Tektronix RSA 306 | GPS Receiver and Clock Reference

The Tektronix RSA 306 is a great low-cost USB 3.0 spectrum analyzer for up to 6 GHz in an ultra low-profile format. If paired with the RSA Map function of SignalVu-PC and a GPS receiver, it becomes an extremely powerful tool for interference hunting and signal strength mapping. This article shows a possible way to supply position and timing information to the SignalVu / RSA 306 set-up from an external source.

In recent weeks I had to do quite a bit of interference hunting for some government agencies such as a local volunteer fire department with harmful interference on their analog repeater network. When working with government agencies, one of the most important thing is documentation. One can’t just point antennas around ans say “I think it’s coming from that house over there.” This is obvious because this kind of interference will likely kick off some sort of enforcement action or even criminal sanctions if the interference is malicious and intentional. To fulfill this documentation requirement I used my RSA 306 spectrum analyzer, SignalVu-PC and RSA Map.

Since the temperature in the car and outside fluctuates quite a bit, I wanted to make sure that my frequency readouts are as accurate as possible. Especially right now where it’s icy cold outside and the car heater is on full throttle, it was apparent that the reference oscillator frequency would drift each time I would open the door to run a test with a directional antenna. Likely, this drift may not cause all too much trouble but I wanted to be absolutely spot on, especially since this was for government agencies. Since the RSA 306 has a connector for an external 10 MHz reference clock, this was a piece of cake to achieve.

Tektronix RSA 306 | External Reference connected

Tektronix RSA 306 | External Reference connected

A perfect solution for supplying both the 10 MHz reference signal and GPS position information that was the GPS-Locked TCXO evaluation board from Jackson Labs that I reviewed about two years ago. This board served me very well as timing reference for my lab until it was recently replaced by a Jackson Labs Chip Scale Atomic Clock and an Analog devices PLL. The GPS-Locked TCXO board was perfect in many ways for this job. It’s small, USB-powered and inexpensive. The latter is important since items used in the field tend to get broken or lost.

Jackson Labs GPSTCXO Eval Board

Jackson Labs GPSTCXO Eval Board

CAUTION: The RSA 306 expects the level of the reference signal to be no greater than 10 dBm. That's 2 Vpp max. The GPSTCXO's output level is, however, 5 V CMOS level.

Despite the level difference, setting up the 10 MHz reference is a piece of cake; Simply connect the 10 MHz out from the GPSTCXO Evaluation board to the 10 MHz reference input on the RSA 306 through a 10 dB attenuator. The attenuator I used is a Mini Circuits HAT-10+.

By default the GPS-Disciplined TCXO Eval board uses a baud rate of 115200 bps. For some reason the RSA Map software only supports speeds up to 38400 bps. Also, by default, the board doesn’t put out NMEA (=position information) data over the serial port. So we have to change the baud rate and set up the GPSTCXO board to spit out NMEA sentences periodically. These settings are modified by sending SCPI (Standard Commands for Programmable Instrumentation) commands to the board. To do this, you will have to use a terminal program to talk to the board and send commands. I used the freely available software Putty. Set the Terminal program to 115200 baud, 8 data bits, N parity bit, 1 stop bit. To verify that you are successfully connected, type the SCPI command “help?” or “syst:stat” (or any other valid command) and see if you get a reply. If everything went right, type the following SCPI commands, one after another and press enter after each line.

GPS:GPGGA 2
GPS:GPRMC 2
SYSTem:COMMunicate:SERial:BAUD 9600

NMEA Output From the GPS locked TCXO

NMEA Output From the GPS locked TCXO

The first two commands enable GPRMC and GPGGA NMEA sentences. The ‘2’ is the interval in which the system will provide us with updates, in this case 2 seconds. We don’t really need a high update rate at all. Even while driving around the county I don’t usually take snapshots more often than every minute or so. But if the NMEA sentence interval is set to anything less than 3, the RSA Map keeps switching back and forth between “GPS locked” and “GPS unlocked.” That’s why I left it at 2 to keep the software happy. After typing the first or second command, you should see NMEA messages being outputted into the terminal you are using to send commands. The last command obviously sets the baud rate. The Jackson Labs GPSTCXO Evaluation Board supports the following baud rates: 9600, 19200, 38400, 57600 and 115200 bps. I was somewhat surprised that it doesn’t support 4800 – a fairly standard speed for navigational receivers. But that doesn’t matter to us, pick any rate you please between 9600, 19200 and 38400 for compatibility with RSA Map. Remember to adjust your terminal’s baudrate to the new rate. Else you won’t be able to communicate with the board.

