Anritsu MF76A Repair - Part I

I recently acquired a nice Anritsu MF76A 18GHz frequency counter in a non-working state off of eBay. While older, this is still a very usable counter that goes for quite a bit of money when working. This one sold for cheap so I figured why not, I'm sure I'll be able to get it working. I didn't really need it as I have a good EIP 545A 18GHz counter, but this one looked fun to fix. The images of it indicated that it was powering up and had the cpu portions functional as the display went through its power up cycle and displayed all zeros on the display. Typically with frequency counters in this state and not counting, you either have a failed 10MHz reference or a damaged front end from over-driving the input. I was hoping for the former as I already have a stable rubidium reference I use with all of my gear.

Anritsu equipment is notoriously difficult to repair due to the lack of service manual availability. I ran into the same issue with my second MT8801C communication analyzer / spectrum analyzer that I had purchased in a non-booting state. I was ultimately able to acquire a service manual for it but it took some serious searching and asking around. Once I obtained it, it was nothing like the older HP and Tektronix manuals I was used to. This manual went through a basic troubleshooting process to identify the issue to a board level, no further schematics or information was available (in the manual I had anyway).

Upon arriving, I powered this MF76A up on the bench, it was in good overall condition and did power up as it was supposed to. Upon applying a RF source to either the 200MHz or 18GHz inputs resulted in no counting. In a way this was good, it could indicate an issue further down then the first converter stages. The back of the unit has a switch to either output the internal 10MHz reference or accept an external one. Checking this with a counter, the internal reference is working as expected. Damn. Time to open it up.

The back of the unit had the calibration stickers broken which was expected, this came from a calibration house / test equipment reseller and I'm sure someone mucked around inside with some attempt to repair it. With covers off, everything looked at least complete, this unit was constructed very nicely.

The power supply is tucked into the back of the unit, not the easiest to reach to check voltages, but the power comes off of it and is transferred to the backplane which has wider traces for increased current making it easy to identify. There are no voltage markings or test points on the boards but it was pretty easy to determine which were power. Probing voltages, everything looked fine except for one strange voltage reading which was around +6.8 volts. Also noticing that a -12V rail was present but no +12, I was betting that something simple like a bad cap was holding the +12V rail down to +6.8 volts, very similar to my EIP 545A repair.

Pulling all the boards and reinserting them to locate the source of the issue resulted in the +12V voltage drop being caused by the input IF brick at the front of the unit. This was fed directly from the 18GHz input, the 200MHz input bypassed it and went right to one of the digital boards. As a test I left the 18GHz section unplugged and ran a 50MHz source into the other input. At first there was no response but I realized that one of the coax connectors coming off of the 200MHz input was not connected in the correct location. Whomever had been in this unit messing around didn't put it back together correctly. Once it was where it was supposed to be, success. 50MHz displaying correctly.

So the counter itself is working fine, just the 18GHz converter is causing problems. This was always a concern, you see this often from someone providing too much input and damaging the input section. So while 200MHz is great, 18GHz is why I wanted this counter. At this point I pulled the input brick out and started taking the covers off of it. The cables for this are long enough to allow you to work on it outside of the unit, I so love designs that allow for repairs.

Each side of this brick was made up of multiple boards all of which actually had labeled power input pins. This made locating the problem board very easy. Measuring +12V to ground I was reading roughly 1.2K ohms through all boards. Through process of elimination I was able to isolate the boards that were sinking the +12V down to two boards, both exactly the same. Pulling the +12V cable off of each board returned the rail to normal levels. Both boards were labeled '342U7363', each containing a handful of passives, some transistors and MMICs, along with what looked like a large thick-film precision resistor.

By following the traces on the board and divide and conquering by cutting some traces, I discovered the +12V sinking was actually coming from this large mystery device package. Looking it up, it is an NEC MC-5156 broadband amplifier in a sip package and was nearly shorted from vcc to ground.

