50W QRP Amplifier – PCB Layout Video

This past weekend, I started on the process of laying out the 50W QRP Amplifier project as a PCB. Small PCBs can be remarkably inexpensive these days – $10-$15 for 5 pieces of say 4″x4″, shipped in 2-3 weeks. I’m treating this amplifier project as a chance to experiment with different, similar FETs to learn about critical power MOSFET properties, and also as an opportunity to brush up my layout skills that I haven’t used in awhile.

As the first step of PCB design, I captured the schematic of the amplifier as built in AutoDesk Eagle. I did this on a livestream on YouTube, the first time I’ve tried such a thing. It was great fun! Kenneth W6KWF stopped by to lend advice – he deals with prototype PCBs as part of his day job, though he has team members to do most of the actual layouts when needed. We’ve had a great deal of fun over the years, including building a cloud chamber for seeing charged ions in high school.

Here’s the full (2h45m!) livestream in all its glory! There’s a recap and full-circuit overview at 2h41m for those who want to see the final circuit.

Hear you on the air!

73

This post is cross-posted to my more general-purpose nerdery blog, jeff.glass/blog.

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50W QRP Amplifier – First Demonstration

This post is cross-posted to my more general-purpose nerdery blog, jeff.glass/blog.

As I alluded to last week, I’ve been working on a simple “QRP Amplifier” to kick my power up from 5W to something a little more punchy. Specifically, an amp I can still use when portable. There’s something wonderful about achieving a contact with only 5W, but there’s also the frustration of getting into the field and having band conditions just wreck your day. It’d be nice to have the power to crank up the juice for special occasions.

While I have awhile to go before this project is wrapped up with a bow and ready for field use, here’s a brief video about my first successful test. 5W in, 50-60W out when run off two 13.8V sources in series:

More technical details to come, but for now, I consider this a really successful validation of the idea! Like I say, a few more critical steps to come, including an input 50-ohm pad, a low pass filter, and a case, but this is enough of a proof of concept to move forward.

Hear you on the air!

73

A Portable 20/30/40m Vertical Antenna

This post is cross-posted to my more general-purpose nerdery blog, jeff.glass/blog.

Following the Forth of July, I took a few days off of work to recuperate from a grueling work project that we pushed over the finish line on the third. And what better way to relax in the wake of a heatwave than getting out in the beautiful, low-70’s weather and working on a new portable HF antenna.

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From eBay listing

The heart of the antenna is an inexpensive ‘7.2m’ telescoping fishing pole, which can be had for less than $30 with Prime shipping or for less than $10 if you don’t mind waiting. The pole weighs about 10 ounces, comes with a small fabric sheath, and collapses down to about 24″. I’ve been wanting to try out something like this since I stumbled across VK3YE and his squid-pole setups awhile back – Peter’s also featured these particular poles in another video. Be aware, a pole called “7.2m” may not actually be 7.2 meters from end to end: check the listings carefully:

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Note the difference between the “stretch” column and the “specification” column.

The length of the pole is enough for a quarter-wave vertical for 20m with some room to spare on either end. To allow for multi-band operation, I added a QRPGuys Tri-Band Vertical accessory to the bottom of the antenna. The piece is essentially just two loading-coils (in this case, iron-powder toroids) with slide-switches to short them out. The 20m configuration is a true quarter-wave vertical; one of the toroids is switched in series for 30m, and both are placed in series for 40m. Ultimately, not a complicated setup. While it would be easy enough to homebrew, the ergonomics of the switches, the hardware to attach the antenna wire and radials, and the clever PCB setup are enough to make it worth the $15 to just buy the darn thing. It even has little notches on the edges of the PCB for straps/ties/rubberbands to attach it to the vertical.

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From QRPGuys.com

Tuning the antenna is straightforward: you cut a piece of wire (a bit long) for a 20m quarter-wave, and lay out four 10′ radials. Then, you bit-by-bit trim down the vertical element to resonate at the desired point in the 20m band. Then you switch in the 30m coil and compress/expand its turns without changing the antenna length to resonate on 30m. Finally, switch in both coils and adjust the second coil to resonate on 40m without changing either the antenna length or the first coil. Voila, a tri-band, base-loaded antenna.

