bandwidth of about 2 kHz andto straddle the two operating frequenciesfairly evenly. For the 40 meter version, re-ceiver filter response is –35 dB at 7100 kHzand up. Although this value of ultimate re-jection is unacceptably poor for typical crys-tal filters, here it needs to be only goodenough to yield significant improvement inIMD performance.

The mixer IC, U1, converts the receivedsignal from the operating frequency to au-dio; that signal appears at pins 4 and 5 of U1.C4 provides some low-pass filtering to cutunwanted audio hiss. U2 is a garden-varietydual op-amp (one-half is unused) configuredfor a gain of about 200 (46 dB). This booststhe mixer’s output audio to headphone-us-able levels. Capacitor C6 provides an addi-tional pole of audio low-pass filtering.

Transistor Q1 provides a simple mutefunction to reduce the amount of keydownthump. It disconnects the audio amplifierfrom the headphones whenever the rig iskeyed. The large (transmitted) signal appear-ing at the receiver during key-down yields adc offset at the mixer output, which is ampli-fied to a large transient by the audio ampli-fier. The muting isn’t perfect but it’s a lot lessfatiguing than none at all. Key-up recoverytime is set by C9 –– this value may be re-duced if you prefer quicker QSK (break-in).U3 is a 12C508A microcontroller device andhas been custom programmed to provideiambic keyer (Mode B) and frequency shiftfunctions. U3 pins 6 and 7 are typically con-nected to a pair of paddle inputs to providethe keyer functions. Ground one of those twoinputs during rig power up and the RockMitewill use the other input for the straight key ormore capable external keyer.

There are two operator controls on theRockMite and they’re both implemented viaa push-button switch closure, in order toground controller pin 4. The two functionsare discriminated by the duration of theswitch closure.

A brief (< 250 ms) closure on the switchreverses the offset to provide a second oper-ating frequency. When you wish to workanother station, use this function to selectthe higher of the two pitches on a receivedsignal. Note that the pitch at the conversesetting is a measure of how close to zero-beat you are; ideally it would be just a low-frequency thump. If the two selections yielda high pitch and a still higher pitch, you prob-ably won’t be able to work the other station.

A longer closure on the switch input putsthe keyer in a speed-adjustment mode. TheRockMite outputs a Morse code S to ac-knowledge entry into this mode. Tapping(or holding) the dot paddle speeds up thekeyer; the same operation on the dash paddleslows it down. The default (power-up) speed

is approximately 16 WPM and the speedrange is about 5 to 40 WPM. If no dot/dashinputs are received after about 1 second, theRockMite outputs a lower frequency toneand reverts to normal operation. The MorseS and subsequent tones are not transmittedon the air.

ModificationsThe idea of a transceiver whose only con-

trol is a pushbutton switch probably flies inthe face of recent trends in transceiver de-sign. If you feel the need to “manage” yourradio, resistor R5 may be replaced with a 1MΩ audio taper potentiometer (wiper andone end-terminal used) to serve as a volumecontrol. Keep the leads short.

Sidetone level can be altered by changingthe value of C8. Note: the “raspy” nature ofthe RockMite sidetone is caused by thesquare-wave nature of the signal. One ormore R-C networks (series-R, shunt-C toground) in the path from U3 pin 5 to C8 willsoften the tone. A good starting point for thisfilter is 10 Ω/10 µF.

Adding a 1N4001 diode in series with thepower supply (V+) feed will preclude re-verse-polarity mishaps. (The banded endgoes toward the RockMite board.) Or betteryet, use a 1N5818 Schottky diode for lowervoltage drop. Any of the diodes 1N4001-1N4007 or 1N5818-1N5820 series is fine.They’re noncritical and all overkill for thisapplication.

The RockMite will run on a 9 V battery ifR1 and R8 are changed from 1 kΩ to 470 Ω.This change increases receiver current con-sumption from 25 mA to 40 mA when usinga 12-14 V supply.

TroubleshootingDetailed troubleshooting information can

be found in the file RMhelps.pdf, availablefor download from the Small Wonder LabsWeb site (see note 1). Here are some of themore common problems.

AC hum: Make sure Y2’s case isgrounded. The RockMite has a lot of audiogain. You may experience difficulty whenusing an unregulated power supply or walltransformer to power the rig. A regulatedsupply will help considerably in this regard.If in doubt, try a battery supply. You mayuse a 9 V battery temporarily to check outthe difference.

Howl in headphones: (Make sure you’renot in straight-key mode with the key down.That’s the sidetone.) The combination ofhigh audio gain and wire lead treatments canyield an audio oscillation or “howl,” al-though this has not been reported often. Hereare some things to try.

• If using a battery supply, make sure it’sreasonably well charged. A nearly-ex-hausted battery may cause howl or

motorboating.• Provide separation between wires run to

and from the RockMite board and the boardcomponents. Close lead proximity affordsmore chances for unwanted signal crosstalk.Where wires do need to cross, keep them atright angles to one another to minimize thecoupling.

• Don’t count on the enclosure itself toprovide ground return continuity. It may behelpful to run a ground return wire from theboard to the headphone jack ground lug andfrom there to the dc power return. If you dothis, continue to use a wire from boardground to the main dc power return. Youwant to avoid conditions where one circuitpath is carrying both audio and dc groundcurrents, and for that matter, RF as well.

• Ensure that the ground braid is used onthe coax connecting the PC board to the an-tenna jack. Connect the rig end to a conve-nient ground point near the antenna pad andbe sure the other end makes connection tothe antenna jack ground lug.

