Homebuilt 10GHz SSB Transverter project.pdf

5/12/2015 Homebuilt 10GHz SSB Transverter project

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10GHz SSBTransverter

BACKGROUNDREUSE OF LNBs BLOCK DIAGRAM ANDDESCRIPTIONMIX UNITLO MULTIPLIER92MHz OSCILLATORPA UNIT AND ANTENNA RELAYAUX UNITASSEMBLYUPGRADES TESTS AND CONCLUSIONS

This project by IW9ARO is based on the reuse of commercial SatTV LNBs for Ku band, often sold at lowprices at ham fests or sometimes recovered from dismantled dishes. Those LNBs are a great source of ready-to-work 10GHz RF blocks, that can be combined together to build complex systems with minimal know-howof microwave circuits design.

BACKGROUND

Today, to build a 10GHz SSB transverter is much easier than, say, 10 years ago. This is for two main reasons: there aregood commercial kits on the market for medium-skilled hobbyists, and the significant price drop of 10-12 GHz activecomponents (GaAsFETs, Schottky Diodes) now widely used in Sat-TV equipments. If a Kit is still too expensive foryour budget, there are also a lot of projects to try, but all requiring a quite high level of expertise and instrumentation,plus a good experience in precision PCB design and etching.

But there is a third choice, that I discovered after visiting the interesting website of G4HJW (www.g4hjw.metahusky.net)that shows the many uses of sections taken from commercial Ku band LNBs. There are many other projects fromdifferent sources of microwave projects built with modified LNBs, so you may think that I'm inventing the "Hotwater",but when I opened a Cambridge dual output LNB, I said: "Hey, this can become my first 10GHz transverter!"

This approach requires a deep tailoring of the project to the characteristics of the LNBs you can find, and is not easybecause it needs a lot of time for adjustments and optimization. Really good is that with little microwave designexperience you can get decent results if you have the right instrumentation to verify what you are doing. Perhaps you willnever copy this project, but at least I hope to give some good ideas.

-IMPORTANT- I did not believe, at the beginning, but it is absolutely true; GaAsFETS are veeeeery delicate. Anythingmetallic that gets in touch with their leads (no matter if they are on the PCB or not) can damage them. Use only non-

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metallic tools, hot-air soldering station and ESD precautions (the simplest one is not to wear shoes). Luckily, you can getplenty of low-power GaAsFETs if you have many LNBs to dismantle.

REUSE OF LNBs

This topic is well covered in the G4HJW website, so here I focus more on what has helped me to build my transverter.No matter which are the LNB sections you want to reuse and/or modify, you also need to arrange a solid mechanicalarrangement while providing all necessary coaxial ports. In fact, all LNBs were not originally made to be used as 10GHzsignal processing modules.

My choice has been to put SMA female connectors to arrange thenecessary transitions from the stripline tracks, and at 10GHz it isnot simple. The best solution I have seen is to drill a 1mm holetrough the PCB, exactly at the end of the RF stripline we want toport out. Then, flip the PCB with the ground plane facing upwards,and with a 3mm drilling bit carefully rotated by hand, mill about0.5mm of GND copper around the hole. Prepare a female SMA forPCB vertical mount, removing all 4 GND corner pins and filing thelittle flange to be totally flat. Place the SMA connector with its centerpin through the hole. and the ground flange well flat on the GND plane. Cut the center pin leaving no more than 0.5mm extra lengthover the PCB and solder it to the stripline with minimum solderquantity. Flip the PCB and solder the GND flange letting the solderfill any gap between the GND plane and the flange.With all SMA ports in place, soldered on the GND side of the PCB,you cannot remount the PCB into the original shell, unless you makesome holes for the SMAs. But this can be very unpractical because ofwall thickness and because the feedhorn waveguide (in one with theshell) is always where it should not be... Better to build a customizedcase for the PCB, and here I explain my way to do it. All LNBs I've seen are built as a "sandwich", where the PCB is between the outer diecast aluminum shell and the internal aluminumcover shaped to keep each section shielded from the others. Someself-screwing screws keep those three things together. The principleis to keep this sandwich structure, but replacing the outer shell withan aluminum plate that has been formerly cut and drilled toaccommodate SMA, alignment pins (if any) and the screws. Platethickness can be 2 or 2.5mm, and if you cut it few mm longer thanthe PCB length or width you also get the room to drill the extraholes for screw mounting to a chassis (see picture).

