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.

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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.

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.

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.

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!

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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:

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.

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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.

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.

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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

Beach 40 Build: Part 6 – T/R Switching

This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.

The T/R switching for the Beach 40 is pretty straightforward.A single DPDT relay is controlled directly through the PTT  switch on the mic. Half of the relay switches the low pass filter connection between the transmit and receive sections. The other half routes power to the audio amplifier on receive and to the mic amp and RF amps on transmit.

Or it would be, it would have been straightforward if I hadn’t soldered the connections incorrectly twice. The first time, I turned the relay 180-degrees before soldering it down, so that the control voltage across the Normally Open pins, which obviously didn’t do anything. The second time, I reversed the coil and normally closed (receive) sections, so while I was able to test the receiver, the transmitter section would never get power.

But at long last, this section is complete. It’s a pretty adorable little relay, an NTE R40-11D2-12. It’s rated at 2A, which is way overkill for this project, but it’s what was available at Microcenter. The 12V coil voltage is driven directly from the battery input voltage.

It’s very satisfying to hold a mic in my hand for the first time on this rig, and hear the audio click in and out in the headphones when pressing the push-to-talk button. After a couple weeks away from this project, it feels like I’m closing in on completion.

Hear you on the air!

73

Beach 40 Build: Part 5 – Mic Amplifier

This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.

Moving on to the transmit-only portions of the Beach 40, today I completed the microphone amplifier. Powered on transmit only, it’s responsible for both boosting the microphone input level to inject it into the mixer, and for switching the microphone out of the circuit on receive, as necessary.

One difference that I’m going to introduce into the original VK3YE design is that I’m going to use a dynamic mic instead of an electret mic, specifically one of the mics I picked up at the SMCC Hamfest back in June. This likely means I’ll need a bit more gain in the pre-amp than if I was using an electret element.

Peter mentions the possibility of using a dynamic microphone on his page on the Beach 40. He says:

“The circuit is suitable for an electret microphone. If using a dynamic unit leave out the 22k [bias] resistor and possibly raise the 100n [mic] coupling capacitor value if insufficient or thin audio.”

Leaving the bias resistor out of the original circuit, and replacing a couple of the electrolytic caps with ones from my stock, I ended up with this:

A quick spice simulation shows about 15 dB of gain through the audio spectrum with a slight high-pass characteristic. Note that I’m making a couple of approximations here. I don’t know exactly what the impedance of the dynamic mic is, but varying its input impedance between 100 and 500 ohms shows only a 2dB variation in gain, so I’m not too concerned. Similarly, I’m measuring power into a 50-ohm resistor as a representative of the mixer, instead of modeling the mixer directly.

This circuit came together very quickly on a piece of copper-clad, just five pads and a few minutes of soldering. The next step will be attaching the mic amp to the mixer and determining whether I do in fact need more gain. I’ll also need to experiment with the microphone input cap to see if I need more low-end response from the mic. But this is a start!

Hear you on the air.

73

 

Beach 40 Build – Part 4: Low Pass Filter

This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.

Next on the bench is the low-pass filter, which is mostly responsible for curtailing the harmonics generated by the VFO and mixer. Since limiting out-of-band signals and noise is also useful on receive, the LPF is connected directly to the antenna socket to be useful in both modes.

This LPF is particularly simple, using just three RF chokes and six capacitors. VK3YE’s original low pass filter used a couple components I didn’t have (namely the 820pF caps), so I played around a little in LTSpice to work out a viable filter with the components I had. Here’s what I ended up with:

SPICE simulations show that this filter maintains a similar low-pass characteristic to the original VK3YE design. Moreover, the insertion loss is approximately equal. The match to 50-ohms isn’t great, but is acceptable. And perhaps most importantly, attenuation at the second harmonic is strong at about 42dB:

Constructing the LPF was simple – just six capacitors and three molded chokes that I picked up on a recent trip to California. I laid the filter out on a small piece of scrap copper clad. I enjoy when the layout of a board can highlight the symmetry of circuit itself.

Before attaching the filter to the rest of the project modules, I did a quick manual sweep of it with my SI5351 board. It’s somehow very satisfying to roll the frequency up past the filter knee and see the higher frequencies just drop off. One word of caution – the SI5351 doesn’t have perfectly uniform output power across all frequencies, so I put one channel of the scope on the input of the filter and one on the output, to observe relative attenuation. Of course, terminating the filter is a must.

Filter attenuation at 14 MHz, roughly the second harmonic of the desired transmit frequency.

Hear you on the air!

73

 

 

Beach 40 Build – Part 2: Diode Ring Mixer

This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.

Charging forward with my project to build VK3YE’s ‘Beach 40’ 40m DSB transceiver,  today I tackled the diode ring mixer. The mixer serves as the product detector, converting RF signals down to audio. It also serves as the balanced modulator, turning audio from the microphone into a double-sideband signal at the appropriate frequency.

This is a particularly simple diode ring arrangement, with only one output transformer to wind. No trifilar windings here:

The local oscillator signal is provided by the VFO I built, which has an available power of at least +7 dBm, as required for good linearity in a diode ring. In receive mode, HF signals from the antenna get converted down to baseband (audio) and pass out the audio port. In transmit mode, the signal flow is reverse, and signal from the microphone travels in through the audio port and modulates the local-oscillator, creating a double-sideband suppressed-carrier HF signal, which is then routed to an RF amplifier stage before ending up back at the antenna.

Two balance controls are provided for nulling out the carrier at the RF port – a 200-Ohm trimpot (which also serves as the LO injection point) and a ~40pF trimcap, which balances the nominally 22pF cap on the either side of the ring. No attempt was made to match the 4 diodes, they were just the first four 1n4148s I pulled from the bag.

The following picture pretty well illustrates my process of building one of these little modules. First, I re-draw the schematic by hand, just to make sure I’ve got a handle on it. Then, I’ll identify the different networks of attachment within the circuit, as each one of these will want to be its own isolated pad. I use that to make a rough sketch of the pad layout (bottom-left in this picture). Much erasing and re-doing happens at this stage! Once that’s mostly sorted, I’ll do a more precise sketch of the pad layout using the actual component sizes (bottom right). Then I’ll duplicate that design in pencil on the copper clad, and have at it with the Dremel, followed by the soldering iron.

The completed diode-ring mixer. The RF comes in/out of the yellow 100nF cap on the left. Audio comes in/out of the rectangular pad at the top center. The LO is injected at the center pin of the balance pot. Coupling capacitors got moved to other boards.

This balance pot, by the way, came out of the mysterious silver radio I got at the DeKalb Hamfest back in May. Huzzah for re-using old parts!

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Upon testing, I’m encountering a bit of difficulty totally balancing this mixer. From my understanding of how the diode ring works, with no signals present at the audio port, there should (ideally) be nothing appearing at the RF output port port. I was expecting to see a little feedthrough of the LO, but the lowest I can seem to acheive by tweaking both the balance pot and the trimmer capacitor still leaves about 50 mV P-P of LO signal at the RF port, which seems like a bit much. Then again, the LO signal is at around 4V P-P going into the mixer, so proportionally, that’s not too shabby. (not quite 40 dB). Some of that could also just be radiating straight from the LO, or from the 12″ clip-lead connecting the VFO and the mixer at this point.

The mixer/VFO test setup

 

For the time being, I’m going to note the balance issue and move on. From my reading, I understand the proper termination of the mixer is essential for good balance. Since the surrounding components are not in place yet, I can’t imagine I have proper termination on this thing. I’ll return to this down the road as the transceiver takes shape.

What’s a few millivolts of RF between friends?

Hear you on the air!

73

Beach 40 Build – Part 1: VFO

This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.

After listening some of of the recent episodes of the Soldersmoke podcast, and hearing about their adventures great and small having to do with VU2ESE’s BITX SSB transceiver, I’ve got a hankering to build something from scratch. Rather than jump in at the deep end, as it were, with an SSB transceiver, I’m starting with something more approachable: Peter Parker VK3YE’s “Beach 40” Double Sideband transceiver.

There’s a host of information on the internet about this project, but Peter’s project page, linked above, is a good place to start. You can also find some expanded notes on the build in a write-up of the Beach 40 Lo-Key magazine at the time, several years back.

The project is conceptually simple: A balanced diode ring mixer functions as the product detector on receive, and as the balanced modulator on transmit. Hook up the RF-pre-amp and an LM-386 audio amp, it’s a receiver. Hook up a mic amplifier and an RF power chain, it’s a transmitter. No muss no fuss. A handful of transistors, one IC, about 2-3W RF output.

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Peter’s original design used a ceramic resonator in the VXO, though later versions of his employed a “super-VXO,” in which two or more similar crystals are paralleled for extra pulling range. I decided to use a true LC VFO in mine, as I’d like to cover from 7.125 to 7.300, the entire USA SSB range, if I can.

This is VFO design I started with. It’s a Hartley oscillator straight out of EMRFD. To that, I added a simple common collector buffer stage:

Unfortunately, this design didn’t produce a high enough power level to drive a diode-ring mixer. Traditional diode ring mixers want to be driver with around +7dBm available power on their LO port, and I was only seeing 2 dBm. So I added an additional common emitter power stage to boost the signal level.

Once the thing was oscillating at the proper power level, I spent a good hour tweaking the inductors and capacitors in the oscillator’s tank circuit to get the frequency spread down to the appropriate range in the 40m band. The final result is actually a bit wider than I’d like it to be, so some additional tweaking in the future will be necessary. I also found that, as I added additional shunt capacitance to the tank, I had to increase the coupling capacitor (originally 47pF, now 220pF), to get the circuit to continue to oscillate reliably. Here’s what I finally ended up with:

One thing this process really drove home is the importance of shielding the VFO – the hand capacitance effects alone during debugging were making me a bit crazy. Sometimes just reaching in to hook up a scope probe would cause the thing to stop oscillating or start oscillating, or develop some parasitic oscillation…. woof. I’ll have to build a little box for the thing now that it’s essentially in the right shape. But for the moment, here it is, in all is little glory:

As I’ve documented before, I find that a variation on the Island Squares method of circuit construction is very approachable for this kind of circuit building. Essentially, small isolated pads are cut into a piece of copper-clad board using a dremel, and the components soldered to those. Ground connections attach to the ground plane of the board. It’s easy and quick, and allows for long connections to be made when necessary (the 12V bus on this project spans one whole side of the board, with de-coupling along the way).

I’m planning to build the next stages in the order Peter suggests in his Lo-Key article: Audio output amplifier, the diode-ring mixer, low-pass filter, mic amplifier, and the RF power chain. Since he originally published the design, Peter’s changed from a discrete-component to an LM386 audio amp, added an RF pre-amp, and stuck an L-Match tuner inside the case as well. I’m planning on adding the first two modifications, and perhaps an outboard unit for the third.

Hear you on the Air!

73