Ripples in the Ether

A little while ago, I got a hankering to work on a SSB transceiver. It’s something that I’ve never homebrewed before, and it seemed it like it might be fun to tackle. The BITX20 seemed like an obvious choice, and W7ZOI recently published an improved bidirectional amp which would be nice to try in the rig. But I’m one who likes to try stuff that’s a bit off the beaten path, so I decided to try my hand at something a bit different.

VU2PEP has a lesser-known SSB design on his website, that’s a dual-band transceiver. Besides having 20 and 40 meter capability, it also has a different topology than the BITX series. Instead of reversing the flow of the signal to generate a SSB signal, this design sends the RX and TX signal in the same direction through the IF. Take a look at the schematics to get an idea of what I’m talking about.

I decided to make a “remix” of this design. The basic topology is the same, but most of the circuits are revised. The IF was moved to 4.9152 MHz, and the VFO is heterodyne-style to provide a ~19.12 MHz LO signal. My version is only for 20 meters. The front end has a preamp added and uses a cascode JFET mixer instead of a single JFET. So far, the RX strip and VFO is complete (although I might change the VFO because of some birdie problems), but the transmit amplifiers haven’t been built yet. I got a good chance to work out the RX during Sweepstakes. Check out my YouTube video below to hear me describe the circuit so far and listen to the receiver on SS.

VRX-1 in 4SQRP Clear Top Tin

While I was away on my honeymoon, I noticed that the upcoming kit that I’ve been hinting about for months has finally been released. The Four State QRP Group announced availability of the VRX-1 direct conversion receiver. The VRX-1 is a simple 40 meter VXO-tuned receiver (crystal on 7.030 MHz), but it’s not your typical NE602/LM386 combo. The product detector consists of only a 2N7000 MOSFET, a capacitor, and an inductor. The audio amplifier is a TDA7052 from NXP. This little 8-pin DIP can output 1 watt of clean audio into low impedance headphones or a small speaker. Current consumption is only about 40 mA, which makes the VRX-1 easy on your batteries if you take it out for portable use. The construction of the receiver is done Manhattan-style, but don’t let that put you off if you’ve never built this way before. I provide a precise, detailed layout diagram to show you exactly where each part is placed and how it is oriented. There’s also some very detailed build documentation to walk you through the build, which you can preview at the VRX-1 web page. Even the novice builder can construct this radio!

The VRX-1 was designed to be a companion to the NS-40, or other similar rock-bound 40 meter QRP transmitters. I also include instructions on how to use some of your own parts to modify the VRX-1 for operation on any HF band, so don’t feel like you are stuck on 40 meters if you would like to try to experiment a little. In a future blog post, I’ll walk you through the process of integrating the VRX-1 with a standalone QRP transmitter to make a complete station.

Proceeds from the kit sales go to fund OzarkCon 2010; I don’t make a dime off of it (just the glory, LOL!). So please support the QRP community and try your hand at a new kind of kit. It’s only $25 postage paid in the States, $28 for DX.

20 Watt Broadband Linear Amplifier

QRP is tons of fun on CW, but it gets a bit rough trying to work other stations on SSB with 5W, especially when you are using antennas that are low to the ground. I had been eyeballing the nice RF MOSFETs from Mitsubishi for a while, and since I got a hankering to get a bit more active on SSB, I took the plunge and ordered five of the RD15HVF1 devices. At a current price of $5.25 at RF Parts, they are a bit more expensive than the IRF510 that you see in a lot of 20-40 watt range linears, but these devices have a few advantages over the IRF5xx series. One of the biggest, in my opinion, is that these RF transistors are designed to run off of a 12 volt drain voltage, unlike the IRF510 amps which don’t really work well until they get around 24 volts on the drain. These things can also take quite a beating from poor mismatches, and have the convienice of having the source connected to the metal tab on the case, making for a nice solid ground connection.