For an overview of all possible SCPI commands and setting I recommend you review the comprehensive user manual [2].

GPS Settings in RSA Map

GPS Settings in RSA Map

Now that we have the GPS receiver functionality of the GPSTCXO set up, we need to tell RSA Map where to find the navigation data. In order to do this you need to know the COM Port number and the baud rate you just set via SCPI command. Just enter it into the GPS Setup dialog, check “Enable” and hit “Apply”. If everything went right, valid GPS data should be displayed in the “status” section of the dialog. If so, click “okay” and start hunting interference

RSA Map: Two measurements on the imported map

RSA Map: Two measurements on the imported map

Just as a side note: This set-up of course also works with different Tektronix instruments such as the MDO4000(B) or the higher level RSA5000 / RSA6000 and SPECMON product line. While they are less portable, they do the job just as fine. And before the RSA306 was released to the market I would actually use my MDO4104B-6 in the car for interference hunting. When you power these instruments through an inverter, make sure it’s of high-quality. The last thing you want is a cheap power inverter to kill these sensitive instruments.

Links and Sources:
[1] Jackson Labs http://www.jackson-labs.com/
[2] Jackson Labs http://www.jackson-labs.com/

 

Importing Maps Into Tektronix RSA Map

The Tektronix Vector Signal Analysis Software SignalVu has a great mapping tool, Tektronix RSA Map. Unfortunately, the default map supplied is extremely coarse and not really useful for real world measurements. This short article is going to show how to acquire free map files from openly available sources.

If you use Tektronix SignalVu, odds are that you also have the RSA Map option installed. SignalVu comes installed on some instrument (e.g. RSA5000, RSA6000 or SPECMON) or as SignalVu-PC for use with entry-level instruments (e.g. RSA 306, MDO3000 or MDO4000) on a PC. RSA Map is a mapping tool for SignalVu. RSA Map makes it easy to map different measurements, such as spectrum, spectrogram and amplitude over time measurements. The measurement locations can me either set manually or automatically via GPS synchronized location information. It is even possible to add directional information (=azimuth) if a directional antenna is used. Therefore, this is a perfect tool to hunt down interference issues or to create coverage maps.

Default Map of Tektronix RSA Map

Default Map of Tektronix RSA Map

Obviously, no mapping software is very useful if it doesn’t have a good map. Out of the box RSA Map only comes with a very coarse world map. While it has the ability to zoom in, no features other than the continental outlines are shown in the default map. So unless you intent to fly around the world with an RSA306 in your carry on luggage maybe, this is not very useful. In case you wonder why Tektronix doesn’t include better maps, just imagine how expensive it would be to gather fully licensed maps for the whole world. Also, the file size would be ridiculously huge. It’s just unrealistic for Tektronix to do. So bottom line, we need to acquire our own maps if we want to use RSA Map. Luckily, there are some free online resources available to source some free maps.

One of those free resources is OpenStreetMaps [1].

Open Street Map Export Button

Open Street Map Export Button

Downloading a map is rather simple. Simply adjust your map view to the area you would like to be covered in RSA Map and click the “Export” button. But there’s one more step needed to make the map compatible with the RSA Map software. RSA Map requires files in the MapInfo Interchange Format (*.mif) but OpenStreetMap exports the files in its own format, *.osm. But no need to worry, there’s a free online tool called GeoConverter [2].

GeoConverter converts OSM files to MIF files

GeoConverter converts OSM files to MIF files

Converting the file is super simple, just select the *.OSM file you downloaded from OpenStreetView and select “Map Info interchange Format (MIF/MID)” as export format. To import the map into RSA Map, simply click on “File -> Load Map…” and select the *.MIF file you downloaded from GeoConverter. That’s it.

RSA Map: Two measurements on the imported map

RSA Map: Two measurements on the imported map

The only issue I have with this map source is that it appears to only map houses, not the streets or street names. But since it’s free I won’t complain. The maps are more than useful for my purposes the way they are. However, if you know of a better map source, please let us know. The above image shows a real measurement that I have performed and documented with RSA Map. With just two directional measurements I was able to track down a leaky cable TV distribution amplifier. It caused severe noise on portions of the VHF aviation and amateur radio bands.
Links and Sources:

[1] OpenStreetMap: http://www.openstreetmap.org/
[2] GeoConverter: http://geoconverter.hsr.ch/

 

DipTrace Export Settings for Dirty PCB

Dirty PCB offers – as the name implies – dirt cheap PCB manufacturing. The trick is a very low profile and a straight-forward system that needs minimum administration. This article is going to show how to set-up DipTrace to export a PCB layout correctly for Dirty PCB.

