Wednesday, November 12, 2014

10GHz Transverter Design and Testing


Ham radio operation on 10GHz and above is one of the many things that finally persuaded me to get my ham license. I love RF design, specially in the microwave bands, so allowing me to do so and at the same time travel to hilltops to work line of sight contacts just sounded awesome.

10GHz is the easiest place to start as surplus (and new) components are plentiful. Most of my station was built through surplus parts purchased on eBay and from the Dayton Hamvention. The rest I milled and assembled out of various parts I had laying around.

The best part about 10GHz and up is there is no off the shelf hardware available to work these bands. There are some commercial bits available such as Kuhne transverters and such that can simplify the design greatly, but you still have to homebrew the rest of the radio. This is what makes 10GHz and up so neat, there are no two radios that are alike. Each and every design is different based on the components available and how you want to make it. With most of the assembly complete, here is my stations final build:

KD8TBD 10GHz Radio

My design is based on the following:

10GHz transverter
Please excuse the crudeness of the above block diagram, I have a better one somewhere, but this one is pretty accurate to the final design. The design in its simplest form consists of a mixed upconverter / downconverter handing the signal transversion between a 144Mhz IF to the 10.368GHz RF. A PLL brick oscillator provides the 10.224Ghz LO signal. RF switches with a custom designed TX/RX sequencer handle the path switching as well as the power switching for the power amplifier.

The transmit and receive paths each have their own device chain separating them from each other. I went back and forth on this decision as I could have easily used a single mixer with additional RF switching, but I chose the dual paths as it actually simplified the design and I had the mixers available. So with this decision, each path contains its own mixer, both driven by the same local oscillator.



The local oscillator itself is a PLL brick that uses the x13 harmonic off of an ovenized crystal. The original output frequency was slightly off of 10.224GHz but was tuneable to my desired LO frequency. A nice feature of this brick is that it contains two outputs, each which provides more than adequate drive of about +13dBm for both mixers. A downside of this LO is that its stability is not perfect. Warm up time takes about 10 minutes or more until it has a stable frequency without drift. Another issue it is is very difficult to tune precisely which results in it being a few KHz off of my desired frequency. This ultimately plagued me during testing.

RF switching between each pathway is handled by a SPDT failsafe RF switch good up to 18GHz. A  switch is located on each end (at the antenna and radio). Being failsafe switches, upon any failure of the TX/RX sequencer the relays will fail to the TX side preventing me from accidentally transmitting into the RX path. The switches need +28VDC drive to function which is provided by a small DC to DC boost power supply.

Filtering occurs in many places and is a necessary requirement for this device to function. The most critical filtering has to occur out of the RF side of the mixers before the PA to allow the 10.368GHz RF signal to pass while blocking the 10.224GHz LO mixer leakage among other spurs. Additional filtering is also needed at the output of the RX mixers IF side to filter additional spurs and other high freq signals along with filtering on the RF side of the RX amplifier.

Low frequency filtering is relatively simple to accomplish as off the shelf filters for 144MHz are easy to source. The 10.368GHz filters are much more challenging. Many solutions are out there including building copper pipe cap based filters. I tried building a few of these with somewhat success. The filters did work but tuning was difficult and the passband was much wider than I wanted resulted in some 10.224GHz leakage. They have been proven to work and with some more tweaking (adjusting probe length and spacing) along with cascading some of them in series I know they could be used, but I ultimately decided to go with a different solution. I came across a couple nice Harris Farinon 10Ghz cavity waveguide filters that can be tuned to 10.368Ghz and have a very narrow passband. Based on the excellent performance of these filters they would be used in the final design.

10.368GHz Waveguide Cavity Filters

Due to the fact that this entire setup will be portable,  power would be provided by a 90 amp hour 12VDC sealed AGM battery. I have several of these and they are terrific mobile power sources. This would power both my entire transverter along with the Yaesu FT-290R II radio. Many voltages are required to power all of the necessary components within the transverter so a DC to DC boost converter is used along with various linear regulators to provide the necessary components their required voltage and current.

Automatic TX /RX sequencing was a necessary requirement for this transverter to function, having to manually switch the signal chain before each transmission was just not a feasible option. To do do this I would need a 2M radio capable of indicating when it was transmitting so I could interface directly off of it. I chose a  Yaesu FT-290R II for this transverter as I already had one and it is a terrific all mode 2M portable radio. An options with this radio was an additional external amplifier that clipped onto the back of it which was good as it provided a way of indicating when the radio was switching into TX for this external amp. I used this output to drive the input on my sequencer.