20 Watt Broadband Linear Amp - Inside

Having the appropriate parts in hand and some designs on the internet to steal from, I set out to build my own linear. There isn’t a ton of creativity to be used when designing a linear of this class (Push-pull Class-AB). Every design that I’ve seen looks nearly the same. Not surprisingly, the real focus of the design is in optimising the input and output networks. Feeling lazy and anxious to just get on the air, I pretty much did “cut and paste” from some different circuits to find out what works best. I know, not the best method, but sometimes the desire to just put out some RF trumps proper procedure. I don’t have a scehematic to post at the moment, but if you click through on the photo to the right, you can see a close-up with descriptions of major circuit blocks. Below, I’ve posted links to the two circuit resources that I used the most for this design. I’ll have more details about the designs to comment on at a later date, when I can pull some proper notes together.

• HPSDR Pennywhistle
• TF3LJ SoftRock Amplifier

One of my weakest homebrewing areas is in the mechanical engineering, but now that I have a bit of a real “shop” in my garage, things have been getting better. A bit of scrounging at the surplus stores around town led me to some cheap heat sinks that looked like they might be suitable for this project. After attacking them with an angle grinder to get a lip off of the bottom side, I was able to bolt two of them to the lid of an aluminum Hammond enclosure. I nibbled a nice square area right out of the middle of the copper clad I used to build on, soldered the RD15HVF1 devices to some pads etched out with a Dremel, then bolted the MOSFETs and copper clad directly to the lid of the enclosure. Drilling the holes for the BNCs and the LED was a piece of cake with the aluminium box material.

Without getting into too many details at this point, I was able to get the amplifier working right off the bat. I didn’t get quite as much output power as I initially liked (only got about 10 watts), but the amp was working correctly. More troubling was the fact that output on 6 meters was only 2.8 watts. Not too great when you are putting in 2.5 watts. I figured it had to be something with the input or output network. The input return loss measured quite good; -15 to -20 dB across all the bands. So I figured that left the output network. My initial iteration of the amp used a transformer similar to the one in the Pennywhistle amp (this is a configuration that I’ve also successfully used before in a push-pull class-C CW amp). Without doing any actual measurements and calculations, I dropped in the broadband transformer pair used in the TF3LJ amp, and immediately improved my output power by a few watts. But I was still a bit low on 6 meters. A bit more searching showed that I might need another compensation cap on the output, so I experimented a bit more until I found that a 1200 pF silver mica in series with the drain transformer outputs worked wonders and boosted my power on 6 meters to nearly 15 watts CW. I haven’t done any analysis to see why this helped. I know, sloppy…but sometimes expedience wins.

Since there’s no output filtering built into the amplifier enclosure, I had to assemble some outboard filters in order to get this thing on the air. I was going to use 7-pole low-pass filters until I realized that everybody else uses 5-pole filters because push-pull amps already suppress the even-ordered harmonics by at least -30 dBc. A bit of work with the new LADPAC software in EMRFD enabled me to crank out a table of filters for all of the bands (160 m – 6 m) using the silver mica caps in my junkbox plus T68-6 toroids. If you click through the photo below, you can get a glimpse of the copper clad enclosure sticking off the output of the amp.

Backyard Linear Test

Last Monday, after a bit of checking of the signal purity with my dummy load and scope, I was satisfied that everything was working OK and took the amp out for a spin on the back porch. I set up the Buddipole in Versatee Vertical configuration with the Low Band Coil. It tuned right up on the upper end of 75 meters, and I had no problems at all checking into the Oregon Emergency Net. One watt out of the 817 gave me about 25 watts out of the linear on 75 meters. I was too busy to do much else with the amplifier until today (the following Sunday), but I was excited to give the amp a try on 6 meters, since that was one of my biggest motiviations for building the thing. The Buddipole was set up in a simple 6 meter dipole configuration about 10 feet above the ground and I parked the 817 on 50.125 MHz. It didn’t take long before I heard N6OR booming into Beaverton from Southern California (grid DM12). I snagged him on the first call, getting a 57 signal report in return and a report of good, clean audio when asked. He was running 100 watts into a quad, which you can see on his QRZ page. I was really thrilled since this was not only a victory for my mad homebrewing skillz, but was also my first 6 meter QSO!

I’ve been parked on 50.125 for most of the afternoon here at the NT7S shack and have picked up a few more QSOs. So far, all reports of the audio quality of the linear have been FB, so I’m satisfied that it (and the LPF) are working as they should be. I think I’ve about worn out my keys on this post, so I’ll wrap it up for today (I always start with modest ambitions on these posts, they they grow exponentially). I’m having way more fun than I should be, and I’m very pleased to be back out of my ham radio funk.