Unfortunately, my antenna analyzer is old school, and doesn’t have a frequency readout. It’s an old MFJ-207 that I scooped up at the SMCC Hamfest in 2016, and while it does have a port to attach a portable frequency counter, I couldn’t find my cheapie one on the day. But I do have a nice Heathkit IM-2420 Frequency Counter with an internal OXCO that I scored an amazing deal on at a hamfest last year (it had an intermittent power switch). So, I attached the MFJ to the antenna, tuned its analog VFO for lowest SWR, walked inside without touching the dail and hooked it up to the frequency counter to see where the center frequency was. Repeat for say the upper and lower 2:1 SWR ranges. Trim the antenna a little, and repeat measurements. Once 20m is tuned, repeat with adjusting the coils for 30m and 40m. A fairly cumbersome process, but for three frequency ranges on one antenna, it was a half-hour project at most.

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In the end, the antenna is less than 2:1 SWR across all of the 20m band, all of the 30m band, and all but the top 50 Khz of the 40m band.

I’ve glossed over the mechanical details of the antenna to this point – the base of the telescoping pole fits snugly-yet-easily into the a piece of 1″ schedule-40 PVC pipe. I bought a 5′ section from the local big-box store and cut off a ~10″ section to hold the antenna. I strapped two ground-stakes that I got at Hamvention this year to the bottom with a couple zip-ties and a couple rubber-bands. Finally,  I threaded a long 3/8″ eye-bolt though a matching hole about 2″ from the bottom of the pipe and secured it with a nut on either side – this acts as both a stop for the pole so it doesn’t fall out the bottom, and provides an easy hand- or foot-hold for pressing the stakes into the ground.

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The finished antenna mount. It may be getting a coat of high-vis paint in the near future.

The setup for the radials was something I stumbled across by chance while buying the PVC pipe. Our local big-box hardware store was having a sale on these RECOIL Brand cable winders that are meant for headphones or charging cables or similar. I’ve found that they can almost hold four 10′, 24-guage speaker wire radials. This is solving the problem of wires-getting-tangled-in-a-bag that I’ve had with all my antennas to date. Thank goodness!

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A few loose ends are worth it for the assurance that the wires won’t get tangled in transit.

It takes about 6 minutes to setup or tear down the antenna:

  • The stakes are driven into the ground with a firm foot.
  • The telescoping pole is unwrapped and placed in the base
  • The antenna wire is unwound from the QRPGuys winder and tied to the tip of the telescoping pole with a small bit of cotton-wrapped nylon line (what we’d call tie line). The top section the pole is very flimsy, so I add a second tie to the next-largest section.
  • The pole is pushed up to full height, taking the antenna wire with it, which leaves the QRPGuys rig hanging about 2′ off the ground.
  • The QRPGuys rig is tied to the pole with another bit of tieline.
  • The radials are unstrung from their winder, pinched to the ground terminal on the QRPGuys rig, and spread out.
  • Run coax to a nearby table/seat/rock.
  • Set up radio, battery, key, antenna, and logbook.
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The antenna fully set up and freestanding.

Of course, my very first time away from home with the antenna… I forgot the radial wires. D’oh! I was way out in the suburbs, too. I wasn’t about to drive an hour home and an hour back for 4 bits of wire, so I first tried out the antenna with no radials (just the coax as a counterpoise). This worked alright – I picked up K2D in CT in the 13 Colonies event on the second call (this at 5W QRP with the ATS-4), but was having trouble with other contacts.

Since I planned to swing by the local Fry’s Electronics on this adventure, I decided to pause operating for a while and make that run. Mostly I was picking up parts for an amplifier project (more on that to come), but while I was there, I looked for solutions to my radial problem. I found a 10′ section of RJ11 phone cord with 4 wires for $1.69 – perfect! Back out in a new park, I stripped the wires out of their jacket, spread them on the ground, and tied them to the antenna’s ground terminal. Instant radials!

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And such colorful radials too!

With the antenna back to spec, things really picked up – surely, being on 20m at sundown didn’t hurt either. I scooped K2A (NY), K2B (VA), K2H (MA), K2L (SC), and K2M (PA), as well as the 13-cols bonus station WM3PEN in Philly. Many of these I got on the first or second call, though K2L was a real struggle. There was a very patient operator on the other end though.

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The 13 Colonies special operating event runs each year for a week around the 4th of July in the US.

I picked up a couple of other interesting stations along the way: PJ2/KB7Q out of Curacao (though the license gives away that he’s either an ex-pat or visiting), and CQ918FWC from Madeira Island (!) off the coast of Portugal. There were a number of these World Cup special stations on the bands this week as we close in on the finals. At 3800+ miles away, this was my best DX of the day, and a great proof of concept for the new antenna.