RF TX / RX Sequencer
The sequencer is very basic, it's just detecting the TX logic from the Yause to switch some relays for the RF switches and PA power in order. It also provides some led status indicators to verify its operation. There are two additional locations for relays that are not used, they were for the original design using four RF relays instead of two.

Amplification occurs at two points, a preamp directly off of the RX chain from the antenna and a power amp at the end of the TX signal chain. The RX preamp is a harris unit with +33dBm of gain at 10.368Ghz. The power amp is another Harris unit that can provide up to 100mw of output, this is lower than I had hoped but it will do just fine for now. I have 10GHz isolators on the amplifiers to prevent any stray reflections from coming back into the amps.

Physical construction consists of a small metal enclosure to house all of the transverter components. The box is white in color to reflect sunlight off of it during the day to help prevent it from going up in temperature. Inside, most RF components are mounted on a 1/4" aluminum sheet. I milled various other blocks and mounts to hold the other remaining components. This provides a very solid platform for everything in addition to being an adequate heatsink for the RX amplifier mounted directly to it. The PA amplifier is mounted directly to the back of the enclosure and mounted to a nice black Foxconn aluminum heatsink that came off of on old Pentium III Xeon processor. I went oversize on the heatsink as I plan to eventually replace the PA with a more powerful one. The front panel has a volt and amp meter and necessary power switches. The additional pushbutton is for manual RX / TX operation if for some reason the sequencer was not working.

The dish is a RadioWaves 18" aluminum dish with 37.8dBi of gain. It was originally designed for 43GHz operation so I had to cut off the front cover and remove the internal 43Ghz cassegrain feed. I then mounted a WR90 waveguide-to-coax transition with a small feed horn at the feed point with three brass rods. The mount is very rigid and allows for some flexibility in adjusting the feed to be right on the focal point. Very low loss Suhner Sucoflex microwave coax is used to connect the feed to the back of the transverter. The Yaesu radio is mounted directly to the top of the transverter via some velcro.

For mobile use, a motion picture camera tripod was used to mount everything together. It is a perfect mount as it is extremely sturdy but very lightweight since it is made completely out of aluminum. I usually will strap the battery placed underneath it to the bottom support of the tripod to give it some extra stability on windy days, but it has worked excellent for mounting the transverter and antenna. Setup and teardown is very fast as it is all held together with a few threaded rods.



A future post will discuss my first real testing of the unit during this past Septembers ARRL 10GHz and up contest, overall I was happy with the first testing as it almost worked really well. This was a good first real world test that isolated some issues including implementing better filtering and a more stable LO. The winter months will give me some time to get everything perfect before the spring.




Wednesday, May 7, 2014

APRS Station On a Boat - Part 1

Since purchasing my boat last year I have been wanting to add a 2m radio on it to access local repeaters. Recently while playing with my Yaesu VX-8DR HT  I recent began to experiment with APRS. APRS functionality is built into the radio and is fairly easy to setup. While its usefulness with the built in antenna is minimal I was able to reach some digipeaters while outside with the radio. Connecting the radio to my roof mounted 2m J-Pole resulted in a huge improvement, the coverage area was significantly impressive with this setup. At this point I was hooked, I wanted APRS on my boat, if anything it would allow family and friends to see where I'm at via aprs.fi.

For the new station, I wanted a simple reliable system with a permanent and rugged 2m mobile for use on my boat. The Alinco DR-135 was a contender as it has a TNC built directly into the radio. Another unique feature is it has the capability for an APRS module to be directly inserted into the radio itself replacing the TNC. With this installed and configured, I could pull NEMA 0183 data right off my SIMRAD NMEA network into the radio making it a standalone APRS device. Unfortunately none were on the used market at the moment so I will be keeping watch for one in the future.

The next option I liked would be for some older Yaesu mobiles including the FT-2400 and FT-2500m. Both of these radios are built better than the Alinco to military standards and have all necessary external hookups available through the 8pin RJ45 mic connector. This would make it an easy single cable interface to my TNC and laptop. Another option is the Yaseu FT-2600m which I ultimately purchased as I came across one for cheap. The 2600m does not use the older RJ45 mic connector, but it does have a DB9 serial connector on the back which also contains all the connections available for a TNC.