DirtyPCBs.com offers super cheap PCBs for up to 10 cm x 10c m through Chinese fabrication houses [1]. Despite their dirt cheap approach, the options are still plentiful for the average hobbyist. All PCBs, 2-layer or 4-layer, are of FR4 material with selectable thickness, color and finish. Even electroless nickel immersion gold (ENIG) finish is offered. Also available super cheap are solder paste stencils. Dirty PCBs stresses that you shouldn’t expect much and that this is a real “as cheap as it gets” approach. It’s a great service for prototyping inexpensive prototyping and hobbyist use though.

Dirty PCB differentiates between the different layers by the file extension of the Gerber files. The following extensions are used for each layer:

Extension – Layer
GTO – Top Silkscreen (text)
GTS – Top Soldermask (the ‘green’ stuff)
GTL – Top Copper (conducting layer)
GBL – Bottom Copper
GBS – Bottom Soldermask
GBO – Bottom Silkscreen
GML/GKO/GBR* – Board Outline*
TXT – Routing and Drill (the holes and slots)
*Required

This is different from the default DipTrace setup. By default, DipTrace uses the .gbr extension for all Gerber files and differentiates between layers by file name. So before exporting one needs to click on the “Files” button in the Gerber export menu and enter the above nomenclature. Furthermore, please deselect layers that are unnecessary. The following picture shows the correct setup:

Correct Gerber Nomenclature for Dirty PCBs

Correct Gerber Nomenclature for Dirty PCBs

The most important file is the board outline. According to Dirty PCBs, forgetting to include this file is a common error that leads to designs being rejected. Technically speaking, a board outline is the only required file. However, this would be an extremely boring board. But it might be useful if you need someone to cut some FR4 material to size for some strange reason.

The next common problem Dirty PCBs mentions is missing drill files. They require an Excellon drill file (also called NC drill file) with either a .TXT or .DRI file extension and embedded tooling information. DipTrace accomplishes this as follows:

N/C Drill File Export for Dirty PCBs

N/C Drill File Export for Dirty PCBs

All the files generated by the Gerber and Excellon export need to be combined in a .zip archive. That is really all of the magic behind exporting a layout for Dirty PCBs. Just upload the .zip archive to Dirty PCBs, select a package size and send off the order. Dirty PCBs will offer you a preview of your uploaded design. They stress that this feature is experimental, however, it is probably a useful tool to see if something went horribly wrong.

Dirty PCBs Preview

Dirty PCBs Preview

One really cool feature of Dirty PCBs is the ability to share one’s design with the whole wide world. For instance, my design from a previous article on how to use DipTrace can be ordered here. Perfect for Open Source Hardware (OSHW). You can even collect $1 every time somebody orders your design. But don’t be greedy; Dirty PCBs doesn’t make much money of orders. If you intent to get rich with your design, I suggest you order a larger quantity and sell it at a profit yourself.

Another feature is the fact that you can upload new design files at all times until Dirty PCBs sends them off to the fabrication house. Seems like no big deal but every design engineer knows that almost always at least one improvement comes to mind just after submitting the design files. As you can see from the previous image, I actually revised the uploaded design files 3 times. It’s a real life saver.

That’s it. Assuming that your design is abiding their design rules, not much can go wrong from here. Enjoy!

Links and Sources:

[1] Dirty PCBs: http://dirtypcbs.com//

 

PCB Layout Design with DipTrace – An Overview

DipTrace is a sophisticated schematic and PCB layout software. I had it installed on my computer for over a year but have never really used it. Since I needed a super simple PCB for some 78XX / 79XX type linear regulators, I decided to use this opportunity for my first steps with DipTrace.

DipTrace offers different prices for different packages [1]. The cheapest version, DipTrace Starter (300 pins, 2 signal layers), is available for $ 75. DipTrace Full offers unlimited pins and unlimited signal layers for $ 895. All versions of DipTrace come with a humongous parts library and 3D models for a large amount of general purpose parts. In case a part or footprint is not included in the libraries, DipTrace offers both a component editor and a pattern editor for custom parts.

I needed a super small PCB for 78XX type voltage regulators. This mini project was perfect for my first steps with DipTrace. Please be aware that this article is going to be extremely coarse as it is supposed to be a general overview of the DipTrace software. The functionality of this software is rather complex as this is a professional design tool. More detailed articles will follow.

