Santa Baby!

rigol_ds1102Santa Claus brought me a very nice gift this year. He used his excellent connections with eBay and got me a Rigol DS1102E oscilloscope for Christmas! In my letter to him, I told him of the great reviews I’ve found on it; in particular several videos by Dave Jones of EEVblog and also the personal experience of Bill Meara N2CQR as described in his SolderSmoke podcasts. Bill also has a video showing the DS1102 in action displaying a frequency sweep of a bandpass IF filter: Sweeping a Filter with a FeelTech Sig Gen and a Rigol ‘scope.”

In a few SolderSmoke podcasts, Bill also talked about using the FFT (“fast-fourier transform”) feature on the Rigol’s math menu as a “good-enough” spectrum analyzer. That’s what sold me on the DS1102E.

After a quick trip to the local Fry’s Electronics to pick up a 10x scope probe (Santa forgot that), I gave my Rigol a little test using the built-in 1KHz square-wave generator that’s used to adjust the compensation capacitor in the probe. After adjusting the probe for nice sharp corners on the square wave with no over or under shoot, I turned on the FFT function.

screenshot_squarewave_1khz_fftI was delighted to find just what I expected in the spectral content of the square wave: pronounced peaks on each of the odd harmonics (1st, 3rd, 5th, etc.) and only a gradual decline in the amplitude of each. Here’s the screenshot made with another nice feature of the DS1102E: screen capture as a bmp file and savable to a USB drive.

You can see Cursor A on the fundamental 1KHz wave, with Cursor B on the 7th harmonic at 7KHz. The vertical scale of the FFT portion of the screen is 20dB per division. Notice that by the 25th harmonic (!!) at the far right of the display that it’s down by only about 20dB (for more on the harmonic content of square waves see and

Here’s an especially-cool simulation that shows a series of sin waves at odd harmonics adding up to a square wave: Sine Wave to Square Wave using Fourier Series. This one is even more cool: Fourier Series Animation (Square Wave).

Tomorrow I hope to pump some RF into the Rigol and see how it likes it. I’m very eager to know how well the FFT function works on the HF-bands frequencies. ..73 de K7TFC

LTC6431 Low IMD IF Amp

Recently, there was a bit of traffic on the EMRFD* Yahoo group about a new IF amp chip from Linear Technology: the LTC6431 ( The main interest in this device is its low intermodulation distortion (IM) as expressed by its third-order intercept point–in this case on the output (abbreviated as OIP3**): 47dBm OIP3 at 240MHz into a 50? Load. Its specified bandwidth is 20 to 1700MHz and it provides 15.5dB of power gain.

LTC6431_breakoutThomas Knutsen, LA3PNA, quickly designed up a breakout board for the LTC6431 and made it available at OSH Park for anyone to order ( I ordered a set of three for a mere $10.30USD. They were so cheap because OSH Park waits to fill a pcb panel with enough other boards to share the production costs. Once a panel is full, they run it and then distribute the boards to those who ordered them. It’s a great idea, and in this case it only took about ten days for the finished pcbs to show up in my mail. Their quality is excellent. This was my first experience with OSH Park, and I’m pretty pleased.

I haven’t populated the boards yet with LTC6431s, but once I do and play around with them, I’ll post an update.


*EMRFD stands for the title of Wes Hayward’s popular book, Experimental Methods for Radio Frequency Design, published by the ARRL. The Yahoo group is devoted to discussions based on the book and its design examples.

**See For a not-too-difficult discussion of IM, see Doug Smith, KF6DX, “Improved Dynamic-Range Testing” QEX July/August 2002 (


Vackar VFOs

Over on the QRP-Tech Yahoo group, Ed Edwards, AE7TE, posted a link to an article ( by Kenneth Gordon, W7EKB, on Vackar-design VFOs designed in the 1950s for vacuum-tube applications. The Czech engineer Jiri Vackar introduced the design in 1949*.  The schematic example Kenneth shows uses a 12AT7 dual triode in common-cathode mode with both plates at 255VDC. The driver is cathode-keyed. The output triode is arranged as a cathode follower. The VFO is series-tuned.

