10GHz Station Upgrades

With the 2015 September 10GHz and up contest approaching fast, I am in process of making some performance improvements to my 10GHz station. I had already missed the first round of the August 10GHz and up, so I wanted to make sure to complete a few necessary items before the 2nd round is here.

First on the agenda is to upgrade my PLL frequency synth that runs my LO to use low-noise regulators. This was a design error on my part, when I originally designed these freq synths, they had less than ideal phase noise as a result. The noise itself was determined to be coming from the 78nn series of linear regulators I had originally spec'd out for the design for the 5v vco, and dual 3.3v analog and digital supplies. A test of an unused synth I had built (for a beacon project) proved to show quite an impressive improvement once I replaced the regulators:

ADF4107 Phase Noise

Phase noise is shown here on the left with the original noise regulators, and a much cleaner output is shown on the right after the low-noise regulator replacement.

Next item on my list was to add some additional attenuation into the LO path that drives the mixers. Originally when I had a phased lock PLL brick, LO drive power through both outputs was approximately 12dBm. This was ideal for the two Magnum Microwave mixers I was using. After switching to the PLL synth and x4 multiplier, my output to the LO's through a splitter was a little hot at 15dBm. This was the upper max limit of power in the datasheet. This as a result was causing some additional spurs on the RF side, which while immediately filtered was still resulting in some spur leakage. A 3dB pad in line with the output of the x4 multiplier cleaned the excessive spurs up nicely.

The final improvement is in regards to the actual frequency reference itself for the LO PLL freq synth. I had designed my synth to use an on board TCXO protected by a shielded enclosure for both rf shielding and hopefully help stabilize the temperature. Temperature drift is critical in any design, this one was particularly touchy to temperature. When the synth was running and locked to my 2556GHz output frequency, simply blowing air across the TCXO caused the output frequency to start drifting. The specs for the particular crystal I had chosen were not that great at ±2.5ppm over the specified temperature range, I would definitely need to do better for good stability.

I have a rubidium 10MHz reference I use at my bench. I didn't want to devote this just for my mobile station as I use the reference for all of my test equipment. I would also need to frequency double it to work with my ADF4107 PLL as it requires a minimum 20MHz reference. As a compromise, I decided to go with a crystal oven oscillator (OCXO). I had a 20MHz version available that runs off of 12V which will be perfect for this mobile 10GHz station. Details of this will probably be in a separate post due to the difference being pretty interesting.

Decreasing Phase Noise with Low Noise Regulators

I have posted several times in the past on the PLL frequency synthesizer I designed and built based on the Analog Devices ADF4107. The overall design is a platform for a fractional PLL frequency synthesizer for any frequency range up to about 5GHz.  A single frequency or range could be generated simply by changing out the VCO and loop filter and reprogramming the ADF4107s registers. The design overall has worked very well, I have used it as a LO for a 1.42GHz hydrogen line radio telescope, a 2556GHz LO for my 10GHz ham station, and a 5.4GHz LO for some specific satellite downlinks.

One element of the design that has been less than ideal was the devices phase noise. My specific PLL was on average about 10dB to 20dB under spec of what the documented phase noise should be for similar designs using the ADF4107 and Z-Comm VCOs. After reading to the application notes some more and a recommendation via twitter from Tony (KC6QHP) who suggested looking into using very low noise regulators for the design, I decided to make the change.

Searching regulator semiconductor manufacturers for very low noise versions is not an easy task, often, the noise levels are not available in any parametric search. So to keep things simple, I just went with the ADP150 which is what Analog Devices recommends for their own designs including the ADF4107. Now this is definitely something I should have considered to begin with in the design, but it was my fault for not reading the docs and assuming the basic ST Micro KFNN regulators which I often use would be suitable for a project like this. Looking at the datasheets, the stated noise levels of each are quite a bit apart:

KF33:

OUTPUT NOISE  10 Hz to 100 KHz 50 µV rms

ADP150:

OUTPUT NOISE  10 Hz to 100 KHz  9µV rms

The issue I now have is I had designed the board for standard DPAK package regulators, the ADP150 used tiny TSOT packages. Because of this I would have to be creative in mounting the devices in the DPAK footprints. This turned out to not be too bad of a task although not the most elegant solution.

The results speak for themselves, after replacing the regulators with the ADP150s, phase noise has considerably decreased. I have already started on a version 2 of this synthesizer and I will be definitely switching to these regulators for all future versions.

Standard KF33 and 7805 regulators on the left, low noise ADP150 regulators on the right.

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.

Experiencing the Dayton Hamvention 2013

The Dayton Hamvention was this past weekend (May 17th - 19th) and after many years of wanting to go and not going for various reasons, I finally planned a day to spend there. This was also the first one since I received my ham radio license last year which gave me another excuse to go.