DipTrace Schematic Overview

DipTrace Schematic Overview

The starting point for my project was the “Schematic Capture” module. It looks very neat and very well ordered. The positioning of all toolbars and elements seems very intuitive. I selected a 78XX voltage regulator, 1 shunt capacitor for the input side and 2 shunt capacitors for the output. I like to put a small capacitor (~ 100 nF) and a larger capacitor (~10 μF) on the output of my linear regulator circuits. Additionally I put a screw terminal each on the input and output side. It took less than 5 minutes to get the circuit together. To be fair, I totally forgot to enter component values in the schematic capture as I was rushing through it.

Convert to PCB

Convert to PCB

After the schematic is complete, the next step is to convert it to a PCB layout. The parts library links schematic symbols and PCB footprints. So when you click on “Convert to PCB”, the layout will be pre-loaded with the right footprints for the selected parts.

Initial Conversion View

Initial Conversion View

The initial part placement is of course anything else than ideal. Although it seems that DipTrace’s PCB layout module tries to place the parts the same way they were input in the schematic capture. A keyboard shortcut for rotating parts “Ctrl + R” makes it easy to place the parts in the correct location. If you would like to manually route your PCB from this point on, you can do so. But the more comfortable way is to feed the autorouter with design rules and let it suggest a layout. If you don’t define a board outline beforehand, the autorouter will actually take care of that. In my case I selected a board outline beforehand and then let the autorouter run. It actually created a very usable layout. Nonetheless I fine-tuned the layout manually. After that I placed a copper pour on the bottom layer of the PCB. One setting for the copper pour, the Net it gets connected to, is hidden away and I totally oversaw it at first. As a result, my copper pur wasn’t connected to ground. I had to go back and set the pour to be connected to Net 0 in the “Connectivity” tab.

Finished PCB Layout

Finished PCB Layout

After The final design is completed, DipTrace offers a 3D preview of the board. This is a very nice feature for collision avoidance and to detect physical spacing problems between parts early in the design stage. You’d be surprised how often it happens even win professional designs that some clearance issue renders a PCB revision useless. So make deliberate use of the 3D modelling if 3D layouts are available for the parts you are using.

3D render of the PCB

3D render of the PCB

The 3D part library is very large but DipTrace of course offers the option to upload custom 3D models as industry standard .wrl files. If everything goes well through this step, it’s time to get the PCB to the PCb manufacturer. DipTrace actually has an inbuilt “Order PCB” function.

BayArea Circuits Order Menu

BayArea Circuits Order Menu

The inbuilt “Order PCB” function is a service offered through BayArea PCB. As the name suggests, it’s a California based fabrication house [2]. Unfortunately, they don’t offer any options, just a standard 1.6 mm thick FR4 board with silk screen on both sides and HASL finish is offered. But the prices are more than fair for a US based manufacturer. 25 PCBs of my design cost $ 3 each or $ 75 total for instance.

But that’s not the fabrication house I wanted to use. For a long time I wanted to try the services of Dirty PCBs ([3]). Dirty PCB sends your order off to various Chinese fabrication houses, depending on what kind of board (e.g. 2-layer or 4-layer) you are submitting. They promise an ultra low-profile system with minimum customer service and options but extremely low, unbeatable prices. But more about them in a different article, let’s get back to the layout export.

Correct Gerber Nomenclature for Dirty PCB

Correct Gerber Nomenclature for Dirty PCB

The de facto industry standard for exporting PCB layouts are Gerber files. Different PCB manufacturers use different naming schemes for Gerber files. Some expect the Gerber files to have a specific name, others expect any name but with a specific extension per layer and some want both. Make sure you know the exact specifications for your PCB manufacturer. The above is the correct extension nomenclature for Dirty PCBs.

N/C Drill File Export for Dirty PCB

N/C Drill File Export for Dirty PCB

Lastly, don’t forget to export the drill files. The common format for drill files is the Excellon Drill (also called NC Drill) format. DipTrace names it N/C Drill in its export menu. You can select tools manually or have DipTrace auto-select the tools. An Excellon file is a very simplistic text file containing tool (hole-) sizes and coordinates (=where to drill) for each tool. The Excellon file for the above layout looks like this:

M48
INCH
T01C0.0354
T02C0.0394
T03C0.0433
T04C0.05
%
T01
X+017315Y+009190
X+017315Y+007190
T02
X+008940Y+007190
X+008940Y+008190
X+014940Y+008190
X+014940Y+009190
T03
X+005690Y+005815
X+005690Y+007193
X+012190Y+007190
X+012190Y+008190
X+012190Y+009190
X+020565Y+009193
X+020565Y+007815
T04
X+004690Y+010940
X+007940Y+004565
X+022065Y+004690
X+022065Y+010940
T00
M30

An Excellon file can not only contain holes but complete routings. Like a routed air gap for isolation in high-voltage circuits. But that’s a whole other story. At this point there’s nothing left to do other than sending the Gerber and drill files to a PCB manufacturer of your choice. Happy designing!

