Dave Richards AA7EE

July 27, 2012

Using the NM0S Hi-Per-Mite Filter From 4SQRP To Make A Simple 40M DC CW RX

When 4SQRP brought out a version of the David Cripe NM0S-designed Hi-Per-Mite Filter, I noticed that it could be configured to give up to 50dB of gain. Like many others, I’m sure, my next thought was that it would make the process of building a simple direct conversion receiver even simpler. I decided that at some point, I needed to use this little filter board as the audio chain in a direct conversion receiver for CW.  I wanted to see for myself how it would sound.

In the meantime, my preoccupation with direct conversion receivers continued, as I embarked on building a G3WPO/G4JST DSB80. The receiver in that DSB rig uses an SBL1-8 DBM. That particular mixer package is out of production, but I used an ADE-1 and was pleasantly surprised by the receiver. I had some problems with the TX section, so put it aside and started building a ZL2BMI DSB transceiver.  I’d known about this little rig for a while from the pages of SPRAT, but a comment on Twitter from NT7S encouraged me to have a go at building it. The receiver is a simple NE602/LM386 combination with just a single-tuned filter on the antenna input. Of course, the receiver is not THAT simple, as each chip contains more complex circuitry, but the schematic at the chip level looks almost too simple to work.  As with the DSB80, I had problems with my ZL2BMI rig transmitting a sizeable residual carrier, but was surprised that the receiver sounded pretty good. The seed was sown…..

The ZL2BMI DSB rig was actually only the second time I had built an NE602/LM386 direct conversion receiver. The first was about 20 years ago when I bought a kit for the Sudden receiver on 20M. At the time, I wasn’t that keen on it and it soon ended up in pieces. I’m not exactly sure why I wasn’t taken by it, but I think it was a combination of factors. Firstly, I was living in an apartment in Hollywood with no outdoor antenna and was using just a short length of wire indoors as an antenna. It’s also highly possible that band conditions weren’t great, but I just remember not hearing many signals and also not liking the fact that a half-turn of the variable capacitor covered the entire 20M band. Because of this one not so great experience, I have since harbored a bias against using NE602′s in direct conversion receivers, an attitude that I now realize was unfair.

After achieving only partial success with both these rigs but enjoying the fact that the direct conversion receivers in both of them worked quite well, I wanted to build a simple receiver and use the Hi-Per-Mite kit that had been sitting patiently on my shelf for the last few weeks.

Look at the schematic for any NE602/LM386 DC receiver, and they are almost all the same, differing only in whether they use a crystal, ceramic resonator, or free-running VFO for frequency control, whether they have a single-tuned or double-tuned antenna input filter, or whether they use a differential or single-ended audio output.  The circuit is so simple that there is only so much room for variation, but there were a couple of things I had in mind for my mine.  While the receiver portion of the ZL2BMI rig only has a single-tuned filter on the antenna input, I wanted to do what G3RJV did with the Sudden, and place a double-tuned filter there. It’s an easy thing to do, and living in a built-up urban area, I have a lot more RF around me than ZL2BMI does when he’s using his rig in the bush, so a bit of extra bandpass filtering certainly couldn’t hurt. The other decision to be made was the method of frequency control. I was curious to see how the internal oscillator in the NE602 would work as a free-running VFO.  My apologies to you folk who have built umpteen simple NE602-based receivers and have-been-there-and-done-that.  I guess I need to get this out of my system! Another thing that I had seen in other circuits but hadn’t tried was the use of a 1N4001 diode as a varactor. Some call it the poor man’s varactor. Sounds like it was custom-made for me!

