Saturday, 3 October 2015

The URMSTON Regen Receiver

Now that the cat has been let out of the bag on the Kanga website, I guess it is OK to tell you - the second of the two receivers I was playing with a few weeks back is now revealed to be a collaboration with George, g3rjv, and friends at the G-QRP club...

As you see above, the receiver looks pretty in her first production build (even though I say so myself).

The new receiver has been developed particularly for the Buildathon at this year's Rishworth mini-convention, so the PCB layout has been made with special attention to the needs of first-time builders. There's lots of space between the components and plenty of elbow room for the soldering iron - no surface mount technologies here!

Also, I wanted participants at the Buildathon to be able to put together a practical receiver with controls on a real front panel. I  like the form-factor of some of Tim Walford, g3pcj's kits - but didn't think that a fixed front panel would be the best option for a raw tyro in a Buildathon, where time is tight and quick access for trouble-shooting might be invaluable. So I came up with a design that could be made in two parts and quickly plugged together - or pulled apart.

The result is seen here from behind...

The system relies on simple latching connectors for the controls (the female headers are supplied with lengths of cable ready-connected in the kit, so the builder only has to trim to length and solder to the controls) and brackets to fix the main PCB to the front panel PCB. But what could I use for the brackets?

Well - modesty almost forbids me from telling you,

After looking around the workshop for a while and scratching my head, I settled on the idea of using these abominations...

They are - I believe - called "modesty blocks" and they stand in place of carpentry skills in this era of particle board furniture.

They are available in a bag of a hundred for the price of a pint of fancy beer in a city-centre drinking establishment - which is to say they cost a few pence each, retail. They are made of a hard plastic and have two holes on 18mm centres in one plane and a single hole, midway between the other two, normal to them.

They make perfect brackets for the two PCBs of the little regen...

Here's a rather immodest view of the assembled unit...

In times of trouble, unscrewing two screws and pulling off five header connectors will separate the two sub-assemblies. But when fixed together, the blocks make the front panel perfectly rigid.

The front-panel ground-plane is connected to the rest of the receiver via one of the leads - without which the notorious hand-capacitance allows the unit to revert again to its secondary role as a Theremin!

The receiver is great fun to operate - pulling in AM, CW and (if you are patient and skillful) SSB signals on a piece of wet string. It is very sensitive. It is also VERY unlike the other types of receiver we are more used to playing with today. Fortunately I had developed some experience with regens from playing with my Paraset, but for regen virgins, this could be a surprise.

It has been a joy to be working directly with one of George's designs, on an "official" commission from George and Graham at the G-QRP club.

As a further joy (or embarrassment), Dennis at Kanga has named the kit The "URMSTON" - which is the postal town near which I live. Presumably the word is stuck in his mind after mailing so many things to me.

The kit is going to be launched at the G-QRP Buildathon on Friday 23rd October and will be available for general sale at the mini-convention on Saturday 24th and thereafter from Kanga.

I am pleased that my two receivers will be "cannon fodder" at the two big forthcoming Buildathons at UK Radio events this month - I only hope the participants will be pleased too!

...-.- de m0xpd

Sunday, 20 September 2015

Acorn II

Eagle-eyed readers in the UK with their fingers on the pulse will have noticed that one of the receivers discussed in prototype on the bench a few weeks back, the "Acorn II", is to be the subject of the buildathon at next month's RSGB Convention.

Those wishing to learn more about the Acorn II should see Dan, m0tgn's build video. Those wishing to get hold of one can do so at (or after) Kanga's launch at next week's National Hamfest.

One of the key features of the Acorn II is the provision of the external oscillator input. Having provided the input, I thought I better show the world how to make best use of it - so I have written some code to drive an Si5351 to work as a local oscillator, with rather more flexibility than the temporary, jury-rigged arrangement I had set up before...

Here's the Acorn II set up on the bench, along with an adjustable L.O. system (so much better than a fixed crystal HI HI)...

The oscillator consists of a Kanga/m0xpd Si5351 shield (of course - I'm biased) sitting on an Arduino UNO, with a user interface formed by a rotary encoder and an alphanumeric display.

The encoder allows the user to tune the radio across all of the 40m band (and the code includes all the support to switch to other amateur bands - but these require the two push buttons of the standard m0xpd interface used on all my rigs and VFOs, which I hadn't hooked up - no problem as the Acorn II only suppots 40m on the board, other bands needing external filters).

The display shows the effective LO frequency, but the oscillator produces the signal at four times this frequency, as is required to drive the standard quadrature detector on the SDR.

Here's the display (which is notoriously difficult to photograph), set to be appropriate to listen at the CW end of the 40m band...

While writing this post, I changed the oscillator up to 7.13 and listened to gb2eaa - the Essex Air Ambulance Special Event Station - on 7.12 MHz LSB. Just the sort of thing that is infinitely easy to do with this flexible oscillator - and (potentially) impossible to do with fixed crystals.

The code is not ready for public release yet - but I shall make a copy available eventually.

As the bands are a bit of a movable feast - particularly in a buildathon context, I have also knocked up a system I've called the "Buildathon Beacon", which just pumps out "QRP" in CW on 7.03 MHz directly from the output of a DDS module. There's enough radiation from a four inch length of wire for a receiver in the same room to pick it up easily.

Here's the "Buildathon Beacon", with its massive antenna looming towards the camera...

As you see, it is made from an early prototype of the m0xpd DDS shield atop a MEGA - these happened to be lying around.

Here's a screenshot of the output of the Acorn II running into HDSDR, with two foot of wire for an antenna, receiving the Buildathon Beacon...

I had done nothing either to match the two audio channels of the receiver (there's a ten-turn pot for gain adjustment) or to up the software to correct for gain imbalance - I just turned it on and "ran". Accordingly, you can see an attenuated image of the beacon at 7.05.

The Buildathon Beacon was actually developed to assist with the second of my two receivers - but that is a piece of news which hasn't broken yet.

Watch this space!

...-.- de m0xpd

Saturday, 19 September 2015

Bridging the Gap

The best things in life are either free or simple. Here's an example of the latter...

I'm a big fan of Return Loss Bridges, enjoying the fact that they give you useful information pretty much for free. They are simple instruments, which teach you while you're using them - rather like dip oscillators and slide rules. 

I've been using my old, home-brewed return loss bridge for a long time...

Whilst it hasn't had a write-up in these pages, it did make a brief appearance back here in the description of my experiments with mag loops.

I've been intending to return to return loss bridges (!) again and could have used the old warrior above. But the form-factor isn't quite right - I wanted something a little easier to incorporate into a different kind of experimental context. So...

First try was a little plug-in module, designed to fit into my beloved solderless breadboards...

As you see, input is applied at the left side (in the orientation of the photograph) and the load is connected at the right side. The output from the detector is output from the male header pins on the right side of the board. 

This 0v1 build worked fine - but did not achieve the sort of Directivity I was aiming for and certainly did not match the Directivity of my trusty old RLB. 

For readers who don't know about these sort of things, Directivity (in this context) is a single figure-of-merit for return loss bridges, describing the ratio of the detector output when the bridge is operated into an open circuit to the output when the bridge is terminated by a reference impedance equal to the characteristic impedance the system is designed for - in my case, 50 Ohms. To be practically useful, the bridge needs a Directivity of greater than 40dB.

Not satisfied with 0v1, I made 0v2, which features a physically smaller transformer and bridge resistors...

These improvements gave the required increase in directivity - to comfortably over 50dB at HF frequencies.

However, it doesn't take good eyesight to spot the bigger difference between the two versions - the later bridge also features a relay, by which the output can automatically be switched from the load to open-circuit conditions, through the application of an active-high logic input. 

For readers who don't know about these sort of things, the return loss bridge can be used to derive Voltage Standing Wave Ratios from return loss measurements in i) loaded and ii) open-circuit conditions. Having the ability automatically to switch between these two loading conditions will obviously allow a machine to make such measurements without the intervention of human hands.

The 0v2 hardware also incorporates an on-board reference impedance (a swell name for another parallel pair of 100 Ohm resistors), which can be used as a load just by putting a jumper between the two header pins on the board. I was tired of fishing around for a pair of external resistors.

Here's the new bridge in its native environment...

Notice that the Bridge PCB is dimensioned such that the rows of input and output pins make best possible use of the "blocks" of contacts on the breadboard. 

Off to the left of the new bridge you can see a little QRPppp linear, providing the power to drive the load. 

It all works well - watch this space (and some other UK radio events in the forthcoming weeks) for developments.

...-.- de m0xpd 

Saturday, 29 August 2015

Stand-alone Total Response Measurement

I've now implemented a stand-alone measurement system to measure the overall RF to AF response of a receiver...

The method, first introduced in this post then used extensively in my presentation and associated written paper at the QRP ARCI Four Days in May event in Dayton, generates an RF sweep and observes the resulting audio response of an HF receiver.

Previous implementations of the method relied upon an expensive electronic voltmeter to transduce the audio response of the receiver into a dc voltage which an Arduino could sample and a computer to plot the results. The new system uses a home-brew AF detector and displays the results on a little TFT screen, making a self-contained, stand-alone system...

The eagle-eyed reader will notice that the Arduino is a DUE, which is a few clicks up from the humbler devices which were used to run this method in its earlier incarnations. This isn't because I'm expecting to do any heavy lifting in the processing - rather I wanted simplicity in interfacing to the 3v3 display (and the DDS module, come to that), I wanted plenty of ports to play with and I wanted plenty of code space for the display library and other future developments of this measurement approach (watch this space). So - the DUE gave all this flexibility over an UNO or a MEGA.

The detector is inspired by an interesting twist on a precision rectifier circuit published by Dave Johnson. Unlike the original circuit, which uses an exotic (i.e. expensive) rail-to-rail op-amp and a single-sided supply, my version uses bog-standard components and generates its own local negative rail from the standard "shack" 12V supply...

I didn't have any 1% resistors in stock for the two critical potential dividers in the system - so I used trimmers and patience.

The measured performance is pretty good, giving me 50 dB of usable dynamic range ...

This isn't a true RMS detector - but I'm only firing sinusoids at the system (as long as I don't overload anything and ignore any but the "wanted" modulation product - which is the only one inside the audio pass-band if everything is working correctly), so it is good enough and the price was right.

The rest of the system follows the strategy already described - generate a 10 kHz downwards RF sweep which starts at the frequency to which the radio is tuned. In a LSB receiver, this will map to a sweep from 0 to 10 kHz of Audio Frequencies - you hear an audio sweep from the speakers and the system observes the speaker voltage, to derive the overall receiving response of the system.

It is important  that the output of the DDS module is appropriately attenuated before it is applied to the receiver's input - otherwise the latter will be grossly overloaded and distortion will ruin the measurement. This is achieved with a couple of attenuators, one of which is visible in the photo above - and any RF controls on the radio under test.

Here's the measured response of the Norcal 40, seen in the photo above...

The little 2.4 inch screen is really difficult to photograph in such a way as to do its image justice.

Here's a measure of the same system's response, made with the "old" system, as reported at FDIM...

The Norcal 40 has conventional, continuous, analog tuning and an internal input RF attenuator, so the "scales" on the above two graphs shouldn't be expected to align perfectly. But you can see that the essential shape and width is there on the little TFT screen, 12 seconds after a button push to start the measurement - all with no involvement of a PC!

The system can measure any other radio-under-test, such as my KD1JV Tri-Bander from Hendricks QRP Kits, which has the American Morse Paddle permanently riding shotgun...

 Here's the overall response of this little rig...

Again, a nice, tight, narrow response peaking around 700 Hz, as is pleasant and appropriate for CW (in my opinion).

The spuriae at higher frequencies are glitches from other electrical noise sources around the place - NOT from the radio (in this case it was the tumble drier downstairs doing its work on a batch of laundry). Unlike the Bruel and Kjaer voltmeter which I was using before, which has a response limited between 20 and 20kHz, my new detector is wide-open - calling it an "AF Detector" is a misnomer, because it still responds pretty well at 100 kHz! I'll get round to making a band-limiting front end at some point in a later revision.

This is a neat system, which will form a foundation on which to build some more interesting measurements. I'm not going to be posting any more details (of code, detector circuits etc) at this stage as this material is destined to be published elsewhere - so please don't ask.

But please do watch out for those other measurement ideas - I think you'll like them.

...-.- de m0xpd

Friday, 7 August 2015

Sandford Wattmeter

Just made up the Sandford Wattmeter kit from Kanga UK, which has been sitting in my "in tray" since last year...

This meter uses some 50 Ohm thick-film resistors to form a 50-Ohm dummy load and a conventional detector to drive the meter.

I made up the instrument according to the clear instructions - with the exception that I substituted a 1N5711 for the 1N4148 specified in the words and music (as I intend to use the Sandford's detector in some comparisons with a digital meter I've been playing with).

You can see the thick-film power resistors and the (blue) 5711 diode here...

My diode substitution required me to make a little modification to the metering circuit to allow the system to be calibrated correctly - but everything was soon back on track. The instructions give a nice little wheeze for calibrating with a d.c. source - useful for those with limited test equipment.

I needed some application to test the new meter on - so thoughts turned to the recently-acquired Patriot...

I measured power output generated on key-down in CW mode across both 40 and 20m bands and show the results in the graphs below...

This is a neat little meter and a nice addition to the test bench - certainly easier than getting out a dummy load and hooking up to the scope (or choosing a dummy load with an integral detector and connecting to a multimeter) and doing the math.

Perhaps not so much fun, though!

...-.- de m0xpd

Saturday, 1 August 2015

Two Receivers

Two receivers are being prototyped on the bench here at m0xpd. Interestingly, they sit close to opposite ends of the spectrum. Not the frequency spectrum - but what might be called the spectrum of sophistication...

At one end is a re-work of a Software-Defined Radio...

The original Acorn is a simple SDR, designed by g0nqe and available from Kanga UK. It uses only through-hole components, which differentiates it from many other SDR systems and makes it attractive to novice builders who shy away from surface mount components - for perfectly good reasons.

I've re-worked the Acorn for Kanga, to provide the "Acorn II" prototype seen above. It retains the "though-hole" philosophy and improves the connectors on the board. Support for an on-board crystal is retained but the main upgrade is the provision of an external clock input. You can see both the (plug in) crystal and the external clock inputs on this picture.

The external input liberates the Acorn II to tune anywhere, given an appropriate source - such as that conveniently provided by my Si5351 board...

In the picture above, the Acorn II is powering an Arduino UNO (through the red and black wires), which is hosting a Kanga/m0xpd Si5351 shield.

The shield is generating a 28.6 MHz clock (applied through the green and black wires), to place the SDR in the middle of the 40m voice band - I was listening to David, g3sqa, chatting on 7,160 after calling "CQ WAB" at the time the photo was taken...

Notice that I hadn't changed the LO setting on WinRad to reflect this tuning!

At the moment I have the two "amplified" output channels of the Si5351 shield set to two frequencies appropriate for 40m use - one at 28.6 MHz (as above) for voice and the other at 28.08 MHz for CW. Given a few more moments spare time, I'll add a rotary encoder to allow changes of the LO setting - but you all know how life is too short, etc etc.

The Acorn II also inherits from the original Acorn an on-board input band-pass filter. The Acorn II will be delivered with components to build this filter for 40m. The external clock input allows the radio to tune other bands - but the radio should use an input filter appropriate to these bands. Such filters have to be constructed off-board and there are routing jumpers to steer the signal to external filters as required.

The Acorn II will soon be available from Kanga UK, along with code to set up the Kanga/m0xpd Si5351 shield to generate LO signals.

I mentioned at the top of the post that there were TWO receivers on the bench at present...

At the lower-tech end of the spectrum is a Theremin Regenerative Receiver I'm putting together for a little event later in the year (watch this space)...

Here's a rear view, which shows more of the giblets...

The Theremin gag is a reference to the "charming" consequences of hand capacitance that you get with these old-time toys. I'd nearly forgotten about all that stuff since playing with sensible, isochronous oscillators, like the AD9850 and the Si5351.

It is nice to have both ends of the "sophistication" spectrum on the go at the same time. Nice - but it does rather squeeze available time for other projects.

...-.- de m0xpd

Saturday, 25 July 2015

Patriotic Display

My TenTec Patriot is usable but user-hostile without a display. Fortunately, adding a display is the work of moments...

I haven't had many moments to spare since getting back home from the USA, but I found the few required recently and so - as you see above - the Patriot can now tell me what it is up to.

Adding the display is easy, as the heavy lifting is done for you - if you are content to attach a 20*4 LCD alphanumeric display over the standard ("Hitachi") interface. The interface is already accommodated in both hardware and the factory code. There is even a (poor) description of what to do in the user manual and a (much better) description in the comments within the factory code.

Here is a blow-by-blow account of how to do it, with some pictures to make it clearer for those who are not so familiar with these things.

Here is the relevant extract from wa4cdm's code ...

As you see, you need power and six lines to the display.

Here is the wiring you must arrange at the display end...

The LCD will have the usual 14 pins of the "Hitachi" interface plus two more for the LED backlight. Only four lines of the eight-bit data bus are used - the rest are left unconnected.

A simple trimmer potentiometer is used to set a bias voltage which makes the display readable. The comments in the code specify this as 10k - but the value isn't critical - I used 1k, because I had one at hand. Adjust this to get best results on the display. Too far one way and you'll see solid "bricks", too far the other and you'll see nothing.

You will need to run eight wires to the inside of the Patriot case - and you'll have to take the case top off to do this (remove the four screws at the side plus the nut and washer on the antenna socket). I had a scrap of ten-way ribbon cable about a foot long to hand, so I used that. I left all ten ways in place - perhaps I'll add some more push buttons or something in future with the extra two wires.

At the "Patriot" end of the wires you'll need to terminate on a 1*6 male 0.1 inch header strip and a 1*2 strip (or individual pins).

The relevant connection points - which are female headers -  are seen on this annotated section of the board layout...

and on this photo of the same area (the relevant connections are identified by the red boxes)...

Pass your cable through the rectangular aperture in the back panel (of the case top) and plug into the relevant connections, as shown below...

then reassemble your case.

Assuming your wiring is all correct, the display will work with the "factory" code - with no modifications required to the software at all - the factory software is already sending all the configuration commands and data (thanks to Stan, AK0B, for pointing out that the original version of this post didn't say anything about this important dimension).

The Patriot is a whole lot more user-friendly with a display.

Makes you wonder why it didn't come with a display (obviously the price of the case would increase dramatically) and why the instructions for adding one are presented in a manner clearly visible to those who already know how to go about doing it for themselves, but somewhat obscured to newcomers.

Funny old world.

...-.- de m0xpd