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At QRP wattage ,this 3800 mile contact was made at 760 miles/watt.

Hear you on the air!

73

Dayton Hamvention 2018

This post is cross-posted to my more general-purpose nerdery blog, jeff.glass/blog.

While I’m still exhausted from the travel and the good times, I wanted to put up a little note from this year’s Hamvention, the largest annual gathering of ham radio operators in North America. I only decided a week ago that I was going – I left Friday after work, drove 5 hours to Dayton OH (well, Xenia), and crashed at a hotel. Up bright and early, spent the day at the convention and checked out some local beer and grub in Dayton. Sunday, caught an early breakfest with some friends new and old, then got on the road back to Chicago. What a ride!

Others are doing fuller summaries of the convention – SWL-ing post always does a photo wrap-up of both the indoor and outdoor experience, and the Ham Radio Workbench podcast from the event is now up.

Hamvention is a great place for meeting the hams you’ve yet to meet, and seeing again those you already know. I spent most of Saturday hanging out with the Workbench crew, but I also ran into hams that I knew from elsewhere. Plus this guy, who falls into both categories:

It’s wild that 15 years after I accidentally introduced the future W6KWF to ham radio we hung out together at giant swapmeet in the middle of Ohio. Totally wild.

The flea market was certainly the biggest radio swapmeet I’ve ever been to – it’s probably bigger than that De Anza flea market by a good 300% – but it wasn’t all that special. I would say there was the usual assortment of used radios, test year, bits and parts, old tools… nothing super intriguing. Of course, I did get there on Saturday, so perhaps all the interesting things were just scooped on Friday.

It was neat to see a lot of the vendor products in person that I’d only heard about, but since I wasn’t in the market for anything in particular, I didn’t linger too long at any of the booths. Except Elecraft – those are some very, very attractive radios. I chatted with Wayne N6KR (one of Elecraft’s founders) for about 15 minutes about the KX2 and its SDR structure, which, not to be a fanboy, was pretty exciting.

In the end, I don’t know if I would go back the very next year – it was a really neat experience, and I’d go to see the people, but in this age of eBay, Amazon, and vendor websites, seeing everything in person and picking through the fleamarket feel just a little bit like a relic of the days when everything had to be done in person.

That said, I did find a few treasures… here’s this year’s haul:

Roughly from left to right:

  • Some panel-mount SMA connectors
  • An old automatic shutter trigger/timer
  • A tube of TFM-2LH Level 10 2Mhz-1000Mhz mixers from Minicircuits ($15 for 20, a steal!)
  • Six interesting potentiometers (dual with concentric controls or dual with concentric switch)
  • A bag of assorted HF/VHF H49 Crystals
  • An old Collins 250Khz crystal filter
  • A ZUMspot DMR hotspot/raspberry pi kit
  • A pair of QRP-Labs filters, both a low-pass and a bandpass filter for 40 meters.

Sometimes, you get back from a big trip or conference or meetup thinking Boy, am I worn out, I don’t need to do any more of that thing for awhile. This time, I came how itching to get back to work, revive some projects that had been dormant for awhile, and make things. So for that, at least, Hamvention 2018 was worth it.

Video – Introduction to Termination Insensitive Amplifiers

As I’ve alluded to previously, I’m currently working on a BITX-style transceiver, using bi-directional termination insensitive amplifiers in the vein of N6QWs ZIA rig and N2CQR’s BITX DIGI-TIA, with the added challenge of implementing a variable-bandwidth crystal filter. In the process of putting this thing together, I’ve been learning about and experimenting with these so-called termination insensitive amplifiers, and I’ve put together a little video about their characteristics and uses:

This is my first foray into video-making, and I had a great time putting it together. Looking forward to making a few more down the road.

See you on the air!

73

Hamfesters’ Hamfest 2017; Homebrew RF Power Meter

This morning, I trekked out to the Hamfesters’ Hamfest in Peotone, IL for the second year in a row. Though the rain threatened to put a damper on the day, the sun had burned through by 10am and left everyone alone.

There was an interesting mix of vendors at this year’s Hamfesters – lots of assorted “hardware” (tool, dental picks, kitchen sink), but also a good number of radios new and old. There was a table absolutely stacked with older 2M gear, mostly Heathkit. A couple newer ICOM rigs, the usual FT-101e’s… nothing too exciting. I scooped up a handful of beefy heatsinks meant for CPU’s, but will be re-purposing them as LED heatsinks in future projects.

I did find two real scores at the Hamfest today. First, from Patrick-J, a secondhand, gov’t-surplus Tektronix TDS644a. It’s a 4-channel digitizing scope, with 500MHz bandwidth at 2GS/s, and I got it for a song.

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Not unexpectedly, when I got it home and opened it up, it’s in need of a good re-cap-ing (see, for example, W6KWF’s recent video on this subject.) I’ve already got two packs of electrolytics on their way from the wide wide eBay, so I’ll pick this project back up on a future weekend.

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The second find was a small, slant-top box with a 1mA DC meter movement already built into the top. It was configured originally to be a shunt ammeter, but I really just wanted the nice box and meter. Instead of measuring AC current, it’s now set up to measure RF power.

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The original unmodified meter – something out of some other amateur’s homebrew shack.

The only external sign that the box has been altered is the presence of a small SMA jack, but the internals are entirely different. I populated and installed one of the AD8307-based power meter PCB’s that I cooked up back in March, fed directly from the external SMA connector. The output feeds the mA meter through an op-amp buffer – I started with a direct connection, but the AD8307, with its 10k-15K output impedance, wasn’t up to the task. The voltage output from the AD8307 is also directly fed to the red external binding post, for measurement with a DMM. I added a little green power LED to remind me to turn the darn thing off. I left the original switch in place, just because it feels right.

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The final meter, with an SMA/BNC adapter hanging off it.

The schematic for this project is essentially the same as W7ZOI’s Power Meter published in QST – see that article for details, as well as Wes’ further errata. The meter covers the range of the AD8307’s output voltage, from about -70dBm to +20dBm. As I’ve been working on putting together my BITX homebuild (more on that soon!), I’ve been thinking that having an RF power meter with an analog movement would be very helpful in peaking filters and looking at relative input and output power. And now, I have one!

See you at the ‘fest!

73

ARRL Field Day 2017 – QRP Portable

In contrast to my field day adventures last year, in which I hung out with the folks from the North Shore Radio Club and ran 100W on a K3S, this year I opted for a decided more small-scale approach. Using my ATS-4 receiver, I ran 3.5W CW on 15m, 20m, and 40m for about four hours Saturday afternoon in the lakefront park here in Chicago.

My antenna was a single 40-some-odd-foot wire hung from a tree, fed by my ZM-2 antenna tuner. I think when I built the ZM-2 last year, I goofed something up in the SWR circuitry – the built-in LED should go out at minimum SWR, but mine seems to be brightest at low SWR. To compensate for this, I brought along the MFJ-207 Antenna Analyzer I got at the SMCC Hamfest last year and used that to adjust the antenna tuner.

With the big analog, multi-octave dial on the front of the 207, I found it useful to first tuner the ATS to the desired frequency (say, 14.030) while attached to the antenna and adjust the analyzer until I heard the “WHOOP” of its signal generator in my headphones. With the frequency of the analyzer and receiver close to matching, I’d move the coax back from the tuner to the analyzer and adjust the ATU until the 207 showed lowest SWR. Reconnect the ATS, and away we go!

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The MFJ-207 Antenna Analyzer covers up to an octave with a half-turn of the variable cap. Listening for its signal on a receiver made it much easier to find a specific frequency.

I used a new battery setup for this outing – a 12V, 6000mAh TalentCell lithium-ion pack that I borrowed from work. I love this pack – a little less than a pound, charges from a wall-wart, and has a built-in barrel connector and on-off switch. There are also models with a built-in 5V USB charging port, for topping off cell phones and other devices on the go. While the ATS is designed to run at 12V MAX (not 13.8V), I found that the 12.2V the pack was putting out proved to be fine – it seems the limitation is in the heat dissapation from the BS170 finals, and running relatively low duty-cycle search and pounce that wasn’t an issue. I’ll be ordering one for my own use soon. (Or perhaps the even more compact 3000mAh version – the size of a deck of cards!)

The rest of the pack list included:

  • A HamKey brand iambic paddle
  • A golfball and a kite-string winder for getting the antenna wire into a tree
  • A pair of Koss UR-20 headphones
  • A small battery-powered speaker with 1/8″ aux input
  • Variable DC and Coax jumpers
  • A notebook and pen
  • A folding camp chair.

All of the above fit into a small laptop bag, along with a few other tools and bits I didn’t end up needing.

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The full field day setup, minus the antenna.

I managed 69 contacts, all QRP CW hunt-and-pounce, during my operating time – no tremendous DX, but I did hit a couple of rocky-mountain states and a plethora of sections up and down the East Coast. Final score was just over 1000 pts – that QRP multiplier really stacks up!

I also couldn’t have asked for better weather on the day – 72 degrees and slightly cloudy with a pleasant breeze. Simply stupendous.

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Hear you on the air!

73

SI5351 Signal Generator

As I alluded to in my March bench report, I’ve been working on packaging up my old SI5351 VFO that sat on a breadboard for two years into a functional signal generator for the bench. That project is now essentially complete, though there are a couple of planned improvements in the works.

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The heart of this project is a Silicon Labs Si5351 clock-generator IC, on a little interface module from Adafruit.  With an I2C interface, it can output square-wave signals at roughly 6dBm anywhere from 8Khz to 160 MHz. This nifty little chip has been around for a few years, and has been used in many a homebrew radio rig, including the work of N6QW, N2CQR, M0XPD, and other. Jason NT7S was essentially in building some of the earlier Arduino-friendly libraries for controlling the Si5351. With such a wide frequency range and the confidence that comes from XTAL-derived, PLL frequency generation, this little chip has breathed life into rigs both new and old.

With the hardest part of building a SigGen (actually producing the darn frequency) neatly packed away in silicon, what remains for a builder is to wrap the thing in packaging to promote ease of use and flexibility. That’s where the fun of this project lay. The parts list ended up like so:

The project box and the LCD display were the most expensive parts of this build by a wide margin, although both would be available for quite a bit less with a longer lead time from oversees.

The SigGen has 4 different software modes:

  • VFO – The primary output frequency matched the displayed frequency. Frequency steps are 10kHz, 1kHz, 100Hz, and 10Hz.
  • Test – Same as VFO, but steps are 10MHz, 5 MHz, 1 MHz, 500kHz, 100kHz, 10kHz, 1kHz, 100Hz, 10Hz, and 1Hz.
  • Polyakov – Primary output frequency is half of the displayed frequency, for experimenting with half-frequency-driven mixers. Step size is the same as VFO mode.
  • BFO – Adjusts the secondary output frequency, for using the SigGen to drive a Superhet directly. Steps are the same as in Test mode.

The SigGen’s three pushbuttons (one built into the rotary encoder and two next to the LCD screen) do essentially the same thing in each mode:

  • Encoder: Changes the frequency step size, per the step options above.
  • Mode: Changes through the software modes, per above.
  • Band: Advances to the next highest Amateur band frequency, to aid with band-changes especially in VFO mode. For example, if the current frequency is 5.72MHz, pressing the Band button sets the frequency to 7.0Mhz, the bottom of the 40m band.

There are still a couple improvements I’d like to make, namely replacing the cheapo detent-based rotary encoder with a nice smooth continuous one, which can be had for about $10 on eBay – using a detent-based encoder as a VFO is a pain in the patoot. I’d also like to implement a sweep functionality to sweep through various filters, but that would require a bit more UI work as well.

This gadget has been complete on my bench for a month of so, and has already been useful in working on my variable-bandwidth superhet… but that’s a story for another post.

Hear you on the air!

73

 

SMCC Hamfest 2017

Sunday morning I attended the Six Meter Club of Chicago’s annual hamfest in Wheton, IL. Much like my visit to them last year, it was a well attended, fairly well stocked event, with perhaps 4 or 5 dozen tables outside and the usual smattering of vendors indoors. I was a bit of a lazybird this morning and din’t get there till close to 9am, when the flea market folks were already making noises about packing up. Thankfully, most stuck around until after the 11am auction.

There were a number of tube testers at the fest today, both at the flea market and indoors. But the bell of the ball was an old Rocketest freestanding model that showed up at the auction. I think it went for $10 to a fellow who really just wanted the interesting nameplate. Sad to see it get parted out, but it really has reached the end of its time.

Speaking of the auction, that’s where I picked up most of my haul today. All in all, I can home with:

  • A Tenma 72-475 sweep generator (working!)
  • A ‘Model 175’ Oscillator Comparator (not yet tested)
  • From some old homebrewer’s box of bits at the auction, $1 each:
    • An inductive SWR bridge
    • A resistive SWR bridge with detector circuit and DC amplifer
    • Another SWR bridge, I think?
    • A 1/4″ headphone extender with volume adjustment
  • Five sheets of some thin, flexible copper clad for experimenting (a dollar a sheet!)
  • Another breadboard (can never have too many)
  • A spare set of flush cutters

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I haven’t been able to find an available online manual for the 72-475 sweep generator (just this datasheet for a similar model) but it seems to be a relatively straightforward device. The main dial selects between 0.0 and ~2.3 continously, and 7 range switches select between 1Hz, 10Hz, 100Hz, 1KHz 10KHz, 100KHz, and 1MHz multiplies.

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In manual mode, the device is simpler a function generator, with sine, sawtooth, and squarewave outputs. In swept mode, the device will sweep over about 3 decades below the selected range. So, for example, with the dial at “1.0” and the “1MHz” range selected, the automatic sweep will go from 1 Khz to 1 Mhz. It can sweep linearly over the range or logarithmic-ly, and total sweep time is adjustable from about 0.3 seconds to 15 seconds. There are also adjustments for the output amplitude, DC offset, and overall sweep time. In addition, a dedicated “TTL/CMOS” BNC jack is mounted next to the the main output for driving digital circuits.

As a preliminary smoke test (since I bought this thing as-is-no-test), and after verifying that the Power On LED lights (good sign!), I tried passing the signal through an old Vectronics 821 Super CW Filter that I got from another Hamfest. The 821 is a variable audio filter with a 750 Hz center frequency and selectable 180, 110, and 80 Hz filter ranges. Setting the sweep generator up to sweep from 1Hz to 1000Hz, the peak of the filter was audible, though not as pronounced as I would like. I feel this is a good smoke test of the sweep gen, and a questionable test on the filter. (Looking at the sweep into a 50-ohm load on an oscilloscope also showed promising behavior.)


The Oscillator Comparator is a nifty piece of kit from the 70’s, designed to allow a homebrewer or other ham to calibrate a frequency source against the (then-ubiquitous) 3.579 NTSC “Colorbust” frequency. The idea in broad strokes (outlined in a 1975 QST article rubber-banded to my purchase) is phase lock an internal 3.579 MHz VCXO to your input signal, which then generates a color-bar test pattern for viewing on a TV.. By connecting this signal to an analog TV’s Chroma input, if the derives 3.5795454… frequency exactly matches the colorburst frequency that the TV is receiving on one of it’s analog “Network” channels, the set of bars will not drift an its colors will be stable. If the VXCO frequency differents from 3.5795454 MHz (because your input signal is not exactly 2.5/5/10 MHz), the color bars will change in appearance over time.

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The advantage of this somewhat cumbersome system of calibration is that the Colorburst frequencies embedded in network television signals would presumably be of very high precision. Probably from a Rubidium source or better, or possibly derived from another atomic source where possible. So the home experimenter would have access to this high-precision time-base “over the air,” as it were.

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Interestingly, the manual notes that a perfectly-still color bar pattern does not, in fact, represent a perfectly calibrated signal. The networks, apparently, offset their Colorburst frequencies below the National Broadcasting System standards. Roughly 300 parts in 1010, to be specific, with some variation by network. Thus, a perfect 10Mhz signal, for example, should cause the rainbow color cycle to slowly change with a period of roughly 9.3 seconds.

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It’s unclear from some quick investigation whether there are any NTSC signals still on the air in the states. All mainstream broadcasting has gone ATSC or other digial format, but it seems there may still be some low-power stations in major metropolitan areas still transmitting in NTSC. Being in the city limits of Chicago, I hope to find these signals, if they still exist.

Finally, one humorous note from the manual:

“In order to make meaningful use of the comparator, the user must be certain that the received T.V. signal is of network original. Since most of the daytime programs, especially the soap operas, are, this task is relatively simple.”

Hear you at the bench!

73

Bench Report – HamRadio360 Podcast, Beach-40, SigGen, Power Meter Projects

A few weeks, back, I spent about half an hour chatting with Jeremy, KF7IJZ, for a listener-projects segment on the HamRadio360 Workbench podcast. That episode just went live this morning, if you want to take a listen online. Jeremy and I had a swell time chatting about the QST 40-9er Transceiver project I built awhile back, and ended up packaging with a laser-cut faceplate.

Seeing the podcast pop up got me thinking – from the last few months of posts, you’d think that I’d been spending all my time operating and that my bench was empty. Not so! There are three major projects milling about the bench these days. None are complete enough to merit a full post, but consolidating them all in to one post feels right.


First on the bench is the continuation of my Beach 40 Transceiver project, that I’ve been working on since the Fall. I’ve been making steady progress recently on the penultimate stage, the RF amplifier, which will take the ~10dBm modulated voice signal from the balanced modulator and turn it into ~33dBm (2W) of RF output power to send to the LPF and out into the world. The amplifier is a three-stage design, with a 2n3904 buffer feeding a BD139 driver and another BD139 final.

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Beach40 PA Schematic and Layout, in Graphite-O-Vision.

The module is assembled manhattan-style on single-sided copper glad, just like the rest of the project. The layout surrounds the +12V-on-transmit bus down the center of the PCB… more on that error later.

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The Beach40 PA mid-assembly – you can see the +12VT buss running down the middle, and the buffer and driver stages starting to take shape.

The module assembly and testing went fairly smoothly – using a variable-voltage power supply, it’s easy to vary the output power of the amplifier, and to observe the changes in PA heating that this causes. You can see the two large screw-on heat sinks on the BD139’s in the following pictures, as well as the the large ferrite bead which feeds the Final. There’s also a chunky 100uF cap at the DC input to the board for additional decoupling.

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The assembled Beach40 PA.

 

The problem I’m having now is the same one that plagued me way back in my assembly of the Virgin Receiver, my first first homebrew receiver: this amplifier would really love to be an oscillator. With the PA voltage set at 12V, any time the signal imput is sufficiently strong, the PA will fall hard into oscillation, and won’t stop until the PA voltage is brought way down or cut. I’ve been experimenting with emitter degeneration for the final BD139 (the original schematic has the emitter directly grounded). That’s helped, but not much. I’ve also changed the RF-carrying wires from hookup wire to coax, to mitigate feedback through other modules. Another help, but so far, the oscillation problem is still there. The next step will be to move the modules themselves around, to limit the amount that the high-power RF coming from the PA has to pass by earlier stages of the transmit chain (VFO, mic amp, balanced modulator) to try to eliminate feedback that way. Fingers crossed!


Next on the bench is a repackaging of an old project – my SI5351-based “VFO,” which for the better part of a year has been serving as my primary signal generator for experimental projects, spread out in all its Al-Fresco glory on a breadboard. With the amount of troubleshooting I’ve been going through on the Beach 40 project, I’ve decided to finally box up the SI5351, Arduino, and display into a proper project box, and make the thing a real SI5351-based Signal Generator. The schematic is essentially unchanged (minus the PA) since I used the project in my SI5351-based transmitter project, which is reproduced below:

CW Transmitter sCHEMATIC

To that end, I purchased an inexpensive nibbling tool and an expensive project box from the local Fry’s electronics, and have been working in the past couple weeks to marry the spread-out guts of the previous project with the clean lines of the enclosure. Biting out the large hole for the display 1/8″ at a time was time-consuming and strangely soothing.

At this point, the display and Arduino are connected, but I have yet to wire the two control buttons, rotary encoder, and Si5351 breakout board back into the Arduino. I also need a provision for getting the signal out the front of the darn thing, so I’m waiting on a shipment of SMA connectors and a jumper to go from the SI5351 breakout board to the front panel. So far, so good.

The code for this project is still on Github.


The last project on the bench is an RF power meter circuit, based on a circuit by W7ZOI from 2001. While the old scope-probe-across-a-50-ohm-load technique has proven very useful, I’ve found myself wanting a way to more reliably measure RF power at low levels, and to do it in a way that could be interfaced to a microcontroller or computer. To that end, I plan on using the W7ZOI circuit connected to an Arduino, much like Vu2ESE’s Sweeperino, to make digital power measurements.

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I rolled a little PCB for this project from OSHPark, and since that service provides PCBs in multiples of 3, I figure I’ll connect one board to an analog meter (for that old-school feel), one to an independent Arduino (for digital measurements), and embed one inside the SigGen project, for marrying signal generator and power measurement at specific frequencies. This last project, I figure, will be especially useful for examining filter behavior at HF and piping the information to a PC for display an analysis.


So that’s what’s on the bench at the end of March 2017. All of these will hopefully merit full posts in future as the projects come to fruition, but for now, the is the smorgasbord that is my bench.

Hear you on the air!

73