So that was my configuration – double-tuned input filter (just like the Sudden) and a free-running VFO tuned with a 1N4001 diode.  From what I’d read, I knew that I should be able to get enough capacitance swing from the 1N4001 to cover at least the bottom 30-50KHz of 40M. Here’s what I ended up with:

There’s nothing original in this schematic  but as you’ll see soon, it did turn out quite well, so I decided to name mine “The Rugster” after my cat, whose full name is Chloe-Rug, but who gets called several different variants of that name, of which “The Rugster” is one.  For the input bandpass filter, I used the vales of L and C that are used in the GQRP 40M Sudden Receiver. The pre-wound inductors in that design have a value of 5.3uH. I decided that I wanted to use T37-2 cores, so I used the calculator on W8DIZ’s site to figure out that I’d need about 36 turns on a T37-2 core to give 5.3uH of inductance. I fiddled around a bit with the values of the parallel fixed capacitors before settling on 47pF for those. I had originally thought that a higher value of around 68pF should work, but that didn’t put the peak of the filter somewhere in the mid-range of the adjustment of the trimcaps.  Some experimentation revealed that 47pF fixed caps achieved this.  Your value may vary depending on what value of trimcap you are using.  For the VFO inductor, I found a design for a 40M DC RX on the internet that was tuned by a 1N4001, noted the value of the inductance, decided that I wanted to use a T50-7 core (for stability) and once again, used Diz’ site to calculate the number of turns needed.

When building receivers, I like to start with the audio chain first. It’s quite affirming to be able to touch the input and hear that reassuring loud hum and mixture of AM broadcast stations in the speaker! So I got out the 4SQRP Hi-Per-Mite kit. Close inspection of the PCB revealed a small problem that was easily remedied. There was a very, very narrow copper trace connected to the ground plane that had been bent so that it was contacting the input pad.  A DVM test confirmed that the input pad was shorted to ground. I didn’t take this picture until I was part of the way through removing the offending trace, but you should be able to see the problem:

A minute or two with a sharp craft knife and that pesky little trace was history. Here’s the finished board:

The only thing that prevented me from putting this kit together even more quickly was my own compulsion to solder the components into the board so that they line up as straight as possible. The holes for some of the capacitors were a bit larger that they needed to be so that if you just plug them into the holes and solder, they’ll be a bit off-square. It’s not a big deal, and most builders won’t be bothered by it, but I just can’t control my OCD tendencies sometimes. Anywhere, I finally got there:

Although with modern low-flux solders, there is no need to remove the flux, it sure does make a board look nicer. I recently started using flux remover on my boards so that I can show off the underside too. At the suggestion of W2EAW, I apply it with a cheap plumber’s flux brush obtained (for 19c) from the local hardware store. Look at that nice clean board!

Trader Joe’s sells Green Tea Mints in a clear-top case that fits the Hi-Per-Mite board well if you’re looking to use it as a stand-alone device. There’s also room for power and in/out connectors (though you might need to cut off the top corners of the board in order to make room for the connectors.)  The tin in this shot had been knocking around my bench for a while, so the clear-top is a bit scratched up, but you can see the potential:

The next step was to build the direct conversion receiver with the NE602 and some of W1REX’s MePADS and MeSQUARES. The most frequency-sensitive part of the VFO circuit was built ugly-style, with the help of a 10M resistor for a stand-off, to help the stability. I don’t have any pictures of the board without the Hi-Per-Mite audio filter/amp added, so here’s the finished board. In the earlier shots, you can see the VFO toroid before I decided to slather it in hot glue in a bid to make it a bit more stable. There is only one component in the final receiver that I missed from the schematic, and that is a 1N4001 diode in series with the 12V supply for reverse-voltage protection.  I’ve blown too many fuses in my shack supply to not use these little fellows in the future :-)

Another shot from basically the same end of the board, showing the input bandpass filter on the left, VFO toroid on the left at the back and the DC input circuit nearest you on the right (reading from right to left, you see the reverse-polarity protection diode,  10uF input smoothing capacitor, 78L08 voltage regulator, and the 0.1uF DC output bypass capacitor. I’ve had those green trimcaps for a long time and it was time they were used. They don’t take solder well (I think I got them from Radio Shack about 10-15 years ago) but it seemed a shame not to use them in something:

Here you can see the varactor tuning circuit built ugly-style just to the left of the VFO toroid. The 3 connecting cables are for the AF gain pot (which is connected in place of R11 and R12 in the Hi-Per-Mite, as per the Hi-Per-Mite instructions), multi-turn tuning pot, and AF output jack. I’ve used lavalier mic cable that I bought from a local pro-audio store. It has 2 inner conductors shielded by a single outer braid and is very flexible, which makes it useful for wiring up small radios.  RF connections (which you’ll see in later photos) will be made with Belden 8216 coax:

At this point, I was very gratified to discover that the receiver actually worked, and seemed to work quite well too. That fact gave me enough inspiration to start building an enclosure to house it in.  The next shot isn’t ideal, as the back of the case is a little out of focus. You’ll notice that the front, and the two bolts sticking up on the right side are in focus, but the back of the case (and especially the bolt at the back left) isn’t. When I realized this, I was too far along installing the board to go back and take another shot and besides, sometimes I just have to control myself and keep forging ahead:

A view of the board installed in the case, with the lid:

A closer view from the top, in which you can see the VFO toroid now doused thoroughly in hot glue.  I’m not sure if it helped the long-term stability at all, though it did help to make the VFO more resistant to physical shock. Another way of securing the VFO toroid in a vertical position would have been to drill 2 small holes in the PCB and use a small nylon tie:

The coax that leads from the 1K log RF attenuator pot to the board is a little long, and that is because the one you see in the photos is a 10K linear pot. It was all I had at the time of building.  I am going to swap it out for a 1K log pot when the next order arrives from Mouser in a few days, and kept that lead a little long in case I need to chop some of the end off when changing the pot out.

Here’s another view of the innards. I am quite pleased with the way this little receiver turned out. You can see one of the 4 brass nuts that are used to secure the lid to the rest of the case. I use brass, as I can solder to it with regular solder. When placing the nut, and before soldering, I screw the 4-40 machine screw through the side of the case into it so I know the nut is in the right place.  If I were to get a little over-zealous when applying solder and accidentally get solder on the screw it wouldn’t stick, as the screw is made of stainless steel:

This is what The Rugster looks like in her case with the lid on:

If you look closely at these pictures, you’ll see that the PCB panels don’t line up perfectly and that the edges are just a little rough.  I am somewhat detail-oriented, but I am not a craftsman by any means, and I know my limits.  I made a conscious decision not to spend a lot of time filing and sanding edges. After making deep scores in the board and breaking it, I ran the edges across a file on the bench a few times and left it at that. Good enough is good enough in this case, and it’s certainly good enough for me.

A view of the underside:

I’ve spent a few evenings listening to it and most of the signals that my K2 could hear, this little receiver could hear almost as well. With the very weak signals the K2 won, of course.  A small part  of this could be due to the fact that the DC receiver, even with it’s 200Hz-wide filter, is still listening to double that bandwidth, as it is hearing on both sides of the local oscillator.  Naturally, I expected my K2 to be the better receiver (duh!) but was really surprised at what an enjoyable experience listening to this one is for such a simple circuit.  As expected, it does overload in the evenings, but all I have to do to get rid of the breakthrough is to adjust the RF attenuation pot back a little from “full gain”, and the breakthrough disappears.   Elecraft use an NE602 in the front end of the K1, and it has an attenuator for the same purpose. I’m not sure whether the breakthrough is from strong in-band amateur signals or from AM broadcasters – it’s actually hard to hear the content due to the narrow bandwidth of the Hi-Per-Mite. One small adjustment of the RF gain pot and it’s gone – then I just bump up the AF gain a bit to compensate. Volume is enough to drive my MFJ-281 ClearTone speaker.  I normally like to listen to CW with a 500Hz note, but because the response of the ClearTone drops off below 600Hz, I kept the Hi-Per-Mite filter at it’s design center-frequency of 700Hz, and the speaker sounds good. EDIT – If the AM breakthrough is from signals in the 550 – 1700KHz AM broadcast band, which I strongly suspect that it is, a simple high-pass filter with the cut-off set somewhere around 2 or 3MHz should work wonders to eliminate this.