-Hints- To easy locate where to drill the SMA holes in the aluminum plate, I mark the hole centers using the PCB as atemplate just after having drilled on it the 1mm holes for the SMA center pins. You can choose whether to drill roundholes or cut square slots on the plate for the SMAs. I prefer the second choice using a jigsaw and a set of files. PCB material of LNBs is very soft and doesn't like heat too. It is very easy to bend or twist it when the SMA areconnected to the testing instruments, and this will likely damage SMD components. And two further remarks: the screws that now tighten the aluminum plate to the shielding cover (with the PCB in themiddle) can be non-self-screwing, to help mounting and dismounting. Verify if on the GND side of the PCB there areany tracks bringing supply, biases or other signals that will be shorted when the sandwich is assembled. Those tracksmust be covered with good quality insulating tape (I've used Polyimide adhesive tape, that is also needed for otherpurposes in this project).

Another key technique in the art of LNB modification is the "gain block reversal". In all LNBs the signal path goes fromthe antennas to the IF output(s), but in the TX section of a transverter this path is the opposite. The pin arrangement ofGaAsFETs helps a lot to do a clean job: there are two Source pins, with Gate and Drain at the opposite sides of thepackage. This suggests that a GaAsFET can be desoldered and then resoldered rotated by 180 degrees. Yes, theimpedance matching is not the same, but by patient cutting and adding stubs on the striplines you can still rely on 7-8dBgain from a reversed GaAsFET. Of course the Gate and Drain biases must be exchanged, but this is quite easy by cutting and crossing the tracks that gofrom the bias transistor to the RF decoupling capacitors.



In the process of building this transverter I had to lower the resonating frequency of the 11GHz image filters. Thesefilters are often made by coupled tiny halfwave striplines, that cannot be easily made longer. The resonating frequency ofa stripline filter depends on the dielectric constant of the substrate, but a part of the E-field lines also run in air, so thetrick can be to artificially increase the dielectric constant of the air! I easily got good results using little pieces of Polyimideadhesive tape (Epsilon-r ~ 4), sticked here and there over the filter. A single piece covering the whole filter works wellmost of the times, but sometimes the frequency shifts too much and a smaller piece is better. That's real "cut and trial"!

If you like to build a similar transverter, you must understand which LNB type suits well. First of all we have to considerLNBs with dual polarization (they have two LNAs and two mixers), with mixers being of the diode balanced type. It isnot important if they are dual-band or single band because the DRO is unsuitable for narrow-band and it must beremoved, but it is important that it is a dual-output type because there is a PIN diode matrix that can be used as RX/TXIF switch, making life much simpler.

Here is the block diagram of the AE2 LNB by Cambridge Industries that has inspired this project:

Can you see a Transverter? Let me help you:



Well, that's just a part of a complete 3cm Transverter, but the biggest and toughest part is there. LNB modifications arenot easy, but we don't need to design and build the sections at 10GHz that are very critical. The only limitation is that,because of the low Q of image filters and the existing design of PIN diode switch section, a 144MHz IF frequency is toolow. Best would be a 1.2GHz IF, but not all hams (and me either) have SSB radios for this band. With accurate tune-upand some changes in the switch section I got good results with a 432MHz IF, perfectly covered by my FT817.

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BLOCK DIAGRAM AND DESCRIPTION

The Mix Unit we can build with a modified LNB needs few other blocks to make a working transverter. The mostimportant is a precise and low phase-noise Local Oscillator (LO), then we need a PA and an antenna relay. I also addedthe automatic RX/TX switch over the IF cable (requires a modification in the IF transceiver), an output power indicatorthat also serves as a DC voltage check, a remote control connector (for pole mounting in fixed station) and a PLL to lockthe LO to an external 10MHz frequency standard. Here is the block diagram of the complete project:

Let's now see how each section of the transverter has been made.

MIX UnitAs told before, the MIX Unit has been made from a modified AE2 Cambridge LNB. Each block is indicated on thephoto of the PCB (shield is removed), and for some of them I give some comments.