Ji?í Vacká?

Jiri Vackar

According to W7EKB, Vackar VFOs have three important design characteristics: they have a 2.5:1 tuning range, they have a constant signal-level output across that range, and they’re “rock solid” in their frequency output. It so happens I have a bag of 12AT7s just waiting for me to play with. I’ve been wanting to design and build a single-band tube transceiver just for the fun of watching the little buggers glow in the dark. Someday soon I hope to test this Vackar design as part of that longer-term project. I’ll let you know how it comes out.  73.


*For more on Vackar and his design, see (copy and paste text into Google Translate) and For a solid-state version of the Vackar VFO, see Floyd Carter, K6BSU, “Meet the Remarkable but Little-Known Vackar VFO,” QST September 1978: 15.

A Circuit Development Base

DSC00624I hate haywire. I can’t stand jumpers and cables and cords strewn everywhere when I’m working on a project. Psycho-optically, I’m driven nuts by the rat’s nest that results from power and test leads when the project is in full swing.

I’m especially put out by stiff cables knocking the usually-small items I’m working on all over the bench. An Arduino, being small and pretty light, is nothing to a springy USB cable. Coax, even the relatively-thin type such as RG-8x, will be happier straight than bent, and it will likewise toss the thing I’m working on back and forth. Arrrggg!

DSC00625My solution is to get as much anchored and tied down as I can and I’ve found what I’m calling a “development base” to be invaluable. Here you see an Arduino Uno with an Adafruit Proto Shield on which is mounted a small breadboard, an Adafruit breakout board for the Si5351 clock-generator chip, and a rotary encoder. It’s a project I’m working on for a simple antenna analyzer.

You also see a 2×16 LCD display, and both it and the Arduino assembly are mounted on standoffs. The white base is a piece of expanded PVC I had on hand, but it could just as well be anything of a suitable size and heft—including a real breadboard. You’ll also notice I’ve affixed (with repositionable glue stick) a chart of pinouts for easy reference. Other important notes can be placed on the base as well, perhaps even with fine-tipped dry-erase markerDSC00626s or a grease pencil.




Instead of haywire jumpers, I’ve used ribbon cable to the display, and even the jumpers I do use on the Arduino assembly are routed, folded, zip-tied, or even loosely knotted to keep them in check. Neat and tidy.

There’s plenty of real estate left on the base to mount other boards or circuit components, perhaps with nothing more permanent than double-stick tape (not the semi-permanent foam type!), Bostik’s Blu-Tack®, or Elmer’s Poster Tack®.

DSC00627I took the trouble to mount the Arduino and the LCD display with screwed-down standoffs since I’ll be using the same footprints for other development projects. They might even stay there more-or-less permanently, to be replaced by other ones in the finished projects.

The size of the development base is not important except that it be adequate. One other advantage to using a base of this sort is that it can be picked up and moved intact, to be replaced on the bench by another suddenly-more pressing project, or moved out of the way of dinner dishes if it’s on the kitchen table.DSC00628

Amateur Radio Parity Act Reintroduced

ARRL Bulletin 14  ARLB014US_Capitol
From ARRL Headquarters
Newington CT  March 9, 2015
To all radio amateurs


The Amateur Radio Parity Act of 2015 Introduced in Congress

“The Amateur Radio Parity Act of 2015” – H.R.1301 – has been introduced in the US House of Representatives. The measure would direct the FCC to extend its rules relating to reasonable accommodation of Amateur Service communications to private land use restrictions. US Rep Adam Kinzinger (R-IL) introduced the bill March 4 with 12 original co-sponsors from both sides of the aisle – seven Republicans and five Democrats. Kinzinger also sponsored “The Amateur Radio Parity Act of 2014, which died at the end of the 113th Congress. H.R. 1301 is an essentially identical piece of legislation. Continue reading Amateur Radio Parity Act Reintroduced

Adafruit Si5351A 3-Channel Clock Generator

DSC00553_scaledThe other day, I received a breakout board for this critter from Adafruit. I want to experiment with RF applications, in particular an Arduino-controlled VFO. For this purpose, I need to be able to set the PLL and divider parameters in real time on the fly in response to, for instance, the turn of a rotary encoder, or direct keypad entry. The Arduino library provided by Adafruit is good for firm-programed parameters, but not for run-time settings.