Another benefit to the three Yaesu radios is that they can be modded to allow TX from 134Mhz to 170Mhz allowing them to be used as a backup marine radio in an emergency. Transmitting on the marine band through the 2m antenna would not be a great idea, but since the marine and ham radio will be near each other, I could quickly swap antennas to my marine band antenna if the need were to ever occur.

For the TNC I ultimately decided on the TNC-X. It is available in kit form, is inexpensive, supports the KISS protocol, has a USB interface, and has great reviews. I hadn't built a kit in probably 15 years, but I found this kit to be a simple build taking about an hour.

For APRS software I chose Xastir. I found it to be the best Linux based APRS application available and is available built in the Fedora 19 repos, although I ended up building an rpm for the latest version anyways as the builds currently available were a few versions older. For testing, I plugged both my TNC-X and a Garmin GPS receiver into my laptop, fed the speaker output of my Yaesu ft-8100r into the TNC-X temporarily and configured Xastir for the two devices. Once setup I began immediately receiving APRS message data.



Right now I have been spending most of my time with clearing and de-winterizing my boat as I have a scheduled date with my marina next week to place it in the water, so not too much time will be spent installing the station until it is in the water. Once I do set it up, it will consist of a 2m marine antenna going to the Yaesu FT-2600m, the TNC-X, NMEA-0183 GPS data from my existing network, and a laptop running Xastir. While definitely not as compact as the Alinco solution would be, I always have a laptop on my boat anyway so it will not be a big deal.





Monday, January 13, 2014

Tektronix 2445B Capacitor Replacement

Due to the fact that my Tektronix 2246 is currently out of commission (although parts are on the way as I did find a parts unit 2246 scope) I have focused my attention to my other favorite Tektronix scope, the 2445B. I primarily use my 2246 as it was the first 'real' scope I purchased about 10 years ago and is a rock solid analog scope. A few years back I came across a 2445B for a good price and have used it mostly for portable use as it is slightly smaller than my 2246. My 2445B also has Tektronix calibration stickers intact from 2002 indicating at least that no one has been inside mucking around with it.

My specific 2445B is the 150Mhz version and is in excellent condition although there have always been a few issues. The on screen display would jump around and not have a clear focus regardless of the focus adjustment and other internal astig calibration controls. When changing volts or time functions there would also be erratic display behavior. Knowing that the scope was over 20 years old and has unknown operational hours, I decided to start with a full capacitor replacement of the power supply. Electrolytic capacitors do go bad over time from several factors, so replacing them is a common step in all old electronic equipment.

The first step is to take it apart and inspect it. I will say the 2445B is one of the better scope designs I have worked on from Tektronix. The design is very modular with the power supply being very easy to access. Once removed, the power supply pulled apart into two boards shown here:

Tektronix 2445B Power Supply
There are quite a few caps between the two boards, luckily most are radial caps and not axial. As I began inspecting the caps themselves I was surprised to see that some were rated for only 85 Degrees C instead of the higher 105 Degrees C rating. I also noticed that some were definitely showing some 'bulging' which is usually indicative of high heat and ultimate failure, although none were leaking. Some other things I noticed were that all of the high voltage film capacitors looked stressed. They had slightly bulged as well and had crack marks on their casings. A few resistors also looked bad including two 15 Ohm ones within the surge protection circuitry in the line level input stage. One had actually blow open and had a physical crack in it.

Additionally looking at the A5 Control pcb I noticed that there were four surface mount electrolytic capacitors on it. I learned how leaky these caps can be after a complete Tek TDS-420 restoration which needed a total of about 75 of them replaced. Looking closely at the board I could already see they were leaking and causing corrosion to traces, so they would be replaced as well.

Tek 2445B A5 Control PCB Caps

Ultimately I decided to replace every capacitor on the power supply boards and the four smd caps on the control pcb. I would leave the caps on the A1 main pcb alone for now as they are primarily low voltage and would be considerably more work to replace. If there was still issues post-recap I would then consider replacing them as well.

For electrolytic capacitors I always go with Panasonic 105 Degrees C units. I will use Nichicon 105 C unit if a Panasonic value is not available. The film capacitors were replaced with Kemet units which were actually also the originally spec'd components. I upped the blown 15 ohm resistors to 3W units. For those interested in re-capping their own Tek 2445B (or variants) here is my Digikey parts list:

Quantity Part Number Description
6 P13476-ND CAP ALUM 100UF 25V 20% RADIAL
2 P13125-ND CAP ALUM 47UF 25V 20% RADIAL
4 P13123-ND CAP ALUM 330UF 25V 20% RADIAL
2 P13131-ND CAP ALUM 220UF 50V 20% RADIAL
1 P13465-ND CAP ALUM 4.7UF 50V 20% RADIAL
1 P5874-ND CAP ALUM 3.3UF 450V 20% RADIAL
4 P10769-ND CAP ALUM 10UF 100V 20% RADIAL
4 493-10252-1-ND CAP ALUM 1UF 50V 20% RADIAL
2 P15W-3BK-ND RES 15 OHM 3W 5% AXIAL
1 989-1206-1-ND RES 270K OHM 3W 1% AXIAL
2 P4639-ND CAP FILM 0.068UF 250VAC RADIAL
1 399-7494-ND CAP FILM 10000PF 250VAC RADIAL
2 399-5410-ND CAP FILM 2200PF 250VAC RADIAL
1 EF2563-ND CAP FILM 0.056UF 250VDC RADIAL
2 UKL1E101KPDANA-ND CAP ALUM 100UF 25V 10% RADIAL
2 PCE3777CT-ND CAP ALUM 33UF 10V 20% SMD
2 PCE3833CT-ND CAP ALUM 10UF 35V 20% SMD
2 493-1421-ND CAP ALUM 330UF 250V 20% RADIAL



Most of the Tektronix 24NN family (2445, 2455, 2465, 2467, etc) has the exact same power supply, so the component list above should apply to all. Here are the rebuilt power supplies once all components arrived:

Tek 2445B Capacitor Replacement
Once completed there were about 45 components in total that needed to be replaced. While the large Sprague 290uF caps were probably were fine I replaced them for good measure anyway.


Tek 2445B Capacitors

All completed and reassembled my 2445B has a beautiful display and extremely sharp digital readouts. I don't see any need to replace the A1 pcb caps at this time but will still consider it for the future. With a total component cost of $31 and about two hours of time I would recommend this rebuild for any original 24NN scope.

Rebuilt Tektronix 2445B



Monday, January 6, 2014

How I Accidentally Destroyed my Tektronix 2246

I have talked many times about my Tektronix 2246 oscilloscope in the past as it is by far my favorite analog scope. Simple, reliable, and inexpensive and was the first 'real' scope I had purchased used about 10 years ago. So to destroy it, all I had to do was take the enclosure off.

All Tektronix scopes of this era usually have handles attached to them for portability which is a nice feature. My problem was that I never take it anywhere, it stays firmly situated in the center on my bench as it is my go-to scope for most basic troubleshooting. Having the handle attached resulted in one problem, it prevents you from easily stacking anything else on top of it. With an ever growing bench of test equipment I really needed the real estate on top of it, so the handle had to go.

Removing the handle requires unfortunately more work than necessary. There is no external bolts to remove to take it off, instead the bolts attaching it are on the inside requiring the entire enclosure to be removed. While somewhat annoying, the handle has a well designed strong attachment to the scope. If I were to transport my scope around I would have no concerns of the handles integrity.

Tektronix 2246 Handle Mount Inside the Enclosure


Normally the covers of all scopes in this era come off easily, typically there are 4 screws on the back of the scope and maybe one or two on the sides. Once removed the back then falls off and the entire cover slides off the back. Normally I place the scope standing vertically and lift it straight up via the faceplate while the cover then remains on the bench. My Tektronix 2246, 2445B, and TDS-420 all have this very similar design (and have removed the later two without consequence).

So what went wrong? There was unfortunately a few dents on the bottom of the units case that I did not consider to be an issue. When I began sliding the cover off, it slid about 6" or so and had some resistance. Not seeing anything to be a problem I pulled a little harder (not excessively) and the cover did continue to come off as normal. Once removed I saw a small round heatsink laying on the bench. That's odd I thought, as I don't recall hearing anything rattling around inside previously. Once I looked to see where it came from I realized what had happened.