20dB attenuator: it is in place of one of the two INA10386 IF buffers, and works in both RX and TX. It is needed toreduce the power from the IF transceiver to a safe level for the RX/TX diode switch. The attenuation it introduces inRX is recovered by the gain of the IF RX buffer (~20dB). To sustain the 1W max power from the IF (0.5WPEPoperating), the 55ohm resistor is made with 4x220ohm 1/2W SMD resistors in parallel.

RX/TX Switch: the original switch was a matrix arrangement, that has been simplified to a SPDT arrangement. Originaldesign significantly cuts signal below 700MHz, so, to work well at 432MHz, two stripline chokes have been replaced withSMD RF inductors.

6dB attenuator: it is in place of one of the original IF buffers (mark C1H) to further reduce the power from the IFtransceiver to a suitable level for the TX mixer (~2dBm). Low power SMD resistors are OK.

TX Mixer: it works reversed, but no modification is needed because diode mixers are bi-directional. The little piece offiberglass laminate is glued over the stripline balun for a better balance, in a position where the maximum LO rejection isobtained. This trick is not needed for the RX Mixer

Image BPFs: one for RX and one for TX. Both have been re-centered to 10.5GHz using Polyimide adhesive tape asdescribed previously

TX Buffer: it is a gain block that has been reversed as described here. The GaAsFET is not the original one and Drainresistor has been reduced to 27ohm to get a little more power. Two tiny copper foil stubs have been used to optimizegate and drain impedance matching.

TX Driver: another reversed gain block. The GaAsFET is not the original one and Drain resistor has been reduced to10ohm. Luckily no retuning has been needed to reach the target output power (11dBm).

RX LNA: these two stages have not been touched until an upgrade was done. I have just added a 1/4lambda short toGND at the RX IN connector for ESD protection.

IF RX buffer: also this stage has not been touched. Its gain (~20dB) recovers the losses of the 20dB attenuator.

LO splitter: the only modification has been to put the SMA connector to inject the LO frequency. The components ofthe original DR Oscillator have been removed.

GaAsFET bias: apart of the Gate/Drain bias wires swap for the two reversed gain blocks in the TX path, this block hasbeen modified to properly activate the RX and TX RF stages together with the PIN diode RX/TX switch and theAntenna Relay. This modification is quite simple, just requiring to selectively apply the Vbb to the PNP bias transistorsusing the inverters of the 74HC14 (see the two 8.2K resistors and the two brown wires).



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LO GenerationUnfortunately I did not have any LO source ready, like a cavity multiplier, so I had to build a good 10GHz LO sourcefrom the scratch, and this took the larger part of the time I've allocated to the whole project. My approach has been a"counter-stream" design: from the final frequency to the oscillator because I had no previous experience on 10GHzmultipliers and I tend to solve first the biggest problems...

Once determined that the optimal LO power for the Mix Unit was something between 2 and 4dBm, my idea was toreach a 2.5GHz frequency by affordable multiplications, further multiplied by 4 with a portion of LNB implementing asaturated gain stage followed by a stripline filter. This is very similar to what proposed by G4HJW, so I made someexperiments, discovering that this kind of multiplier is quite deaf and critical, but can work, provided that two gain stagesare used and that the stripline filter accepts to be retuned down to 10GHz. With a +4dBm output level at 10GHz,spurious x3 and x5 multiplications were well below -40dB, so next efforts were concentrated on earlier multiplications,oscillator stability and phase noise.

LO MultiplierFor this transverter a good fit is a 5th overtone 92MHz crystal, that multiplied by 108 goes to 9.936GHz: the exact valuefor a 432MHz IF. So the multiplication scheme I chose is:

92x3=276MHz276x3=828MHz828x3=2484MHz2484x4=9936MHz

This is basic schematic of the LO Multiplier:



First two multipliers are a classical design, and have been realized "Manhattan style" on a dual-sided FR4 laminate. The276MHz coils are on the backside, and all other components plus the the 828 MHz filter are on the front side. Last twomultiplications have been implemented reusing some sections of a modified LNB. An interesting point is that the2484MHz filtering is greatly simplified by using miniature monolithic filters for WiFi, that I took from an old dismantledWiFi access point. Two 2484MHz stages were needed to reach enough drive level for the GaAsFET x4 multiplier(~5dBm). This last multiplier is a 2-stage gain, where I added a trimmer to adjust the bias of the two FETs to let themwork as much as possible in a non-linear region. Besides, this trimmer can adjust the LO level to an optimal value: if toolow there will be less TX power and less RX gain, but if too much the LO leakage and the image level will risedramatically. Of course, the image filter frequency has been lowered by putting some dielectric material on it.In later 10GHz LO multipliers I made (for Andrea's transverter and for the CW beacon), I got a much higher efficiencyfor this "LNB multiplier" creating an intermediate "weak" x2 multiplication.

The difference stays in thecoupling between the twoGaAsFETs, that instead ofthe SMD capacitor, is donewith two coupled lines. Thiscoupler is equivalent to acapacitor when the two linesresonate as 1/4 lambda, sothe trick is to make themresonating at about 5GHz.Without any calculation, Isimply took the 1/2 lambdaresonators length of the11GHz image filter, thatmeans 1/4 lambda at halfthe frequency. Then I'veeasily engraved the couplerpattern on the existingtransmission line using acutter, as shown in thepicture. This improvedmultiplier can be driven byjust -2dBm signal at 2484MHz, so only oneMMIC amplifier stage isenough.

92MHz Oscillator



Working at relatively low frequency, this section in not particularly critical, and could be as simple as a one-transistorcrystal oscillator. But for best performance a more complex design is mandatory. The key parameters, as widely describedin literature, are the low phase-noise, the frequency stability and the frequency accuracy. To get the first one I simply tookthe project of a 5th overtone oscillator+buffer by I2SG, who has optimized it for phase-noise performance. Then Ihave added the varicap frequency adjust and a prescaler buffer to phase-lock it to an external 10MHz reference, like agood quality OCXO or my GPS-based Clock Generator. This last would ensure a very high absolute frequency accuracy(around ±100Hz at 10GHz), but to keep a reasonable degree of accuracy when no external source is available, I put asmall TCXO that keeps my transverter within ±10KHz with a very compact design. The TCXO insertion happensautomatically when no external source signal is detected. I've seen in practice the advantages of having a fairly good accuracy without relying on auxiliary equipments during quicklab tests or for line-of sight contacts, but with the possibility to stay within 10ppb when challenged by a long-rangecontact or for beacon monitoring/hunting.

The Oscillator and the PLL have been realized on experimental pre-drilled small boards, mounted inside a tinnedshielding box. An internal shield keeps the oscillator separated from the PLL. The big 92MHz crystal is on the backsideof the oscillator board. Similarly, the TCXO sub-module, that has been added later, has found his place at the backside ofthe PLL board. Room is quite tight, but I like this compact arrangement that still allows a good access to all internalcomponents and adjustments. The TCXO is now replaced by an OCXO following an upgrade work.

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PA Unit and Antenna RelayThe PA unit is built from a part of another LNB. Since only one gain stage is needed, the simplest LNB is the best. Themodification could have been easy, but the 20dBm GaAsFET I had was a flange-mount type and I also wanted to addan RF detector. So I carefully cut a rectangular slot in the PCB exactly the size of the GaAsFET flange (a little, sharpcutter does the job), and now the GaAsFET is tightened to the aluminum plate with two small screws. The RF coupler isdone with a small strip cut from a copper foil, glued to the PCB close to the output stripline. At one end of the addedstrip there is the 50ohm termination, while at the other end there is a microwave schottky diode taken from the unusedmixer of the LNB. During RX, the GaAsFET is forced to cut-off, again by playing with the Vbb of the PNP biastransistor. The GaAsFET matching has been optimized seeking the maximum (stable) power by cut and trial with small stubs cutfrom a copper foil. This technique is normally used when tuning microwave amplifiers, but needs patience and extremeESD precautions (here GaAsFETs start to be highly priced...) . At the end I got the 20dBm with 9dB gain: very close tothe transistor specifications. Here is the picture of the PA unit PCB (without shielding cover):

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A new 2-stages PA has replaced this little boy, and currently output power of the transverter is a respectable 1WPEP. Apicture of the new PA here.