Well, I’ve pretty much put the finishing touches on my secret QRP project for a well-known QRP club. The beta testers are currently working on building it, and hopefully the club can make an announcement about the kit fairly soon. It’s not a very complicated circuit, but it’s somewhat novel and has a bit of the “popcorn” factor. I think it will be a fun build for those who purchase it.

Spring boarding off of this project, I intend on extending some of the circuit elements that came about during the development of the secret project and using them to create something new. I don’t want to say too much at this point, since I don’t want to give any clues which could spoil the QRP club’s kit announcement. At this point I’ll just say that if I can pull this off, it will be a new take on a QRP classic. But I don’t want to hype anything too much. I would rather underpromise and overdeliver. Plus, this thing hasn’t even gotten off of the engineer’s notepad yet. It should be a fun one to try to accomplish.

Last week, I was informed that everyone in our company would be receiving a temporary (but indefinite) 10% reduction in pay. To make matters worse, it looks like we will be taking more mandatory shutdown days in the current quarter; we’ve got five coming up over the next few months. This is also another open-ended cost cutting measure; there is a possibility of more shutdown days in the upcoming quarters. As might be expected, my first reaction was anger and frustration, but I’ve cooled off a bit since then and let rationality replace emotion. The truth of the matter is that they economy sucks real bad, and only appears to be getting worse at the moment. I believe the company leaders when they say that this was done to avert more layoffs. Given the choice between a 10% reduction in pay and a 100% reduction in pay, it’s pretty obvious what the best option is. I still can’t say that I’m terribly thrilled, but this wasn’t entirely unexpected either.

So now the belt needs to be tightened a bit more at our household. Given my personal theories about what’s going on in the world, I seriously doubt that this is the worst that we will see. Let’s just say that I will be extremely grateful if I still have this same job in a year from now. With that in mind, I have been giving serious consideration to getting into the kit-selling business. Not that I expect to ever get rich selling kits to hams, but I really enjoy design and it would be nice to at least be able to supplement my income now that it’s taking a pretty big hit.

Along those lines, one of the reasons why the technical content has been light around here lately is that I have been working on a kit for a well-known QRP club. I don’t know when I can spill the beans or how much I can say about it, but I think it should be an interesting and fun kit for a lot of builders. Keep an eye on the blog for more details about the kit when I’m able to release them.

Furthermore, I have some other kit ideas that sprang from this original design. Those are the ones that I would like to bring to market under my own banner, along with some other unique designs that aren’t currently on the market. At best, it will still be months before you see anything commercial coming out of my lab, but I’m going to do my best to get the ball rolling very soon since I’ve now got a fire lit under my butt. I’m still going to post other technical content when I can, just don’t be surprised if that category is a bit light for the next few months while things spin up over here.

1. You can never have enough low frequency decoupling in an audio amplifier with 100 dB voltage gain.
2. You can never have enough voltage regulation on the VXO you are using to feed your diode mixer. Not enough and you get all kinds of nasty low frequency oscillations in the receiver.

As I mentioned in a previous post, I did the calculations to change the AF filter in the Willamette from a low-pass filter with a 3.3 kHz cutoff to a peaked low-pass filter with a cutoff frequency nearer to 1 kHz. I finally got around to implementing the mod last night and got a chance to listen to it on the air today and take some measurements of the filter response.

First off, let’s take a look at the filter response:

If you compare this to the old response, it probably won’t look drastically different, but it does cut off a bit eariler than the original filter. There is a bit of a peak as predicted, although it’s a bit wider and shallower than expected.

However, the real proof is in the listening. I found (purely qualitatively) that the response of this filter was much tighter sounding than the original. Much of the high frequency interference is gone, and you can tell by tuning through a signal that it drops off much more quickly at the higher AF frequencies. You do lose some of the “crisp” direct conversion sound, but I feel like this is made up for in the utility of having greater filtering.