I’ve done a pretty-good search through the Silicon Labs site and all I found was their Windows app for calculating those parameters for static programing–not for on-the-fly applications. I was hoping to find some formula or algorithm for determining the parameters given a specific frequency to be generated.

After what Lady Ada (Limor Fried) calls “noodling around,” I found a number of Arduino libraries for the 5351 at GitHub. Except for Adafruit’s own respository there, everything else is RF/Ham radio focused. So far,  I’ve experimented with a library developed by Jason Milldrum NT7S for his IndieGoGo Si5351A breakout-board campaign. I found it works flawlessly with the Adafruit breakout as well.

The nice thing about Jason’s library is that it will calculate the PLL and divider frequencies from an output frequency specified by a variable. This is exactly what I wanted. With a few buttons and a rotary encoder or keypad, I can dial in the frequency I want.

Here’s Jason’s GitHub respository:

Adafruit Si5351A Breakout Board w/ArduinoI have yet to explore all the features of the library, but so far I’ve used a variable that allows the xtal-load capacitance to be entered–no doubt to fine-tune the calculation of PLL and divider frequencies. According to the Adafuit document, 10pF is the nominal figure. That’s what I used and it got the closest to the specified output frequency of those I tried.

Another feature I’ve noticed but not yet tried is a variable to enter a correction factor to compensate for manufacturing tolerances of the 25MHz reference xtal. Before I try that, I need to setup a GPS-disciplined frequency standard to properly calibrate my gear.

One odd thing I’ve found is that the official Silicon Labs datasheet for the Si5351A/B/C series lacks information on pinouts–by pin number, that is. They show pin names but no numbers. Strange! I finally found them, though, on Lady Ada’s drawing.

The cool thing about the Si5351 is that it can generate both the VFO and the BFO frequencies for a receiver/transceiver at the same time. If the rig needs a digital clock for other purposes, the 5351 can provide that as well.

As you can see from the waveform on the ‘scope screen, it’s not a perfect sine wave. This is to be expected, and a simple low-pass filter (LPF) can clean it right up.



Here’s the link to Adfafruit’s page for the breakout board:

At $7.95USD, it’s an excellent buy, especially considering it includes pull-ups and a LDO regulator so either 5V or 3.3V can be used for power and pin-high levels.

I’ll report more on the Si5351A in general and the Adafruit breakout in particular in subsequent postings.



!!! THIS JUST IN: I also just received my “perk” for my contribution to NT7S’s IndieGoGo campaign for his Si5351A breakout board. It’s comes as a kit, so I’ll assemble it and post a preliminary report soon.

Antenna Raising Weather is Here!

“The best and most expensive transmitter is but a useless toy if it is connected to a poor antenna.”                                                                               

Bill Orr, W6SAI, S-9 Signals (1959)


Green grass, mild days, and blue skies are sure signs it’s time to climb towers, string wire, and get up on rooftops in the never-satisfied quest for a few more dB of gain.

It’s also time for contest pallor and seasonal-affective disorder to give way to tanned skin and spring fever. IOTA, SOTA, and POTA (Islands, Summits, and Parks on the Air, respectively) activations seem alluring again. Two-meter radio direction finding and foxhunting offer fresh air and at least a little exercise.

It’s good weather to grab your QRP field bag and head to the local park or lake, toss a EFHW antenna up in a tree, and attract both QSOs and curious onlookers. You can boast how many hundreds of miles per watt you get from that little cigarette-pack sized rig.