The dents on the cover that were pushed in had snagged a transistors heatsink on the bottom pcb. This had then put force on a TO-39 transistor ripping one of the leads off of it.



The transistor itself was not salvageable as the missing lead had been physically torn out of the can. Looking into the part itself it looks to not be an easy replacement. The markings on it reference it to a custom Motorola / Tektronix part number of 151-0846-00.


Searching for this part I discovered two things. 1. I'm not the first person who this has happened to. 2. There are no easy to find replacements for this part. The part is critical to the scopes operation as it will not power up without it in place.

The part cross references to a SRF5286 or 2N3866A. The 2N3866A is possible to find and purchase inexpensively, but reports from others say that since it is not an exact replacement for the original Motorola part, it will not work. I'll probably give it a try anyway and also be looking for a 2246 parts unit in the meantime. 






Tuesday, December 3, 2013

ADF4107 PLL Frequency Synthesizer Part II - 1.35Ghz LO for Radio Astronomy

Earlier this year I built the first version of my frequency synthesizer which was to provide a stable local oscillator frequency of 5.4Ghz. It was based on an Analog Devices ADF4107 PLL chip paired with a Z-Comm VCO. Control is provided by a Microchip PIC 18F14K50 microcontroller. The original design I had made was specifically for a Z-Comm V940ME02 VCO to provide the 5.4Ghz LO source that I wanted to use for downconversion of various amateur satellites (FITSAT-1 being one of them). The ADF4107 is a very versatile frequency synthesizer with 7Ghz of bandwidth. Using it I wanted to leave my design flexible for other frequencies for various other designs requiring stable local oscillators. With this in mind, I stuck with the Z-Comm VCO mini-14 form factor to give me the flexibility for many other frequency ranges including the one shown here.

Another design feature is the addition of a serial interface. Analog Devices ADIsim PLL software provides the tools for programming either fixed or tunable PLL designs. While the two units I have now built do not require tuning as they are fixed frequency, having the possibility of tuning or simply a status of frequency lock via RS232 was a nice addition.

The next unit I have made provides a fixed 1.35Ghz LO source that I will be using for radio astronomy. 1.35Ghz will provide a 70Mhz IF from the target 1.42Ghz hydrogen line frequency. Only a few modifications were needed on this second unit from the original design:

1. A new VCO had to be chosen, a Z-Comm V602ME15 was selected with a tunable range of 1100Mhz to 1400Mhz.
2. A new loop filter had to be calculated which ADIsim PLL was able to do for me. Several high frequency capacitors and thin-film resistors were used.
3. The PIC had to be programmed to write the correct register values to the ADF4107. The PLL calculator on Analog Devices website was attempted to be used for this. Interestingly its calculated values were not working as it would not lock with them in place. After manually calculating the values out and programming them in I was able to get a successful lock.
4. The output filter had to be replaced. A 5.4Ghz bandpass filter was easy to source in the original build as that is used in 5Ghz wifi access points as part of the 802.11A and N frequency ranges. Unfortunately a filter centered on 1.35Ghz was not easy to find. For the time being I have bypassed the filter on the board which did end up with a spur on its output (will discuss more in a bit).


ADF4107 Based 1.35Ghz Local Oscillator

While testing the original 5.4Ghz version, I was limited to measuring its performance by my test equipment. I was able to measure the peak frequency output via my EIP 18Ghz frequency counter and its RF output could be measured by my Boonton microwattmeter. Unfortunately my best spectrum analyzer only goes to 3Ghz, so spurs and phase noise would not be measurable. With my new design on 1.35Ghz, these measurements are now easily obtainable.


1.35Ghz LO
Once the ADF4107 registers were programmed correctly, I had an immediate lock and very clean RF output. The Z-Comm VCO has a rated output of 7.5dBm. I have a small pad on the VCO output to both feed the loop filter and stabilize the VCO which results in a final output of roughly .70dBm. Driving my mixer will need a slight higher output so an external amplifier will be used.

Now to check phase noise, I zoomed into a 100Khz and 10Khz span respectably:




Using the 10Khz span to calculate phase noise, my results are -65dBc/Hz. Not quite as good as I would like, although adequate for my needs. There still may be some performance I can get out of the design by adjusting some of the other registers within the ADF4107. 