The Antenna relay is an HP8765A I bought on the surplus market (probably aNOS). It is specified at max 4GHz, but on the the same model HP has a "B"version specified up to 20GHz with slightly derated insertion loss and "just"90dB insulation. So at the SNA bench I verified how the HP8765A works at10GHz, measuring <0.5dB loss and an insulation higher than the maximum Ican measure with my bench (60dB). That clearly says how large are the marginsthat HP takes over their specifications and that I got a good antenna relay forthis transverter. Luckily, with Andrea, we bought enough relays to build anothertransverter for him and keep a couple spare... This relay is of the latching type,with the two coils being specified at 15V. This is not a real issue because at 12Vit still works fine (thanks again, HP), but a static drive wastes a lot of currentand causes severe overheating. Therefore the relay coils of my Transverter arepulse-driven by a dedicated circuit of the AUX Unit.

AUX Unit This last block includes a few circuits that are not part of RF processing:- Antenna relay driver- Generation of -5V supply- Remote control and monitoring- Supply Voltage checkThe antenna relay driver gives a short current pulse to each relay coil by discharging the energy stored in a 220uFcapacitor, that was previously charged at 17V (nominal) through a 1K current limiting resistor . The -5V generator is notstrictly necessary, but I've put it to have the negative supply ready when I (hopefully) will add a more powerful PA. Inthe meanwhile, having the -5V rail put in series with the Transverter supply, I can reach the specified 15V relay coil drivealso when the supply voltage drops from the nominal 12V down to 10V. This gives a good margin for portable operationwith a Pb-gel 12V battery. And for a pushbutton check of the battery voltage, a simple indicator circuit drives the samemicroammeter used for RF TX monitoring. Not strictly necessary is also the DC amplifier block, that I have included to remotely monitor the RF TX power whenthe Transverter is pole-mounted in a fixed station. For a complete operation monitoring, also the LEDs drive voltagesare made available at the Supply/Remote connector.

The AUX unit is also a kind of hub for supply rails and for the connections between the various other units. I built it ona pre-drilled experimental board, and is mounted inside the Transverter without any shielding box.



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ASSEMBLY

Being the only 10GHz Transverter in my ham station, Iwanted it to be a "one size fits all". For portable outdoor useit has to be practical and small, but rugged at the same time,able to be powered by a Pb-gel battery, and to be used withor without an external precision frequency standard. Forfixed home station (that means permanently mountedoutdoor, close to the antenna) it has to be small to fit easilyin an hermetic box, and able to be operated/monitoredremotely. Moreover, being an experimental built, it has tobe easily opened for lab verifications/tuning, with eachsection easy to be disconnected and removed for individualchecks/service.

This form-factor is different from the more common "desktop style", and in fact you cannot stack anything on it, but Ifound it more suitable to be mounted close to the antenna. A foldable collar on the back allows to hang the transverter tothe vertical mast that holds the antenna. Alternatively, with a U-shaped bracket, it can be steadily attached to the back ofa dish, and the two front handles that protect the controls and the connectors can serve to steer the microwave beam...

The enclosure I found perfect for the purpose is a 1590D diecast aluminum box by Hammond, that I have used"reversed". All modules, switches, connectors LEDs and the microammeter are mounted on the box cover that actuallyis the front panel, while the box has become in turn the cover. So, when the enclosure is opened, everything comes out inone piece with the panel: you have 5 sides open, therefore all modules and internal connectors are easily accessible whilethe whole Transverter is still operational.

These are 5 views on the insideof the Transverter. You canidentify each block of thediagram.



And the next two photos have been taken with the MIX Unit removed, to show the inner view of controls andconnectors.

UPGRADES

No project is perfect, and homebrewing gives a lot of room for upgrades. Three main parts of this transverter have beenaddressed for improvements:- TX power- RX conversion gain- Replace the TCXO with an high stability OCXO

The original 100mW PA is a single stage with9dB gain, so to reach a significant increase ofpower it has been replaced by a dual stage PA.This time, the LNB reuse was abandoned, and aspecific teflon PCB from an italian hamfriendwas adopted. Luckily, a very small dual stagePCB was available, so it has been possible toreplace the old 100mW PA keeping the samemechanical arrangement. The new PA delivers1W (30dBm) with the same driving power fromthe Mix Unit (11dBm). Its full project isdescribed in another page of this website, sohere we just show how it fits inside thetransverter.

Because of the architecture chosen in the TX/RX PIN switch section of the Mix Unit, the 20dB gain of the IF bufferjust compensates the attenuation of the 20dB patch from/to the IF transceiver. As a result, overall conversion gain in RX

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is just 12dB, that could be low if the transverter is mounted on the rooftop with a long IF cable. To get a littleimprovement, also on the NF side, the GaAsFETs of the 2-stages RX front-end have been substituted by an higher gainpair. Original transistors of the AE2 Cambridge LNB were FHX06LG (10.5dB gain, 1.1dB NF), now replaced by twoNE32584 (13dB gain, 0.4dB NF). Measured conversion gain shows a 4dB improvement given by the new transistors: nota big jump, but not bad without making radical changes in the architecture, and with a theoretical improvement in theNF.

Last upgrade was the replacement of the TCXOwith an high stability Ovenized Oscillator. Inthe starting concept of this transverter, mostlydriven by a compact design, the only referencefrequency source should be external, like a GPSDisciplined Oscillator. Then, as a back-up, asmall TCXO was placed inside the 92MHz PLLoscillator to give a decent accuracy withoutrelying on an external source. Having recentlybought on e-Bay some nice small-sized OCXOby Piezo Technik, I found a little room insidethe transverter to put one of them and finallyget rid of the GPS DO unit when operatingoutdoor. Furthermore, after warm-up, currentdrain of the OCXO is much less than the GPSDO, leaving longer operation time withbatteries. Of course, the OCXO is routinelycalibrated in my lab with the GPS...

All test results of next section reflect the upgrades just described.

TESTS AND CONCLUSIONS

A thorough test has not been possible, because not all necessary instruments are in our hands. For example, RX NoiseFigure should be measured now that new Low-noise transistors have replaced the original ones. The few tables belowgive the main characteristics of the transverter, plus the breakdown of the contributions to overall signal gains or levels.

CONVERTER/LOHam band covered 10368~10370MHz IF frequency 432~434MHz Usable RF BW >10MHz LO frequency 9936MHz LO level 0~5dBm adjustable10MHz EXT REF lev min 2Vpp

RECEIVER

RX NF <2dB based on orig. LNBspec

RX Gain (RF to IF) 16dB TRANSMITTER

TX driver power 11dBm TX power (PA out) 30dBm TX power (ANT port) 29dBm TX LO leakage (ANT port) -20dBm TX Image rejection -38dBIF TX drive level 0.3~0.5W PEP typ 1W CW max safe level

SUPPLYSupply Voltage 10~15V DC

Supply Current (RX) 400mA After OCXO warm-up

Supply Current (TX) 800mA 1W Pout, after OCXOwarm-up



GAIN BUDGET RXAnt relay -1 dBRF LNA 24 dBImage filter -2 dBMixer -3 dBIF buffer 20 dBPIN diode switch -2 dBIF powerattenuator -20 dB

Total RF to IF gain 16 dB

GAIN BUDGET TXIF powerattenuator -20 dB

PIN diode switch -2 dB2nd IF attenuator -6 dBMixer -3 dBImage filter -2 dBTX Driver 17 dBTX PA 19 dBAnt relay -1 dB

Total TX gain 2 dBIF drive level +27 dBm

Ant. TX power 29 dBm

Another transverter, based on the same concepts and similar architecture, has been built by Andrea (IW9HJV). It'sshown working with the IF transceiver and the GPS frequency reference. With these two transverters we have started ourexperimentation on the 3cm bands, establishing interesting contacts. This activity is described in our Ham Radio pages.

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The projects presented in these pages are our own design and have been tested and verified by ourselves at the best we can.However, they might be inspired by concepts, ideas, solutions coming from knownart or free resources on the Web. We providethem as reference designs to skilled hobbyists and technicians who are willing to reproduce them for noncommercial use. Yourresults might be different from ours and we cannot be considered responsible for that. Similarly, we are not responsible for anydamage or injury you might incur while building, assembling or using the equipments, projects or ideas presented in these pages.The firmware embedded in our projects is our property unless differently stated and, when available in the Download Area, it islicensefree only for noncommercial purposes.

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Homebuilt 10GHz SSB Transverter project.pdf