Removing these two amplifiers from each board returned the +12V rail to normal, so these two parts were at least indicating to be bad. Was anything else bad? Maybe, it is hard to tell for sure and without a full schematic it is really difficult to trace the signal paths between these boards to guess what else may be blown. Luckily these old amplifiers are still available as NOS from some suppliers, so with two ordered we shall see. Right now with these two amplifiers removed, the unit powers up and counts correctly on the 200MHz input. I am also able to see some RF on the input to each now missing amplifier, so RF is at least getting that far. Is this the only problem? Maybe, maybe not as this would be too easy. I'm betting there are more issues than just this but I can't do any more at the moment until the replacement parts arrive.

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.

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. 

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.

Hydrogen Line 1.42Ghz RF Front End - Radio Astronomy

After finally completing the power supply for my RF front end last week, I spent some time tonight assembling the power supply onto the back of the feedhorn assembly and wiring all of the amplifiers and downconverter to the necessary voltage supplies. Here is a look of what it currently looks like:

From the front, you have the feedhorn, LNA, 1420Mhz cavity filter, downconverter, low pass filter, second LNA, then the power supply on the very back. Now that it is completed, I performed some testing tonight to check operation, sensitivity, and power supply stability. So far everything is looking good, the video below shows its operation on my bench for testing purposes:

Radio Telescope Power Supply - Progress

Today I received my power supply boards for the RF frond end of my radio telescope. Due to the number of amplifiers and the voltage requirements of my downconverter, I needed a supply that provided a stable and clean +5V, +10V, +12V, +15V, and adjustable 8V to 15V output.

Current requirements on all outputs is very small (less then 20mA at each respected output) with exception of the 12V supply which powers the mini-circuits ZRL-2400LN low noise amplifier. It has a 350mA @ 12V requirement so I have utilized the aluminum enclosure to sink some heat off of the 12V regulator. I slightly miscalculated the boards size for the aluminum enclosure I had chosen to use for it. Due to the mounting standoffs inside the case, I was not able to mount the board flush against the side of the enclosure... although it was nothing a band saw couldn't fix. All voltage outputs are passed through the enclosure using feed through capacitors.

There is nothing really exciting about this supply other than the fact that it is now completed. Power supplies are probably the least exciting components of any project to build, even though their importance and stability are critical. Now that it is completed I can install it on the back of my RF frontend and start prepping the actual mount for the dish.

Here is the final assembled board mounted in its shielded enclosure:

New 10Ghz Oscillator

I have been needing a stable 10Ghz oscillator for a few projects I have been working on and was able to pick this brick up for pretty cheap. This is a Digital Microwave Corp 10.210763 GHz phase locked oscillator. This unit consists of a dielectric resonator oscillator that is phase locked to the x100 harmonic of a 102.10763 MHz crystal.

 

While not a YIG, it should be more than stable enough for my needs now (I hope). The new 10Ghz project will be coming after I finish up my 1420Mhz front end for the radio telescope which is nearing completion :)

21cm Hydrogen Line Feedhorn Assembly for Radio Astronomy

I am excited to finally be able to say that my radio telescope is starting to come together. This morning I worked on another major component of my receiving system, a 1.420Ghz hydrogen line feedhorn. This has been a project that I have been wanting to build for roughly 16 years and now that things are moving along I am hoping to have a system together ready for testing within a month or so.

Now for the feed, I decided to go with a rectangular design for my feed instead of a circular one for simplicity of assembly. I went back and forth many times on which design I should use but ultimately ended up deciding on the rectangular feed for several reasons. It is easier to assemble (90 degree cuts are easy to mill), it's based on a standard size, and the material was cheap. Rectangular waveguides are polarized, although for radio astronomy purposes this should not matter as any natural occurring signals would in theory have random polarization. Here is the final assembled version:

While not really a feedhorn as of yet (I have not built the horn) it is a nice waveguide to coax adapter that will be used as a feed at the focal point of my dish. As for the horn, I will have to check to see if I will have any benefit of using one. A horn can provide additional gain from the dish, but it also blocks off surface area of the dish in its shadow. A choke ring on a circular feed would have the same effect in blocking the signal, this is just something I need to research more.

