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

 

Easy QRP Low Pass Filters

With an eye toward getting my rudimentary Si5351-based transmitter on the air, I’ve started putting together a number of low pass filters to knock down some of the nasty harmonics that come with using a clock chip as a frequency generator.

The filters are all based on the designs given by W3NQN in the GQRP technical pages from 2015. (This is another point at which I’m taking design inspiration from the QRP-Labs QRSS beacon.) The paper lists both the target values for each of the toroids and capacitors for a 7-pole low pass filter for each band, as well as the required number of turns for each toroid and an appropriate toroid to use. For this project, since my target frequencies range from about 7 MHz to 14 MHz, I’m using all T37-6 toroids from KitsAndParts.com. The basic design of each filter is the same:

All these filters are symetrical, for 50-ohms on either side.

Here’s the values that W3NQN gives for low pass filters for the 20m, 30m, and 40m bands:

Band	C1, C7	C3, C5	L2, L6	L4
20m	180pF	390pF	.773uH	.904uH
30m	270pF	560pF	1.090uH	1.257uH
40m	270pF	680pF	1.380uH	1.698uH

Unfortunately, I don’t have exactly the specified capacitors in my kit, so here’s my approximation to these values. (Note that the number of turns is different at 40m than W3NQN recommends, to more closely match the specified inductance). All inductors are wound on a T37-6:

Band	C1, C7	C3, C5	L2, L6 (turns)	L4 (turns)
20m	1000+220 (series) = 180pF	1000 + 690 (series) = 408 pF	16T = .77uH	17T = 0.87uH
30m	220+47 = 267pF	220+220+100 = 540pF	19T = 1.08uH	20T = 1.20uH
40m	220+47= 267pF	470+220=690pF	21T = 1.32	24T = 1.73uH

Some quick simulations in LT Spice confirms that these little adjustments don’t have an enormous effect on the filter’s behavior. On 40M, for example, it has the effect of moving the cutoff frequency down maybe 100 KHz:

The behavior of the 40m LPF as constructed with the parameters above. Note the nice knee just under 10MHz and that the attenuation is around 40dB at 14Mhz, the second harmonic for this band.

So far, I’ve only constructed the 40m filter. It’s built on a little 2.3cm x 10cm piece of coppper clad. (I’ve got a bulk stock of 7cmx10cm pieces, which cut nicely into threes to make little projects like these).

The 40m filter as built. I love these little screw-on BNC connectors. There’s probably some RF leakage because of them, but they’re so easy to apply and use. And it makes changing male and female BNC connectors as necessary a breeze.

The filter seems to be working great – a little qualitative examination with an oscillloscope shows that the filters are smoothing out the waveform at 7 Mhz quite nicely. Above that, the attention seems to match the slope predicted in the simulation above.

With this filter on my little homebrew transmitter, I finally made my first (official) WSPR contact… but that’s a story for another post.

Hear you on the air!

73

LTSpice – First Steps, Already Helpful

LTSpice is a fairly approachable, very powerful circuit simulation software system. It has a straightforward GUI interface – drop in parts from catalog, draw wires, run simulation – with a strong simulation engine sitting behind it. I downloaded it this week for the first time, and it’s already proved its worth.

Many others have written excellent tutorials about using LTSpice, so I won’t rehash the details here. I found the tutorial on All About Circuits to be very useful, if a little bit hard to follow.

One of sample projects that that tutorial walks you through is analyzing the response of a basic LC bandpass filter using LTSpice’s AC Sweep mode. After working through that, I thought it might be fun to model the front end filter on my little Direct Conversion receiver to see what the actual (expected) frequency response is. Since I had ripped those values straight from the GQRP Sudden, it seemed like a good learning experience.

So, I quickly drew up a little model of that front end filter. It looks something like this:

A pretty-little schematic! The series/parallel capacitors used values I had on had to get close to the GQRP filter values.

And, running a sweep from 1 MHz to 10 Mhz, I got… a steaming pile of what the heck kind of input filter is this??

The first AC sweep of the above schematic. More like a throw-it-in-the-bin-put filter….

-24dB at best, a “peak” somewhere above 25 MHz, there’s no way the the GQRP would have put this out in a kit. I went back through my network, looking for a dropped connection, or a capacitor with a value in farads instead of picofarads (boy will that cut your gain). But no dice, the circuit seemed solid…

…Until the third time I checked the component values, and noticed one fatal slip-up. You see, R1 and R2 in the above schematic represent the source impedance and the input impedance, respectively. While the source impedance could be a nominal 50 ohms, the RF input impedance for the NE602 mixer is at least 1.5 kOhms. Doh!

So, with a slight change to R2 in the schematic, and re-running the simulation:

The same filter as above swept with the correct ~1500ohm output impedance.

Now that’s more like it! A nice peak in the middle – not a ton of attenuation out of band, but pretty good for a little five element filter. We can even zoom in on that peak a little

A zoomed section of the sweep from above. Not quite the ideal response, but pretty close for a first-ever filter.

It looks like my filter’s currently peaked at around 6.63 MHz, instead of the ~7.15 MHz I would have hoped for, but considering the lower and upper boundaries of the US 40m band are only 2 and 4 dB down from the peak respectively, I’d call that a good first start.

A little more messing around showed that using a proper 10pF series capacitor instead of ~9.8pF doesn’t make much difference. The best solution I found to be to reduce the shunt inductors to about 4.8 uF, which puts the filter peak at just over 7 MHz. Of course, this assumes all ideal values and no parasitic effects for any of the components. And that my hand-wound toroids actually match these inductances. But hey, it’s somewhere to start!

—

It turns out, LTSpice isn’t just useful as a software tool – it makes a great rubber-duck debugger as well. While squinting down at the board to double-check the value of those tiny series capacitors, I realized I’d made a horrible assembly error. I should have put those two 200pF shunt capacitors in series on either side, as in the above diagram, but I instead put them in parallel….

When you build it wrong, it won’t do the thing it was supposed to do…

Yeah, forget the audio pre-amp for now. I’m losing 30+ dB to my front end filter. Time to heat up the iron.

73