The fact that the tuning rate is quite low helps a lot in the enjoyment of this receiver too.  The 10-turn pot covers 6999KHz – 7051KHz giving an average tuning rate of about 5KHz/rotation. The VFO shifts in frequency very little indeed when  the receiver is knocked or moved. Drift is not great, but manageable.  Drift in the first 10-15 minutes is only slightly worse than after that initial warm-up period,  so I’ll give you the results from switch-on, which were 90Hz drift in the first hour,  140Hz drift in the second hour, and 110Hz in the 3rd hour. All the drift was downward – there was no upward drift at all. The drift was steady, and with a little bit of compensation, I think those figures could be improved upon quite a bit. However, for casual listening (and this is all I am going to use this for) it is adequate.

This receiver is certainly not perfect, but considering it’s just an NE602 and an LM386 (oh – and a quad op-amp IC for the filtering), it’s not bad at all.

Thanks Dave NM0S and 4SQRP for this neat little audio CW filter kit.

Here are a couple of videos that I just made of The Rugster. Please excuse the poor video quality – it’s an old, cheap camera:

This one is a little shorter:

June 3, 2012

The DSB80 – A Direct Conversion DSB Transceiver for 80M By G4JST and G3WPO – First Stage Of Building

I was 19 years old in March 1983 when the UK magazine Ham Radio Today published the article “A Low Cost DSB/CW Transceiver for 80M”. Being short of cash and wanting to get on the air, I sent away for the kit and soon after, was surfing the phone portion of the UK 80M band on my new DSB rig.  I didn’t get too many QSO’s due probably, to my poor 80M dipole, although G3UMV who lived just a mile down the road heard my home-brew signals and came over to see where they were coming from.  The receiver seemed to work very well, and I spent many hours listening to the chatting between 3.6 and 3.8MHz (80M only goes as high as 3.8MHz in the UK). The whole thing was enclosed in an aluminum case and tuned with a Philmore vernier attached to a polyvaricon.  I don’t remember any drift, so it must have been stable enough for sideband, and it didn’t have any noticeable microphonics either.  As it was my first DC receiver, I didn’t even know that this type of circuit often suffered from microphonics, as this one didn’t have any to speak of.

That little rig made it with me across the Atlantic and met it’s end one day in my apartment just a block from Hollywood Blvd. In a passing wave of nostalgia for my earlier radio days, I hooked it up to 12V DC to see if it still worked. It would have, had I not connected the 12V the wrong way round, and if I’d had the foresight as a kid to provide it with reverse polarity protection. I still don’t know why I didn’t just put it aside so that at a later date I could have replaced the damaged active devices. Unfortunately, I tossed it into the dumpster of my apartment building. What a shame – and it had an SBL1-8 mixer too!

From time to time either when moving or thoroughly tidying my apartment, I come across the copy of the original article that came with the kit. Trouble is, whenever I looked specifically for it, I could never find it, and the only copy of the article I was able to find on the internet is of too low a resolution to be of much use. I’ve been wanting to recreate this rig for a while and recently, when the desire became too strong to ignore, decided that I was going to find that article even if it took a day or two of searching. It did, but I did.

Pure nostalgia wasn’t the main reason for my wanting to build this rig again. A big reason is that I have always been drawn to simple receiver topologies such as regens and direct conversion receivers, yet not all designs are created equal. I remembered this one as working well and on top of that, it used something in the circuit that you don’t see too often in DC receiver designs these days – a passive diode ring mixer package (NE602 anyone?)  I wanted to build a DSB rig that used a diode ring mixer package, so this is why I am here.

The schematics for this rig aren’t that easy to come across.  I eventually a found a low-res version of the article online after some searching but it’s not really good enough to work from.,Ham Radio Today is no longer being published, and the company that sold the kit back in the 80′s, G3WPO Communications,  went out of business a long time ago. On top of that, Tony Bailey G3WPO is no longer an active radio amateur. On this page on his website he gives a link to a reprint of an article about another of his projects, the Minisynth VFO. Judging from this, and what he says in the whole of that 3rd paragraph, I don’t think he’d mind my publishing the schematics for the DSB80 here on my blog.  That is what I am hoping as I’m pretty sure that some readers will want to see the schematics, and there doesn’t seem to be any other way to get them. I’m having a bit of trouble with the VFO (more on that later), so by showing you photos of my construction and the schematics, I’m hoping someone may be able to help me.

The plan is to get this working well as a receiver first, after which I’ll add the transmitter stages.  So to kick things off, here’s a block diagram of the receiver, a pretty standard diagram of a direct conversion receiver:

I built the VFO first of all. It seemed to work OK and be reasonably stable, with drift of less than 80Hz/hour after warm-up. I know that’s not stellar, but a bit of temperature compensation could help that.  Somewhere in between adding the buffer and adding the rest of the receiver, I noticed that the VFO was drifting a bit more and FM’ing, which makes SSB sound pretty bad. However, if I can lower the drift on the VFO and get it to stop FM’ing, I think I’ll have a nice-sounding direct conversion receiver on my hands. There are virtually no microphonics – you have to turn the volume way up and really be listening in order to hear them. For all practical purposes, microphony is just not a problem; something I like very much in a DC receiver.

I’m getting ahead of myself. Here’s the original circuit for the 80M VFO in the DSB80. I did leave out a trimcap, 1N4148 diode and associated components that were used to switch in a CW offset, as I won’t be using this rig for CW:

Important – please note that I experienced instability with the buffer transistor Q2 (it wasn’t doing a lot of buffering). I don’t understand why Q1 was coupled to Q2 with a 100 ohm resistor instead of a capacitor, but at the suggestion of K4AHO, I replaced it with a coupling capacitor (I used a 39pF NPO) and put a 100K resistor from the gate of Q2 to ground. My problems with the buffer cleared up and the receiver sounded really great. I’ll publish the schematic of the entire receiver section of this rig in a future post.

The oscillator is a Colpitt’s configuration and the 260pF variable was, in the original design, a polyvaricon.  I wanted to modify the VFO for varactor tuning (at what point did we stop calling them by the more descriptive term varicaps and start using the name that makes them sound like a prehistoric bird?) and also figured that a 78L08 regulator in place of the 8.2V zener diode could only help. This is what I came up with:

All the caps marked “poly” are polystyrene. I changed the values of the 2 x 1000pF caps to 1200pF simply because that’s what was available.

Projects always look pretty when I first start them, before I’ve had a chance to mess them up -

Here’s the VFO, although I have yet to add the varactor at this point – it will be located at the far right end of the board -

One more view just for the heck of it -

Somehow, by the time I got around to adding the mixer, AF preamp, AF amp and bandpass filter to complete the receiver, the whole thing started to look just a little bit messy. You’ll notice that I ditched the nylon mounting hardware for the VFO toroid in favor of a single nylon strap. I figured it would be one less material in contact with a frequency-determining part of the circuit that might cause drift. I didn’t clean up the board for it’s photo-op, as this is a work-in-progress that may not make it out of the emergency room -

VFO drift was steadily downwards after the initial warm-up and probably something that could be brought to within useable limits with some temperature compensation work.  There are almost no microphonics to speak of (always a good thing in a direct conversion receiver), and only a small amount of broadcast band break-through which is only coming through at the kind of high volumes that will rarely be used. This breakthrough is not coming from the DBM, but from the preamp, which is set for a gain of 1,000 (60dB). If this rig makes it to a later stage of building, I may reduce the gain of that preamp just a bit – we’ll see. The receiver sounds pretty good on 80M SSB with one big problem – the VFO FM’s when receiving signals, and that IS a problem.

The documentation that came with my kit for this rig in 1983 had something to say about  FM’ing of the VFO:

Hmmm….but I was using J310′s in this re-creation and was still getting FM’ing of the VFO.  As far as I can remember, it was not happening in the original version I had.

I decided to build the original VFO circuit with the zener diode regulation and with an air-spaced variable capacitor instead of a varactor to tune the circuit (the first schematic in this post and the second image down). I wanted to do this on a separate board, before connecting it to the rest of the receiver.  This is where it started. It sure was exciting looking at a bare board (blank canvas) with just an air-spaced variable capacitor. The variable capacitor was given to me by a friend and boy, what a great gift. Thank you – you know who you are. I was looking at this thing and thinking of all the possibilities – a signal generator, crystal set, or a regen perhaps?  Air-spaced variable capacitors are very inspiring to me -

Here’s the VFO circuit built – no buffer yet, and you’ll notice that I have not yet installed C2 and C3.  I wanted to see what the frequency coverage was without those capacitors first.  It was pretty wide, so I ended up installing C2 and C3 in the values suggested, and removing a few turns from L1 to achieve coverage of 3600 – 4000 KHz with one gang of the capacitor, which was about 330pF at full capacitance -

And with the buffer added, and temporarily terminated with a 51 ohm resistor for drift testing:

I noticed that on touching the output of the buffer with a small metal screwdriver, the frequency of the VFO changed by about 600Hz when terminated with the 51 ohm resistor. I wonder if this is the reason the VFO I built on the main board FM’s when receiving signals? The only difference between this one and the one that is incorporated into the rest of the circuit as pictured 5 images above, is that the one directly above is tuned by a variable capacitor, whereas the other one is tuned by a varactor. Either way, it suggests to me that I need another stage of buffering. Before I even look at the drift and figure out how to compensate for it, I need to tackle this issue.

To be continued……..(unless another project derails this and it ends up on the shelf, with the variable capacitor used for something else…..)

July 6, 2011

Common-Mode Hum Issue With VRX-1 Fixed and YouTube Video Posted

In the last post I mentioned a hum issue that I’d been having with the VRX-1 that I bought as a kit from 4SQRP. When I related that it only happens on connecting the antenna, Jason NT7S told me it was probably common-mode hum. For a better description, refer to EMRFD, but my layman’s understanding is that it is caused by signal from the local oscillator being radiated by the antenna, picked up by AC wiring in your house, and re-radiated with the 60Hz component from your AC supply imposed on it (which the receiver then picks up).  The solution is to prevent the receiver from radiating, and an RF pre-amp can do this.

The first place I looked for a suitable schematic was Todd VE7BPO’s site and lo and behold, I found one.  It uses a J310 JFET in common gate configuration.

I’m feeling the need to make some kind of excuse for the messy look of the circuits I have built Manhattan style. Part of the reason is that I’m not as disciplined with my building technique as are masters of Manhattan like Jim Kortge K8IQY and Chuck Adams K7QO. The other part of it is that I think I need to recalibrate the way I look at my projects and reign in my OCD tendencies. With an experimental project like the VRX-1, it was inevitable that bits would get added on, and it would be a work in progress for a while.

Here’s the completed RF pre-amp board:

And here it is installed in the VRX-1. It cured my common-mode hum problems beautifully, by the way:

If you’d like to hear how it sounds, I created a YouTube account for my ham radio shenanigans, and the video of the VRX-1 is here:

February 9, 2010

The Fort Tuthill 80 – A Direct Conversion Transceiver For 80M

A week ago, the Arizona ScQRPions released their Fort Tuthill 80M TX/RX kit. I’d been keen to build it ever since I became aware of it (thank you AE5X) due to a long standing interest in DC receivers, and the fact that the first transceiver I ever built was also for 80M and also utilized a DC receiver. That was back in the early 1980′s.  I brought that rig with me to the US when I moved here from the UK in 1987 and then fried several of the active devices in it by accidentally connecting the power the wrong way round.  Yes I know – for some reason I didn’t want to use a diode to polarity-protect it (probably didn’t want to lose the 0.6V forward-bias voltage drop).  At the time I was going through a phase of my life in which I was distinctly uninterested in amateur radio, so rather than replace the fried devices, I threw the whole thing out and somewhat regret that decision to this day.  I don’t remember it suffering from any of the problems normally associated with DC receivers – microphony and hum pickup. However, this could very well be a case of my looking through the past at rose-tinted glasses.  As it was the first transceiver I had ever built, it was the apple of my eye, and well, proud parents can be quite good at ignoring flaws in their offspring.

OK, here’s what you get for your $53.  The silvery bag contains the active devices, and you also get 2 sets of decals:

Considering the amount of work that goes into designing something like this, designing the PCB, sourcing and ordering all the parts, as well as bagging them all up ready for delivery to you, the end consumer, $53 is a steal. Dan N7VE, the designer of this transceiver, knows a great deal about active filtering in receivers and has applied his expertise and knowledge to the design. Look here for a presentation he gave on the subject of active filtering in receivers.

A closer look at the board that Dan designed for this radio:

One of the many great things about the internet is that manuals for kits can be more detailed, with more pictures than ever before.  Dan’s manual makes building this little radio a lot of fun, and with the help of the Yahoo Group, expert advice from the designer, or other builders, is not far away.

The build went smoothly.  As tends to be the case with these things, I stayed up all night to finish it off and ended up finally hitting the hay at 9am.  I find that it’s easy to get so engrossed in a project that I’ve barely noticed that it’s something like 3am.  By that time I’m within spitting distance of finishing – or so it seems.  I’m not fast at doing things, tending to get distracted easily by things like the need for coffee breaks, the urge to look at something on the internet etc, so what some might call an 8 hour kit build, is closer to 3 times that for me. Next thing I knew it was 9am, but the board was finished, and all the external connectors temporarily connected:

The board with external connectors temporarily attached. On the far right just above the middle you can see the 2 PA transistors epoxied to the heatsink. Just above that is the trimpot for controlling output power. The VFO toroid is on the far left about 2/3 of the way up the board.

Initial impressions are favorable. Although some of the capacitors are microphonic (to be expected in a DC receiver), I can tell that this is not going to be a problem in use, especially when the board is mounted on standoffs in a case.  The other main problem with DC receivers is the issue of hum pickup.  I’m a little concerned, because I am getting quite a lot of hum pickup through the antenna connection. I’m hoping that enclosing the board in a metal case will help.  An enclosure will be arriving later this week, so we’ll see how that helps.

The receiver is sensitive and VFO stability seems to be good enough for regular usage.  Using my FT-817 as a reference,  I measured about 60Hz of drift in an hour from the VFO after it had already been on for several hours, operating in a room of reasonably constant temperature. I then measured the frequency drift every hour for 6 hours. After 6 hours, the VFO had drifted 90Hz higher than the original frequency; the maximum drift from the original frequency within the 6 hour period was 150Hz. Not bad! I intend to fix the VFO toroid more firmly to the board with a nylon nut, bolt and washer to help improve the resistance of the VFO to physical impact.

At this point, my main concern is that of hum pickup in the receiver.  We’ll see what happens when I’m able to install it in a metal case. To be continued………..

Postscript – nothing better to do with my time this morning than stare lovingly at the PC board and ponder on what a thing of beauty it is:

The board after disconnecting the temporary knobs, switches, power etc and before mounting in a case.

Hum Problem Solved – I haven’t begun to put the FT80 in a case yet, but the hum issue has already been resolved.  The radio was connected to an antenna via an LDG Z11 tuner, which was powered by an unsmoothed wall wart transformer.  On unplugging the wall wart from the wall, the hum all but disappeared.  There is still a very low level of background mains hum, but only at the level you’d hear in a mains powered receiver with a well smoothed power supply. At this point, the radio is very usable in just the current bare board situation; things can only get better when it is installed in a metal case. Dan N7VE made the point that if you mount the board close to the bottom of the case, as the traces are on the bottom of the board, and the top of the board is mainly ground plane, then the traces will be sandwiched between two ground planes. The FT80 looks like it is going to be a very usable little transceiver. I’m really looking forward to when QRP Kits start stocking versions for other bands.

January 11, 2010

Software Defined or Hardware Defined – Which Way to Go?

The main project going on in the AA7EE  “shack” (for which read “main room of my studio apartment”) has been, and will continue to be, the monumental task of committing my sizable collection of cassettes, CDs, DAT tapes and broadcast carts to backed up hard drives.  This is an ongoing project which I expect to take several years. Over the course of a 22 year career as a DJ, I amassed a lot of stuff that is becoming wieldy and expensive to lug around, so it’s time to start consolidating.

So whenever I’m listening to the radio or soldering something, I’m often also ripping a CD, scanning the artwork, or calling up my friend Antoinette and trying to give her as many of the CD’s that I just ripped as possible. I spent 22 years accumulating stuff (opens in a new browser window) and I now want a good portion of it gone (thanks Antoinette!)

All that aside, the amateur radio goings-on here have included building a neat little direct conversion receiver for 40M – the VRX-1 designed by NT7S and sold by the 4SQRP club. It’s a cool little DC receiver.  I’ve been having a bit of a problem with the input bandpass filter, so it’s on the back burner for a while, but it’s been a fun Manhattan building experience:

NT7S’ fun DC receiver got me all charged up.  The first successful TX/RX I ever built was an 80M DSB TX/RX designed by G4JST and G3WPO and published in the UK magazine “Ham Radio Today” in March 1983. It utilized a direct conversion receiver, to which I added an audio filter built from a 741 op-amp.  It was my first experience with DC receivers, and I remember being surprised that such a simple receiver could sound so good. Jason’s VRX-1 re-introduced me to the pleasures of DC receivers, as well as the technique of Manhattan construction (my first time), so by the time I’d finished construction, I was all primed up and ready to swoon at any direct conversion receiver that might flit it’s tail feathers at me.

John AE5X’s post couldn’t have come at a better time. Allow me to repost this picture of the 80M direct conversion TX/RX that the Arizona ScQRPions will be providing a kit for very soon:

The details are here. I was beyond excited when I found about this (thanks John).

I have a mental list of several different QRP rigs that I want to build, among them the Weber Dual Bander, and either the Elecraft K1 or K2. However……SDR has been on my mind too recently.  I built the SoftRock Lite II for 40M a few months ago and was impressed with the performance for such a simple piece of hardware, and low price too.  Of course, the simplicity of the hardware and the low price is a bit misleading because the signal processing is all done in software.  This is the beauty of SDR though – if a better demodulator or filter is available, you just download it.

I’ve been using the SoftRock to monitor the CW portion of 40M, and then once I see a signal, I can work him on the main rig, which is currently a Norcal 2N2/40.  Yesterday, I built a combined switch and dummy load into an Altoids-type tin so I can easily accomplish switching the antenna between the SoftRock and the 2N2/40.  The 2N2/40 is a cracking little rig – a sensitive low-noise receiver with a stable VFO (after the intial warm-up period).  It has a nice narrow crystal filter too which is great for working people, but not as convenient when you’re trying to find stations to work.

So….what I do is use the SoftRock to look at a wide portion of the band.  With my soundcard, I can look at a 96KHz-wide slice of the band on my screen, and the minute I see a station, flip over to the 2N2/40 and work him. It works well but it got me to thinking – why switch over to a traditional radio to work a station that I find with SDR? Why not just get an SDR transceiver and avoid having to switch over to a hardware defined radio?

Hardly original thinking.  I’m sure it’s the same thought process that has led many an amateur to adopt an SDR rig as their main station radio. FlexRadio are about to introduce their Flex-1500, which is a 5 watt all HF band SDR transceiver.

Mighty tempting and with my limited amateur radio budget, I’m now wondering whether to continue building all the QRP transceiver kits I’ve had my eye on, whether to build the SoftRock v6.3 HF TX/RX, or whether to go for broke and get the Flex-1500 when it comes out.

In the meantime, I have a KD1JV Digital dial on order, which will turn the Norcal 2N2/40 into an even more usable little radio.

Oh, and I have 1,000′s of CD’s to rip and perform hi-res artwork scans.  It’s not as if I’ll be sitting here twiddling my thumbs.

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