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:

Beach 40 Mic Amp-07

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.

Mic Pre-Amp Simulation.PNG

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

 

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Beach 40 Build – Part 3: Audio Amplifier

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

Today I tackled the audio amp section of the Beach 40. And when I say tackled, I think perhaps the player was already down, and I just had to fall on top of him.

The audio amp configuration I’m currently using is the bog-standard, straight-from-the-datasheet LM386 20x or 200x configuration. A SPST switch puts the 10uF cap between pins 1 and 8 of the IC, or takes it out of the circuit.Currently, I don’t even have a volume control pot in place. Peter VK3YE later modified the LM386 configuration for greater gain and less hiss, which is something I’d like to tackle once I’ve gotten the rig working.

Beach 40 Audio Amp-05

Part of what made today so simple is that I’d already built this audio amp section for another project. I’d been using a bit of scrap copper-clad to experiment with coupling between an NE602 and an LM386, to see if passive filtering would be a useful option for future receivers without introducing too much loss. But since I haven’t touched that project in a couple months and this one’s exciting me now, that board hit the chopping block.

IMG_1266
The old receiver experimentation board. What a Frankenstein!

Extracting just the audio portion of the board was simple enough – now that I’m looking at the pictures and seeing the two big bypass capacitors on the red power line reminds me that I need to think about bypassing the power input at the source, probably with something beefy. I’ve had enough LM386’s turning into oscillators, and I’d rather not subject my ears to that too much if I can avoid it.

IMG_1267


With the VFO, mixer, and audio amp “completed,” I now have the bare bones of the receiver portion complete. Of course, there’s exactly zero filtering, either at the front or the audio portion, so it’s not like this would make a particularly reliable bit of kit. But it’s got enough parts that I can string them together and test.

IMG_1276.JPG

While I didn’t exactly succeed in keeping the leads short, using clip leads was just the fastest and simplest way to tie the modules together. RF from the ‘antenna’ (read: long bit of wire strung around the office) is brought into the mixer via the white clip lead at center. The VFO drives the mixer and converts the RF down to audio, where it’s amplified and sent out to the headphones.

To generate a test signal, I used my SI5351-board without an amplifier, with just a short bit of wire hooked directly to the output of the Si5351. I set the output somewhere around 7.150 MHz. Tuning the VFO across the band with the polyvaricon, I found my strong local source no problem. The receiver works!

I can see why Peter later added a fine tuning control to this project – with half a turn of the variable cap, I’m covering around 200Khz of the band, which makes tuning in to an individual station tricky. I do have a rather large panel knob in mind for the cap, which will help, but we’ll see if it’s enough. If I can’t get the VFO drift a little more under control, that fine tuning cap might be necessary to help stay on 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:

Beach 40 Mixer-04

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.

IMG_1271
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!


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.

IMG_1272
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.

IMG_1273
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.


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:

Beach 40 Schematics-01

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.

Beach 40 Schematics-02

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:

Beach 40 Schematics-03

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:

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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).

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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