I am still very happy with the output, the results are a very clean peak near perfect on frequency with no noticeable drift. Looking at a full span of 10Mhz to 3Ghz, there is one noticeable harmonic at the 2x frequency of 2.70Ghz. Due to the fact that I am not using my onboard filter. I will have to add an external lowpass filter to remove it. 




Saturday, June 8, 2013

Analog Devices ADF4107 Based RF Frequency Synthesizer

I have had a few projects recently needing a stable local oscillator (LO) source for downconversion, notably my hydrogen line radio telescope and a few receivers for low earth orbit high frequency satellite transmitting in the 5.8Ghz range. I have attempted the use of various VCOs along with a very stable power supply to produce a stable LO frequency, but this over the long term has proven unsuccessful. My previous designs used a  precision adjustable voltage regulator trying both analog and digital precision trim-pots, but temperature drift among other things resulted with it being too difficult to keep a stable frequency. These solutions would need constant calibration which proved to be unreliable for a true stable LO source. An alternate good solution could be the use of a precision frequency synthesizer / phase-locked loop from a reference source to produce a stable frequency. Researching frequency synthesizers, I came across the Analog Devices ADF series of PLL frequency synthesizers which I ultimately ended up choosing for my design.

I wanted the design to be flexible, that is with a single board I could populate it with a variety of VCOs only having to update the ADF4107 register configuration and the loop filter component values to produce a fixed or tunable frequency. My final design achieved this goal.

ADF4107 based frequency synthesizer
This design was based on the Analog Devices ADF4107 frequency synthesizer. The ADF4107 consists of a low-noise phase frequency detector that uses a programmable reference divider and prescaler to run a VCO via a precision charge pump to produce a stable locked frequency from a fixed reference oscillator. There are many variations within the ADF frequency synth family, I chose the 4107 as it met my needs with a maximum frequency of 7Ghz.

The ADF4107 can utilize a variety of VCOs, I chose to go with a Z-Comm device which I have used in many previous designs. The specific VCO used in this version is a V940ME02. It has an adjustable output frequency range of 5220 to 5420Mhz which fit well into the 5.40Ghz LO needed for my downconverter. The reference input to the PLL is provided by a 32Mhz TCXO crystal, which was a last minute change as I ordered the wrong package type for the original 20Mhz TCXO I had specified for the design. Only some reference counter changes were needed to use the alternate crystal that I had in my parts bin.

The ADF4107 itself is a very complex device looking at both its usage and programming. To help integrate this family into your design, Analog Devices has a great tool called ADIsimPLL to assist with the initial design regarding calculations of output frequency range based on the reference frequency input. It will also assist with selecting the loop filter component values based on a given design among many other useful things.

ADIsimPLL screenshot
My ADF4107 synthesizer is controlled via a SPI bus from a Microchip PIC18F14K50 microcontroller. After the initial power up of the ADF4107, multiple registers must be sequentially loaded to define the prescaler counters among other configuration flags. The PIC stores and handles this configuration. I have also included an external serial interface into the design to allow remote tuning and control of the frequency synthesizer via the PIC, although in this specific design it is not being used as it only needs to provide a single hard-coded fixed frequency.

The design itself follows good mixed-signal design practices. The board is a four-layer design that I had received from my favorite fab house, OSH Park. Upon receiving, I populated by first board by hand and had a working board on power up. ADF4107 register programming took a bit as the specific loading order of the registers was tricky, but once mastered, the PLL immediately locked onto 5.40Ghz according to my frequency counter. I couldn't view it directly on my spectrum analyzer as it only goes to 3Ghz, but the frequency counter was nicely stable at 5.4 Ghz plus or minus a few hundred hz. Each subsequent PLL lock is almost instantaneous. I added two LEDs to some spare PIC outputs, one of them is used to indicate when there is PLL lock and a spare that can be used for any additional status I wish to see.

An output filter is included in the design, in this case it has a center frequency of 5.4Ghz. Filters of this frequency are easy to find as they are used a lot in 802.11a WLAN hardware. I did not include an output amplifier on this board as I had planned to keep the design as simple as possible, instead I would be using an external amplifier. This will need to be the case as the output of this VCO at 5.4Ghz is -15dBm, which is not high enough to provide a LO for most mixers. So on any future redesigns, I may add an amplifier back in.