Here are the steps that you need to take in order to modify your own rig:

• Replace R50, R51, R54, and R55 with 24 kΩ
• Replace C55, C59 with 100 nF
• Remove C56, C57 then place a 1 nF capacitor from Q12 base to ground (in the place where C56 was located)
• Remove C60, C61 then place a 1 nF capacitor from Q13 base to ground (in the place where C60 was located)

One other small thing that you might want to do is replace C65 with a 1 uF capacitor. I noticed that when the AF gain control was set to maximum, that there would be an annoying popping during keying. This change helps to eliminate this problem.

I hope that you enjoy this modification to the rig. In hindsight, I’m not really sure why I designed such a wide open AF filter, although I suspect it was because I wanted to preserve the “DC” sound of the rig. However, I think that utility trumps a nicer sound in this case and will make the rig more usable overall.

Baxter Is Uncertain

I finally got a few days of decent sleep (decent meaning more than 4 hours), so I had a little energy to work on the simple DC transceiver. A few days ago, I got the remainder of the audio chain working. The emitter follower on one of the outputs of the differential mixer was yanked, and I connected a class-A audio amp directly to the mixer. Then I stuck the emitter follower on the output of the class-A amp to enable the receiver to drive low-impedance headphones. No, it’s not extremely efficient, but it is simple and it works. Best of all, no transformers are needed. As an afterthought, I added a simple shunt-to-ground mute circuit with a 2N7000. That might have to be tweaked a bit later

Unnamed on the Bench

The transmitter is also a simple design. The second output of the differential mixer is tapped with an emitter follower that will have its V_CC line keyed to control transmit. Directly following this is a 2N7000 class-C PA. After a bit of work tweaking the impedance matches to get the right amount of drive to the PA, I can easily get 2 watts out of the amp (before low-pass filtering). What’s neat is that the emitter follower puts out about +10 dBm, and it gets amplified up to +33 dBm in one stage. A very compact design that can generate a decent amount of power.

So in order to make this a true transceiver, I have a few things left to do. First thing, of course, is to get a low-pass filter on the transmitter. I’ll also need to provide a keying circuit and T/R switching. I’m still not sure what I’m going to do about a sidetone. I also think that I’ll put an RIT circuit in there and not worry about a fixed transmit offset (that would be very hard to get right in such a simple transceiver). Keep watching for another update, hopefully soon.

Unnamed Simple Discrete 80 Meter Rig

Yes, its a post about another simple, low-performing direct conversion receiver. However, I think that this one is slightly unique. I was inspired to give this a try based on the Flea minimalist transceiver that was introduced on the EMRFD Yahoo group. These little rigs are fun to build in an evening, but just how usable are they? Would you feel comfortable giving it to a new ham and believing that they even had a small chance of success? For me, these Pixie-class rigs are nearly unusable due to the horrible AM broadcast interference that blows right through the rig. While a minimalist rig is an admirable thing, they are only useful in limited circumstances. I figure that a few things have to be added to these rigs in order to make them more than a novelty. KD1JV also shares this viewpoint, and has created his own answer to the Pixie.

I’ve started with a similar philosophy, but built the rig around a different topology. The basic strategy is to use a differential amplifier as an active mixer. The rig is designed for the 80 meter band, which is probably the easiest for homebrewing. The LO is a Colpitts ceramic resonator oscillator, but is not separate from the mixer. Instead, the oscillator is built around the third transistor which acts as the constant current source. I know that this is certainly not a new idea; it’s used all of the time in NE602-based QRP circuits. However, I don’t think this topology is seen very much in discrete component use. It saves quite a bit of circuit space and is composed of very common components.

The rest of the receiver is very simple. I placed a standard double-tuned circuit bandpass filter in front of the RF port of the mixer to filter out all of the AM BCB crud. The output of the mixer feeds a dirt-simple emitter follower to transform the relatively high collector impedance of the diff amp mixer to a low impedance output. I haven’t designed the final AF amp yet, but I don’t think it will take much to get the signal up to headphone levels. When the emitter follower output is connected to my test bench AF amp, I have to have the amplifier AF gain control turned nearly all the way down, lest the whole thing start oscillating wildly.

Tonight, I connected the RX to the bench AF amp and the antenna to see how it would work. Tonight was an excellent night to try, since we are right in the middle of Sweepstakes. Pleasantly, the receiver immediately came to life with a cacophany of CW signals in the unfiltered audio output of the receiver. I’ve attached a recording of the receiver output so that you can get a feel for how well it works for such a minimalistic design. The ceramic resonator osc tunes from nearly 3.500 MHz to 3.580 MHz, and I tune across the entire band in this clip.

All I have to do to finish the receiver is to add on a discrete component AF amp. I think that a single class-A stage of amplification will be enough to get the audio up to headphones level. After that, I’m going to try to tack on a transmitter by picking the VFO signal off of the other unused collector port of the diff amp. I think that I can get away with another emitter follower as a buffer, followed by a class-C PA. I’m shooting for around 1 watt of output power, which is enough to snag QSOs without too much difficulty. I think this could be a lot of fun to build as a kit. It’s will be quite a bit more complex than a Pixie or Flea, but also quite a bit more usable. Stay tuned for further developments on this rig.

Even though a homebrew return loss bridge is a relatively simple piece of gear to build, I’ve never gotten around to building one until now. Which is truly a shame, and something I don’t really have an excuse for, except perhaps laziness. I’m excited to add this essential gear to my stable of test equipment, as I know that I’m going to get a ton of useful data from it in the future. Now that I have all of the necessary equipment for measuring the return loss of the evolving dual gate MOSFET amplifier, it was time to get the job done.

Test Equipment

• Power Meter: M3 Electronix FPM-1
• Voltmeter: Fluke 8840A
• Signal Generator: Tektronix SG503
• Return Loss Bridge: Homebrew (51 Ω, ¼ W resistors, FT37-43 bifilar transformer)

The Circuit

Based on the suggestions made on the EMRFD list, I modified the amplifier to incorporate the best practices that were recommended. First off, I tossed out the 100 kΩ gate 2 resistor, which was serving no useful purpose (except for dumping noise into the amp, according to W7ZOI). The bypass cap leads were clipped very short and soldered directly to gate 2. I’ll probably change this to a 10 nF or 1 nF cap in later iterations, but I took the lazy way out and left it as is for now. The other change was the addition of R2, the swamping resistor for the drain inductor. V_AGC was set for 9 V, a point which should give nearly the full amplifier gain.

Results

Before taking the return loss measurements, I checked out the biasing and gain of the new amplifier configuration. Not surprisingly, the transducer gain dropped to 17.3 dB (-30 dBm input signal at 28.1 MHz), which of course was the whole point of adding R2. The drain current at this bias point is only about 2.5 mA, so there’s still lots of headroom in this amp.

Using the procedure outlined in EMRFD, I measured the amplifier input and output return loss. The SG503 was set to 28.1 MHz with a -10 dBm output level and connected the the RF port of the return loss bridge (RLB). The detector port of the RLB was connected to the FPM-1 and a reading of detector power was made with an open circuit on the unknown port of the RLB. Next, I terminated the output of the amplifier in 50 Ω, connected the unknown port of the RLB to the input of the amp, and applied DC power to the amplifier. The difference between the two readings was the measured input return loss (S11), which was 10.0 dB in this case. Using a handy chart, I translated this into a VSWR of approximately 1.93. Given what I’ve seen in the literature about the poor input return loss characteristics of dual gate MOSFET amplifiers, I was actually pretty pleased with this number.

I repeated the measurement, but this time I terminated the input in 50 Ω and connected the unknown port of the RLB to the amplifier output. This indicated the output return loss (S22) to be 11.9 dB, which is a VSWR of approximately 1.68. This shows that the load transformer is doing a fairly decent job of handling the impedance transformation on the output.

This was a simple test, but the results were satisfying. In the current state, this amplifer should work fine if integrated into a large system, such as a receiver. The real question is the effect that the 2.2 kΩ termination on gate 1 has on noise figure. Speaking of noise figure, I plan to make this measurement next, but I have quite a bit of work to do first. In order to characterize the ENR of my noise generators, I’m going to need to get my hands on a true-RMS detector. I’ll probably end up building the one that Sabin describes in his paper on the EMRFD CD that deals with measuring receiver sensitivity and noise. But that means waiting, since I have to order a few of the parts. I’ve also got more reading to do in order to better understand everything that’s going on with noise figure measurements. I also need to start thinking about how I’m going to get my hands on a spectrum analyzer to make IMD measurements. Which is funny because I work on very nice spectrum analzyers every day at work!