The feed itself is assembled out of 1/4" 6061 aluminum stock that I cut and milled down to size. The dimensions of the opening are 6.5" x 3.25" which is the exact spec of the industry standard WR-650 waveguide which is designed for frequencies between 1.12Ghz and 1.70Ghz. The Hydrogen line of 1.420Ghz fits nearly perfect between these two limits which makes this specific size ideal for radio astronomy. I drilled and tapped 22 holes which have stainless steel hex head screws holding it together. I was very pleased with the final assembly as it has a nice tight fit.

The probe consists of a 4mm section of copper wire which is exactly 1/4 wavelength of 1.420Ghz long and positioned 1/4 wavelength from the back of the feed. The probe terminates to an SMA connector mounted to the top of the feed. I had to mill a small slot into the top of the feed to allow the bottom section of the SMA panel mount jack to lie flush with the inside of the feed.

One note on the WR-650 standard itself. There are commercial feeds available as it is a standard waveguide size, but the cost is extremely high since this this specific size of waveguide does not show up in the surplus market very often. Smaller waveguide standards for higher frequencies like WR-90, WR-42, etc, do show up but it has been extremely hard to find anything WR-650 available for cheap. My total cost to build this feed is about $50.

I have already tested this with my HP 8614A signal generator set at 1.420Ghz and have verified it does indeed work very well. Next steps are to add the mounting brackets to it which will allow me to mount it at the focal point of my dish and also add the additional RF amps, filters, and my downconverter to the back section of the feed. I still also will need to calculate total system gain and noise once completed.

New Spectrum Analyzer: Anritsu MT8801C

The Spectrum Analyzer is by far the most important piece of gear for any RF design work, unfortunately spectrum analyzers are also one of the most costly pieces of gear you can buy (A VNA is right up there too, but that's a different post). I have had access to spectrum analyzers at a few previous jobs which is great whenever you need to test your latest RF design. The problem is when you are at home working at your own bench at 2am,  it's annoying to not be able to have access to this gear all of the time. Buying an analyzer is ideal, but costly. Anything new is pretty much out of the question, so the usual source of eBay is the place to go. Both Tektronix and HP/Agilent have some amazing pieces of gear for an 'affordable' amount (the Tek 49N series and HP 85NN series come to mind), the problem is any of these models can easily cost over $1000 in good working condition. The other issue with this equipment (like all older gear) is their age. Since most were used in a production or lab environment, they have been powered on for 8 hours a day for years. This can result in some crt burn in, the devices being way out of calibration, instabilities and other problems as most of the equipment in this class is 15+ years old (note that makes it affordable). I have had a good run with all my HP / Tek gear as this equipment is really built extremely well. As an example, my HP8614A signal generator was built in the 1960s and still works perfect today. So what happens if you want a spectrum analyzer but don't want to spend $1000+? As I found, there are a few options:

1. Buy a really old analyzer. Some of the older HP models will go for under $500. Keep in mind that these models usually have a max frequency range of no more than a few hundred Mhz.

2. Buy a lesser known brand. There are a handful of analyzers by Chinese companies that go for cheap. They may be perfectly fine, I just prefer to go with a good established brand if I'm going to invest in one.

3. Watch local auctions. There are a ton of company liquidation auction houses such as Dovebid that sell off large companies test equipment assets. These are great places to pick up gear. The issue with this is that there is no guarantee that the gear works (no one tests it) and if it is a valuable piece it will most likely get bid up pretty high. Packing and shipping can cost hundreds of dollars as well if you are not able to pick up the gear locally.

4. My favorite option. Buy gear whose primary purpose is not a spectrum analyzer, but has an analyzer hiding inside it. A lot of communication analyzers and cell phone test sets have an available spectrum analyzer option. I will look up unusual gear on eBay that seems to be selling for cheap and read the product literature on them. You will be surprised on what you will find. I have purchased both of my spectrum analyzers this way.

My first spectrum analyzer that I bought a few years ago is an HP8922H GSM test set. It is designed to replicate a GSM cell station to test GSM cell phones. It also has a bunch of options included one of which is option 006, a 10Mhz to 1Ghz spectrum Analyzer. You can find these for around $500 or less. Now 1Ghz is great, but you eventually reach the limits of what you can do with it. One of my current projects is building a hydrogen line radio telescope which at 1420Mhz is outside of my analyzers reach. I needed something to at least 2Ghz at this point to test my down converter so I began my search again.