Links and Sources:

[1] DipTrace: http://www.diptrace.com/
[2] BayArea Circuits: http://bayareacircuits.com/
[3] Dirty PCBs: http://dirtypcbs.com/

 

Goodbye Radioshack!

On February 5th, 2015 the electronics retailer RadioShack finally filed for Chapter 11 protection under US bankruptcy law after 11 consecutive quarterly losses. Many stores closed as a result of this. Before they closed their doors forever, RadioShack blew out their remaining inventory with great discounts.

When I first heard of the RadioShack bankruptcy, I was not surprised at all. Nor was I bothered by it. RadioShack used to be the go-to place for electronics parts and state of the art electronics. But those times have been long gone. The electronic giant made the fatal business decision to shift more into the cell phone parket. Many of their employees lack knowledge in teh electronics area but will try to sell you a cell phone with every purchase. At least that was my perception. Towards the end RadioShack management realized their mistake and tried to re-align the stores with the maker industry. But just putting Arduinos on the shelf by itself doesn’t do much. The employees aren’t well trained for these kind of products and therefore RadioShack never really lost its image as a cell phone store.

With the closure of many stores and the chapter 11 bankruptcy an era comes to an end. I’m not sure if it the stores in their current state will really be missed.What about you, how do you feel about the RadioShack closure? Please leave your RadioShack related stories and opinions in the comment section below.

Here are some pictures of my final blowout shopping trips:

First RadioShack bankruptcy blowout catch.

First RadioShack bankruptcy blowout catch.

Second RadioShack bankruptcy blowout catch.

Second RadioShack bankruptcy blowout catch.

Pretty much cleared out store

Pretty much cleared out store

Not much left

Not much left

Links and Sources:

[1] QST: http://www.arrl.org/qst

 

Teledyne LeCroy Factory Tour Day 1, Post 1

This week I am at the Teledyne LeCroy factory in Chestnut Ridge, NY. Besides enjoying the cold, rainy and dark weather, I’m going to share lots of pictures and videos with you throughout the next days and weeks. If there are any questions you’d like me to ask during interviews or there’s anything in particular you’d like me to cover, let me know!

Here are a few pictures of this morning’s Factory tour:

IMG_6549

IMG_6581

IMG_6636

IMG_6637

IMG_6662

IMG_6759

IMG_6951

IMG_7029

 

Tektronix RSA306 IF Frequency Verification

After posting the teardown video of the Tektronix RSA306 USB 3.0 Spectrum Analyzer on my YouTube channel, someone asked me to verify that the IF sampled by the Linear Technology 14-Bit / 125 Msps ADC is really at 140 MHz. Using the Tektronix MDO4104B-6 and a passive RF probe, I gladly did so.

With a 100 MHz, 0 dBm signal applied to the spectrum analyzers input and the SignalVu-PC settings set to 100 MHz center frequency, 40 MHz span, the signal at the ADC input looked like this:

Spectrum at the LTC2261-14 input with a 100 MHz, 0 dBm signal

Spectrum at the LTC2261-14 input with a 100 MHz, 0 dBm signal

So indeed, 140 MHz is our strongest peak. 20 dBc lower is a 28 MHz peak. Before taking the picture I of course tried a large span to make sure that there aren’t any relevant signals at a lower frequency. So the next test uses a 105 MHz signal still at 0 dBm.

Spectrum at the LTC2261-14 input with a 105 MHz, 0 dBm signal

Spectrum at the LTC2261-14 input with a 105 MHz, 0 dBm signal

So indeed, it seems that the LTC2261-14 samples the 140 MHz IF directly. How you ask? The answer is undersampling. The ADC is actually sampling at 112 Msamples/s. As a result, it sees an alias of the 140 MHz signal at 28 MHz. That’s all the magic happening here.

This probing expedition also yielded some pretty pictures that I would like to share. Enjoy!

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer