Junkbox

No earth-shattering news to report on the blog, but a few little things to mention (hence the "junkbox" title).

The CC-20 Beta 1 test is proceeding pretty much as planned. As of tonight, AA7EE has his receiver up and running now and a couple of the others are close behind. I'm eagerly awaiting the results of at least a couple of the builds so that I can get moving on the revisions for the Beta 2 circuit (which will hopefully also be the production PCB). I'm anxious to get the business up and running!

I got a very nice mention from Bill Meara on the latest episode of SolderSmoke. He talks up Etherkit and my blog, then mentions that he's going to try to use the single-ended passive MOSFET mixer from the VRX-1 in his homebrew WSPR transceiver. I hope that the experiment works out well for him.

As we approach the halfway point of the gestation of our new little one, I got to thinking about mortality a bit. I hope to be around for a very, very long time to come and have been taking steps to improve my health to make that more probable. But in the awful case that something were to happen to me in an untimely fashion, it seemed that I'd like my family to have a little bit of my own thoughts with which to remember me. At first, I thought that maybe I should do a private journal, but then it occurred to me that wasn't necessary. Barring a complete collapse of civilization, all of my descendants will be able to access an archive of all of my Internet activity. Every blog post, tweet, Google+ post, website comment...and perhaps even my email. If you Google my last name, I'm the first result. I'm active enough online that it's not entirely inconceivable that a reasonable avatar of myself could be created sometime in the distant future (given that Moore's Law holds up in some fashion for the next 50 years or so). Perhaps this is all pie-in-the-sky speculation and will look as foolish as the "flying car future" does to us now, but I'm pretty sure that I'll live on in human information space in some fashion long after I'm gone.

Curing Common Mode Hum in the VRX-1

As I've previously noted, the VRX-1 is a nifty little basic direct conversion receiver, but it has some shortcomings that could be problematic under certain circumstances. Here's a story of one of those issues and the cure that was found.

Dave AA7EE purchased and built a VRX-1 kit a while ago but was never fully satisfied with the performance due to an annoying 60 Hz hum. He and I had briefly traded comments on the topic via Twitter, but I never really seriously took the time to think about it until just recently. Dave had built and placed a peaked lowpass audio filter into the receiver thinking that would help with the hum, but unfortunately it did virtually nothing to help with it.

I was a bit surprised to hear of the hum problem, since I had never encountered any significant amount of hum, nor had I had other complaints of hum. The eureka moment came when Dave had mentioned that the hum went away when he disconnected the antenna, or it decreased in signal strength when he moved away from his home. I had assumed that the hum was a glitch in his audio circuitry, but this reminded me of the problem known as common mode hum. The best description of this phenomena is found on pages 8.8 - 8.9 of Experimental Methods in RF Design, but I can provide a brief overview. Common mode hum is the result of the LO leakage getting out of the antenna port, modulated by a mains power supply (like an old-fashioned model with rectifiers), and then re-received by the radio.

Due to the simple, single-ended mixer design in the VRX-1, I knew that LO-RF isolation was very poor. So the first suggestion to pop in my mind was to tell Dave to try a common-gate JFET preamp on the front end. Although these type of mixers have modest gain, they have a low noise figure, and even more importantly for us, excellent reverse isolation (on the order of 30 dB). This should be enough to kill any significant amounts of LO leakage.

Dave built a circuit from master homebrew experimenter, Todd VE7BPO. It's the last circuit on this page, and it looks rock-solid. A double-tuned circuit on the front and a single-tuned circuit on the output. Sure enough, that ended up doing the trick. Rather than trying to reinterpret Dave's thoughts, go visit that last link, then watch his YouTube video so you can hear the results for yourself:

I'm really pleased to hear that Dave's annoying problem is finally fixed. This makes me wonder, in retrospect, whether I should have just designed in a preamp to the VRX-1. It certainly isn't needed for noise figure purposes, but as you can see it can make a huge difference with those who might have problems with hum. There's also a well-documented problem of a loud impulse generated when the antenna is connected or disconnected during operation. I suspect at the reverse isolation of the preamp would also help this. Hindsight is certainly 20/20. If there is ever a VRX-2, then you can bet that it will get a stock common-gate preamp.

KE7GKM VRX-1 Update

As you may recall in a previous post last December, I made a QSO with, then received a cool e-mail from Bob KE7GKM about his quest to make 100 QSOs using a lashup of a VRX-1 and a homebrew QRP transmitter. Bob kindly sent me an update on his progress:

Hi Jason,

This is just a brief up-date.   I now have a total of 160 qsos with
33states using ur vrx-1.  fun!   My last 60 qsos hv bn on 20meters.

It is too bad that the  vrx-1 "bag of parts kit" is no longer
available to those with weak/novice junk boxes.   (It is hard to do
one-stop shopping when toroids are needed)

My experiment to combine some aspects of the vrx-1 with Campbell's
Binaural I-Q Receiver worked, but not very satisfactorily  as there
was too much noise and the channels were not balanced.

My next project is to construct a better (I am still amazed at the
vrx1) receiver which is essentially discrete.  Any leads?  I am
thinking along the lines of borrowing the rx design from NorCal's
2N2/XX Transceiver .

Tnx
Bob
Boise
KE7GKM

^^^^^^^^^^^^^^^^^^^^^^^^^^^
This   P S.  is just a reminder to you of my set up.   Besides the VRX1 I use a
modest transmitter which serves as the exciter at about 1/4 watt.   It
uses a  XFO. My ICOM  or a
frequency counter is used to set the TX and RX frequencies, giving
a 600HZ offset.  My straight key feeds a double pole relay with one
line to the TX and the other to the side tone, which presently is produced by an
HB practice code oscillator.   [3 toriods, 3 bipolar transistors, 1
mosfet]

The  exciter drives  a W6JL RF amp.  The June QST article on this amp
and the chance construction of the  VRX-1 formed the imputes to
complete my HB station.  [4 toriods, 2 bipolar transistors, 2 mosfets,
pwr from a wheelchair battery. ]   (Unless one is very careful,  one
should have spare mosfets handy.)  I was attracted to the push-pull
design, as I had wished to make  a 5 watt push-pull tube amp some day.

I hv  2 fully home brewed stations -- one fer 40m es one fer 20m.  They
are essentially the same except for filters and antennas.   When
convenient I like to make two of each home brew project so that I can
judge them by comparison. See attached file for pic of 40 m rig
followed by 20 m rig.  The attache case yields lots of room fer mods,
etc.  Both are portable, using either wheel chair batteries or
emergency automotive starter batteries (presently).  The efficiency of
the TX amp is (presently) poor.

Since we last had a QSO, he has doubled his amount of QSOs. Impressive! He's also the first person that I've heard of who has actually modded a VRX-1 for a band other than 40 meters (I added quite explicit instructions on how to do it in the documentation, so I hope there are more out there). Congratulations Bob on your great homebrew accomplishments! They're certainly inspiring!

This Is What Radio Is All About

KE7GKM QSO
KE7GKM QSO

Over the last few days, Jennifer has been off work, so I've been able to spend more time in the shack working on Project X. I only recently made my first QSO with the prototype rig (I think propagation was unfavorable for me when I was trying late in the evenings), so I've been leaving the radio hooked up to my bench AF amp and monitoring 7030 kHz during the day.

Late this afternoon, I heard a very strong station calling CQ just a bit up from 7030. I bumped the VFO up a bit and found that it was KE7GKM calling at a nice, comfortable speed for me (my CW is rusty after quite a few months off the air). While I called him back, the thought occurred to me that the call sounded familiar, but I couldn't remember how. After getting the QSO basics out of the way, I remembered why. Bob said that he was using a VRX-1 and homebrew QRP transmitter combo! Then it hit me that Bob had just e-mailed me about a week ago to ask me a few questions regarding the VRX-1.

I don't get on the air as much as I should (seems like I'm melting solder way more than pounding brass), but when I get a chance, it means so much to me to have a contact with someone who has built one of my radio designs. It's even more special when I get that make that QSO with a homebrewed radio on my own end as well. If I remember correctly, this is only the second time that I've done such a thing.

Bob told me that he is trying to get to 100 QSOs with his VRX-1/HB TX combo, and that I was QSO #80 (if I remember correctly, my notes aren't great). I wish Bob all the best of luck in his endeavor. It certainly looks like he doesn't have much more to do in order to meet his goal.

It's hard to beat an experience like this in capturing the essence of amateur radio for me. It is my hope that more amateurs will homebrew their own gear so that they can get that same thrill.

DIY Isolated Jack

The VRX-1 receiver kit uses a TDA7052 audio amplifier IC, which is a bit different from your run-of-the-mill LM386. Because of its push-pull output, if you use a phones jack, you must isolate it from ground. This isn't something you have to worry about if you use a non-conductive enclosure or if you have an isolated phones jack. However, more often than not, you won't have an isolated jack, and many of the enclosures that we use are metal.

This little hint will allow you to modify a "standard" grounded ring phones jack with a conductive enclosure (such as the 4SQRP Clear Top Tin, like the one I used).

Bill of Materials

  • 3.5 mm phones jack
  • Small length of 3/8" OD (1/4" ID) Polyethylene Tubing
  • 1/4" nylon washer
  • CA glue (optional)

Instructions

Cut off a piece of the 3/8" OD polyethylene tubing approximately 1/16" long (about 1 mm).

Cutting a small piece of the 3/8" OD tubing
Cutting a small piece of the 3/8" OD tubing
Place the small piece of tubing over the phones jack, so that it rests against the body. You may want to glue the tubing to the body of the phones jack.

3/8" OD tubing over jack
3/8" OD tubing over jack
Drill a 3/8" hole in your enclosure to accommodate the tubing, then place your phones jack in the hole so that the 3/8" tubing is centered in the hole. Place the 1/4" nylon washer over the jack from the outside of the enclosure, then use the knurled nut to secure the jack to the enclosure. It's as simple as that!

Isolated jack installed in enclosure
Isolated jack installed in enclosure

Introducing the VRX-1

VRX-1 in 4SQRP Clear Top Tin
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.