While recently looking at more gear that was available I came across an Anritsu MT8801C radio communication analyzer. Not being familiar with Anritsu as most of my gear is HP/ Agilent and Tektronix, I did a bit of research into this particular model and discovered that not only was it an amazing piece of gear, but much like my HP8922H, it has an option for a 300Khz to 3Ghz spectrum analyzer (Option 07):

Being a communications analyzer it has a bunch of other nice features such as a 300Khz to 3Ghz RF frequency generator, and an RF power meter:

 

 

Here is a full span of 300Khz to 3Ghz to my outside wideband antenna:

A couple nice things about this analyzer is that it has a large LCD screen, is capable of displaying a full frequency span, it has a resolution of 1Hz, and a very fast interface. I checked its calibrated accuracy with my HP 8656B RF generator and it was spot on which made me very happy as well:

The additional 3dBm loss above is from the mini-circuits splitter I was using between the generator and analyzer.

Included with this unit was a nice shielded RF test chamber for no extra cost. I can only guess what this had cost new:

 

It will be a great tool for testing devices within a completely shielded environment from external interference.

 

So all said and done this complete setup cost less than $600. Quite a steal considering new this was a $30K+ piece of gear. With that being said I'm finally excited to test my down converter further and also get my RF front end for my radio telescope underway.

Verifying LMR-200 Coax Cable Loss

For years I have used RG-58 cable for my antenna systems, it is inexpensive, easy to work with, and for short runs it works just fine. Last year I started heavily receiving NOAA weather APT imagery using the same RG-58 cable. During this time, pass after satellite pass I noticed my images were not as pristine as they should be. The cable length I was using was just over 100' in length and looking at the loss of my RG-58 it's no surprise that I had such poor images. As a test I replaced all of the cable with a used length on Andrew LMR-400 cable which resulted in an instant improvement.

Now this past week I finally mounted a wide-band scanner antenna on my home and needed a 60' run of cable to reach my RF bench in the basement. Instead of going to my typical spool of RG-58, I wanted something better. LMR-400 was my first choice but is significantly more expensive than my free RG-58. Because of this and the fact that a smaller diameter cable would be better hidden on the outside of the house I chose LMR-200. I purchased 60' of Times Microwave LMR-200 (arguably the best cable you can buy).


With an attenuation of 9.9dB @ 900Mhz, it was significantly better than RG-58. This cable combined with quality Amphenol SMA connectors I was curious to see if the cable would measure up to it's spec sheet. Times Microwave rates it's LMR-200 at 9.9dBm of attenuation at 900Mhz over 100'. At my 60 feet I should only be seeing 5.94dBm of loss which is what I am looking to verify.

The test setup:


An HP 8656B signal generator would be the test source while the spectrum analyzer in my HP 8922H would be used to verify the attenuation of the cable. As a baseline test, I connected a small 1' high quality 50 ohm SMA cable with Amphenol connectors in between the signal generator and spectrum analyzer. Using a -50dBm baseline, the measured signal through the cable was -50.52dBm at 100Mhz. At 900Mhz, it was -51.65dBm, a good start. Now to place the 60' of LMR-200 inline. Unfortunately I did not have a male N to female SMA adapter so I could not test the cable directly inline. To make the test work I placed a mini-circuits ZFSC-2-5-S splitter inline of the cable.


According the the mini-circuits datasheet, at 100Mhz this splitter has a loss on each output of 3.25dBm. At ~900Mhz it has a loss of 3.57dBm, both of which will be compensated for in the results. So testing the 60' of LMR-200 through the splitter at 900Mhz yielded -59.95dBm:


Taking that result and subtracting the loss of the mini-circuits splitter you end up with -56.38dBm, and the difference of that from the original baseline of -50.52dBm at 900Mhz is -5.86dBm of loss at 900Mhz, which is under the spec sheet of -5.94dBm by .08dBm, a very good result indeed. I love it when numbers work out. As a final test I ran a sweep test on the 60' cable using an HP 8754A Vector Network Analyzer. As expected, the loss from 10Mhz to 1300Mhz exactly matched the expected attenuation in the datasheet:



I will be running this cable to my antenna tomorrow. :)