The Hamvention is an enormous ham radio swap meet that has been going on since the 1950s. While ham radio is a huge part of it, general electronic components are everywhere, so it is suitable for anyone who loves electronics. While I would have loved to get there on Friday morning to be one of the first ones looking for deals, prior work commitments forced my visit to occur on Saturday. The day started by leaving my house at roughly 6:00 am from Michigan to drive about 220 miles down to Dayton Ohio. The weather forecast for the day was sketchy at best with a 50% chance of rain all day, I decided to get there as early as possible to hopefully beat any rain. The location was easy to find, just a few miles off of I-75 and the towering antennas made it easy to spot. Upon arriving around 9:30 am, there was no rain and still plenty of parking in the field across from the venue, Hara arena.

After my initial ticket purchase at the front doors, I headed straight for the outside market area as there was no rain at this point and I wanted to start hunting for anything good before all the great items were picked over. Upon walking outside into the huge outdoor swap meet, the initial experience was overwhelming! The size of the outdoor swap meet was just enormous.

I started at the front and started hitting every row of booths on the east lot, followed later by the west lot. There was just so much good stuff! There was a lot of junk there too. Things like late 1990s computer hardware, old giant two-way radios, and random assorted telco hardware from the 1980s. I would say about 75% of the booths (a booth could be just the bed of someones truck piled full of electronics) contained interesting stuff. You would find people with just boxes and boxes of the strangest things. About an hour into the day the rain did start falling, so I browsed into one of the larger tents for a bit.

Meters!

Once the rain subsided, I began more browsing. The selection of hardware and components was terrific. Looking for RF components was not an issue, the stuff was everywhere so I was extremely happy. Everything from new MMICs and cables / connectors to older used random brick oscillators and waveguides.

Later that morning I spent some time browsing the inside exhibits, all the big radio brands were there (Icom, Yaesu, Kenwood, Alinco, etc). The DZKit guys were there too with their Sienna HF Receiver/Transceiver kit which was really impressive. It's a very high-end HF transceiver that you assemble. Most of the smd work comes completed on the boards, another 40 or so hours of assembly is required to complete including stage by state testing.

Mini-circuits had a booth there, being one of my favorite RF component suppliers I had stopped by to talk to them for a bit. I also talked with the Society of Amateur radio Astronomers along with the AMSAT people. I was happy to learn that their upcoming Fox-1 Satellite has an actual launch date scheduled in 2014. This satellite will carry a handful of experiment payloads along with the best part, an FM repeater. It will be nice to have a second working FM sat available besides SO-50.

A little after noon I met up with Chris Gammell of The Amp Hour podcast, it was the first time meeting him which was awesome after listening to the show for almost two years. We browsed around the booths for the rest of the day talking about everything electronic and looking for bargains. A little bit later that day we also met up with Dr. Gregory L. Charvat who was a guest on The Amp Hour which was equally as awesome. I had a bunch of questions for him regarding my radio telescope project, he was really informative and a great person to talk about RF to while browsing random electronic bits.

Towards the end of the show at 5:00 we started looking for any last deals before people started packing up their gear. While I didn't find anything, Chris came across an old HP frequency generator in perfect condition for $10. A lot of exhibitors who were not sticking around for Sunday starting just dumping stuff they didn't want in the trash. Some of it was scavenged by people, but most of it wasn't worth carrying back to the car even for being free.

As for my own purchases, I only ended up buying a few things: some SMA hardlines and cables, a nice brass WR-90 waveguide about 16" long (perfect for a 10Ghz slot antenna), a few mini-circuits attenuators, and a large lot of aluminum hardware (hexagonal standoffs and such). Looking back, I keep thinking about things I should have bought, but did not. I definitely needed more than a single day for this!

After 5:00 when the show was over for the day, a bunch of us headed over to a local bar which Chris had previously planned. There were about 20 people that had showed up total, most are other fellow Amp Hour listeners which was very cool. Shortly after 6:00 I decided to start heading home as I was completely exhausted, but not before Chris tore into his frequency generator to take a look inside.

Lessons learned:

Next year, arrive on Friday morning. I felt like a lot of booths tables had a lot of empty spots where gear had been sold. It seemed like everything good (test equipment and radios) was already gone by Saturday morning.

Bring a rolling luggage cart. I had one but forgot to throw it in my trunk. If you purchase anything heavy, it's a long walk back to the car.

And definitely spend more than one day there. A single day is not enough time to see everything. I probably only saw a quarter of the inside booths and still missed a bunch of stuff outside as well.