Hunting for the source of that new and pesky S7 noise floor in your otherwise-quite neighborhood now takes on the grim-yet-determined character of a military campaign.

Marred only momentarily by the tax-filing deadline, Spring is always something of a happy surprise.

Radio Crystals Developed during War

Here’s a very cool post on the development of radio-crystal manufacturing during the Second World War. Xtals had been used experimentally before the war, but the need for reliable communication transformed amateur radio based mostly on coil and capacitor oscillators to the widespread use of much-more stable crystal ones.




Around the su1873_A._and_C._Black_Map_or_Chart_of_the_Solar_System_-_Geographicus_-_SolarSystem-black-1873n, that is. One revolution a year. Once thought a mere satellite of Earth–the geocentric view–it turned out we’re the satellite ellipsing about in a heliocentric system. At some point in our egocentric lives-somewhere between eight and eighty–we each learn we’re not the center of the universe either.

This time of year, we’re fast approaching perihelion–closest to the sun in spite of the gathering cold. As Hams, we’re S-meter and Sun watchers. Greater activity of the latter leads to higher numbers on the former. Recent sun-spot and solar flux numbers have been some of the best this solar cycle. So far, every predicted (and lamented) downturn in solar activity this cycle has been followed by a flurry of good activity. It has to start declining sometime, so enjoy it while you can.



At our local club’s annual swap meet in September, the technology of all but the first few decades of amateur radio was on display. Vacuum tubes, transmitting coils, high-voltage variable capacitors, quartz crystals, and “boat-anchor” rigs from the 1950s were juxtaposed with software-defined radios, with everything in between in various hybrid combinations. But isn’t all that older stuff “obsolete”? Not on your grandfather’s spark gap!


Is the tuned LC “tank” circuit obsolete?

The thing is, all that earlier technology still works as well as it always has, except I suppose if you think that reducing VFO drift down to 1 ppm over 60 minutes is something you can’t live without. With good ol’ ovenized oscillators and other forms of thermal stabilization, I’m satisfied enough to get on the air without concern over some good, honest drift. If some Barney-Fife operator wants to complain my signal drifts 10Hz per hour, he can kiss my plate chokes, and he’d get what’s coming to him, too.

Not until the twentieth century did “obsolete” enter the everyday talk of ordinary people. Within the scope of what we all call “technology’—all dependent on electricity–change has come at a dizzying pace. In some cases, new developments result in real value added to the human condition, while others only serve to foster dissatisfaction and discontent. And always there’s something else to buy. I’m as fond of technology as the next radio amateur, but I’m more fond of contentment and peace of mind, especially when the proffered advances seem—to me at least—less than compelling.

In gaining some insight into this march into perpetual obsolescence, it’s worthwhile to look at other technologies that have survived the onslaught of the “new and improved.” Painting did not disappear upon the introduction of photography in the middle of the nineteenth century. It’s still alive and well in the art world, and materials for it are available, even at “big-box” superstores. Though typewriters have been mostly supplanted by desk- or laptop computers, pen, pencil, and paper have not. Synthetic textiles are superior (easier to maintain) in most respects to those made of cotton or wool, but natural fibers used before recorded history are still very-much in use. And so on.

Likewise, the technology of the codex ( was a true advance over the book scroll since a scroll can be accessed only in a fixed sequence, while any page of a codex can be accessed immediately. A synoptic skimming of a codex is no more difficult than flipping through its pages. Not so with a scroll. Well, though our tablets and notebook computers are right up to date, we’re now back to scrolling. Not everything new is unassailably better.

I admit this may be much ‘ado about nothing, and in fact it may be only slightly more meaningful than any other garden-variety navel-gazing and pseudo-intellectual self indulgence. But the old boat anchors and hollow-state gear is still around, the coils and capacitors are still used by some to create tuned circuits, and it can all live side-by-side in anyone’s radio shack. It does in mine. It’s one of the “cool” things about Ham Radio.