Broadcast pickup: There are two poten-tial issues with the RockMite. Shortwavebroadcast (SWBC) will be more likely dur-ing the evening hours. Despite the presenceof the front-end crystal filter, some SWBCmay occasionally be heard. A fix involvesreducing the signal levels getting to U1,which can be accomplished by changing R5to a 1 MΩ variable volume control as de-scribed in the previous section. The use of anantenna tuner will also assist in reducing theout-of-band RF energy getting to U1.

Local AM broadcast interference is morelikely during daylight hours when local AMstations are on the air. Install a 1 kΩ resistorat the two unused pads immediately belowD1/D2. Note that this fix does not help withshortwave broadcast. The fix was tested suc-cessfully at the ARRL lab, located withintwo miles of several 5 kW AM broadcaststations.

Very low volume: This is a minimalisttransceiver, so it won’t provide ear-splittingvolume. Even so, with a good antenna andheadphones you should have little troublehearing signals. If everything else checksout, consider the following. Use a resonantantenna, 50 Ω nominal, such as a dipole. Ifthe antenna is nonresonant (random wire,etc), use a tuner to make the antenna looklike 50 Ω at the rig. SWR is not especiallycritical here. The worst that could happen isthe loss of an inexpensive transistor (Q6).

Headphones should be a low-impedancestereo type, such as those used for personalMP3 or CD players. If there are specifica-tions on the package, look for a sensitivityspec of 104 dB/mW or better. And a finalcaution related to audio output: You’d besurprised how often reports of very low au-dio are traced to use of incorrect audio jack

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Transceivers, Transverters and Repeaters 15.19

TransvertersAt VHF, UHF and microwave frequen-

cies, transverters that interact with fac-tory-made transceivers in the HF or VHFrange are common and are often home-built. These units convert the transceivertransmit signal up to a higher frequencyand convert the receive frequency downto the transceiver receive frequency. Theresulting performance and signal qualityat the higher frequencies are enhanced bythe frequency stability and the signal pro-cessing capabilities of the transceiver.For example, SSB and narrowband CWfrom 1.2 to 10 GHz are feasible, and be-coming more popular. Some HF and VHFtransceivers have special provisions suchas connectors, signal-path switching andT/R switching that facilitate use with atransverter.

VHF TRANSVERTERSThe methods of individual circuit de-

sign for a transverter are not much differ-ent than methods that have already been

described. The most informative approachwould be to study carefully an actualproject description.

The interface between the transceiverand transverter requires some carefulplanning. For example, the transceiverpower output must be compatible with thetransverter’s input requirements. Thismay require an attenuator or some modi-fications to a particular transverter ortransceiver.

When receiving, the gain of thetransverter must not be so large that thetransceiver front-end is overdriven (sys-tem IMD is seriously degraded). On theother hand, the transverter gain must behigh enough and its noise figure lowenough so that the overall system noisefigure is within a dB or so of the trans-verter’s own noise figure. The formulasin the Receivers and Transmitters chap-ter for cascaded noise figure and cascadedthird-order intercept points should beused during the design process to assure

good system performance. The trans-ceiver’s performance should be eitherknown or measured to assist inthis effort.

MICROWAVE TRANSVERTERSThe microwave receiver section of the

Receivers and Transmitters chapter inthis Handbook discusses a 10-GHz trans-verter project and provides references tothe QST articles that give a detailed de-scription. The reader is encouraged to re-fer to these articles and to review theprevious material in that chapter.

Other InformationThe ARRL Microwave Projects CD, sev-

eral RSGB publications and VHF/UHFmicrowave conference proceedingscontain additional information abouttransverter design and construction. Moreinformation about these publications isavailable from the ARRL Bookstore atwww.arrl.org/shop.

or plug types. You won’t hear much with theaudio output shorted to ground!

Does It Really Work?The receiver is direct-conversion, so the

audio you hear is busier than what’s typicallyfound in a big rig. There’s some audio low-pass filtering, but it still doesn’t have thesharp roll-off characteristics prevalent withcrystal IF filtering. Because the D-C receiverreceives both sidebands equally well, thereare twice as many signals as you’d expect ofa more capable receiver. Once you get thehang of selecting which of the two operatingfrequencies to call someone on, the opera-tion is pretty straightforward.

A Thriving CommunityThis project started out as a party favor

and indeed, it was initially dubbed “a wire-less code practice oscillator” –– somewhattongue-in-cheek. Once the first sampleswere available, it became clear that theRockMite was a usable radio. Much of thissuccess can be attributed to the QRPcommunity’s use of watering holes. ManyQRPers monitor those frequencies whenthey’re in the shack and your chances ofsuccess with a “CQ” are surprisingly good.

A gallery of construction pictures, modi-fication information, links and related topicsmay be found at www.qsl.net/n0rc/rm/.There’s also a very active user’s group on-

line at groups.yahoo.com/group/Rock-Mite_Group/.

A special thanks to Doug Hendricks,KI6DS, for his material support with thisproject and to Rod Cerkoney, NØRC, for hisenthusiastic Web site support. Thanks alsoto Steve Weber, KD1JV, for design sugges-tions during the development phase.

1Complete parts kits for the RockMite, includ-ing PC board, all on-board parts, a pro-grammed microcontroller and instructions,are available from Small Wonder Labs(www.smallwonderlabs.com). Pro-grammed microcontroller ICs alone arealso available. RockMite object code (.hexfile) may be found in the Templates sectionof the Handbook CD-ROM.

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