Sunday, June 2, 2013

Lowrance Globalmap / LMS GPS Antenna Replacement Modification

The older Lowrance Globalmap and LMS series of marine electronics are terrific GPS receivers. The only common issue I see with them is that the GPS antenna units for them often go bad. The antenna are about 4" round white pucks that you mount high up on your boat that actually contains the GPS antenna / receiver along with an optional DGPS receiver. This then connects to the back of your Lowrance unit supplying power and communication to the antenna from the head unit itself.

There are two series of these GPS antennas depending on how new your Lowrance unit is. The older ones are NMEA-0183 based while the newer stuff is NMEA-2000 based (most models support both). The advantage of the older 0183 protocol is that is uses a simple RS-232 serial interface, while the 2000 protocol is CAN-bus based (not that this is bad, CAN is a great protocol, but for the point of interfacing these units the RS-232 protocol is significantly easier to work with). Both of my units (A Globalmap 3000 and LMS-480) support NMEA-0183.

Repairing the antennas themselves is a huge pain. I have a dead Lowrance LGC-3000 GPS CAN bus antenna that I cracked open to take a look inside, these units are definitely designed to not be opened, it took some serious cutting to get inside.

Inside Lowrance LGC3000

Nothing unexpected found, a basic GPS receiver, small micro, and an impressive watertight enclosure. Typically I see the micro and/or GPS receiver itself is dead. I'm not sure if heat is the cause or possibly static discharge is, but repairing usually isn't really feasible. Just getting into the thing pretty much destroys the nice water-tight enclosure.

So what to do if your GPS antenna is dead and don't want to spend $100+ for a new one? Simply use any GPS receiver that outputs NMEA-0183 data and feed it directly into the Lowrance spare serial port. This is pretty much every GPS receiver / module ever made.

Each device within the Lowrance family has an optional NMEA-0183 input (and sometimes output depending on device) that allows you to daisy-chain units together. So a single unit connected to the GPS antenna can send the GPS information via NMEA-0183 through multiple units allowing you to need only one antenna. Now the cool thing about this is in the Lowrance configuration menu, you can set which serial input you wish to receive the GPS information from along with what GPS strings you want to receive. This means you can feed NMEA-0183 GPS strings directly into the units spare serial input from any 0183 compatible receiver, not needing to use the Lowrance expensive antenna.

My solution as to use an old 12 channel Cirocomm receiver that I have had laying around which cost about $12. This receiver runs off of 3.3V- 5V and outputs TTL level NMEA-0183 GPS strings. The original GPS  module in the LGC puck can also be used as long as it is not dead itself. To use this with the Lowrance, I added an RS-232 level converter and small power supply to run directly off of 12V. I had these boards already designed and built for Xbee use, it was just a matter of wiring in the GPS unit to make it work. The only downside of this is you need to power the GPS unit directly as the Lowrance does not supply power on the spare serial interfaces like it does on the direct GPS interface. The serial interface requires only two wires, TX from the GPS antenna to RX on the Lowrance unit and ground. You could also interface this into the GPS input connector on the Lowrance, but if you are missing the antenna and cable, wiring to the spare serial interface avoids sourcing the expensive twist-lock connector. Once the new receiver is put together the only thing remaining to do is to mount it in a sealed watertight enclosure, something nice looking so it doesn't look stupid. Also preferably something plastic and white to reflect heat and be transparent to the GPS signal.


To use, set the Lowrance to use the NMEA-0183 input for GPS, then set the correct baud rate and GPS strings your receiver supports ($GPGGA and $GPGLL are the important ones). That is all, the Lowrance should respond indicating it is receiving GPS data and begin to populate you satellite information screen. Multiple units can then also be daisy-chained in the exact same manner.

Lowrance Globalmap 3000 NMEA Config
My Cirocomm GPS unit defaults to 4800 8N1, which is fine and sends $GPGLL, $GPGGA, $GPGSA, and $GPRMC strings, so make sure those are selected in the GPS configuration. If daisy-chaining, make sure the NMEA outputs are also enabled and set to the same baud rate on the sending units.

Lowrance LMS-480 NMEA Config
Note that again almost any GPS receiver can be used to supply GPS information to the Lowrance. Something like an old Garmin GPS 12 also works great as a spare if your antenna dies, simply take its NMEA-0183 output and feed it directly into the input of the Lowrance in the exact same manner.

Both units are daisy-chained off of my GPS receiver. Note testing was in my basement, so no GPS lock here