Sunday, 27 July 2014

Boxing Clever

Having dealt with the small amount of electronic preparatory work the time had come to cut some metal and put my experimental HF rig into its new enclosure. Or - to be more accurate - into its old enclosure...


The first task was to make a rectangular cut-out in the 2U front panel to accept the nice new 20*4 LCD display - a task which I tackled on my "trusty rusty" milling machine...


After a few more strategic holes had been drilled (and existing ones enlarged to suit new purposes), I had the beginnings of a rough panel...


Here's the "first fit" of the components into the enclosure, in which I mounted the "RF" system and ran the "digital system" out on the bench...


As you see, the main RF board occupies the greater part of the floor of the enclosure and the I2C controlled band-specific filters sit over at the left hand side of the enclosure, close to the antenna socket. 

Having already occupied all the "ground" area of the enclosure, I had to take inspiration from the Tokyo town planners and extend into the third dimension to accommodate the digital system - it is mounted on a hinged platform in the right-hand-side of the enclosure (when viewed from the front). The hinge allows access to the RF board below, such that modifications / adjustments can be made without completely stripping down the rig.


The view above shows the speaker / phone output (currently via a 1/4 inch jack as I don't have any chassis-mount 1/8 inch jacks in the junk box stores), the inputs for key and paddle, the power input and the antenna connections (via BNC and SO 239).

Here's a shot of the newly enclosed system on the bench...


You can see my home-brewed fist mic at the left hand end. The display - although not very visible in the shot - is very much easier to read and use in vivo. There are two push-buttons just to the right of the display, which are used for various functions within the menu system and the main tuning control is the large black knob. The red push button does nothing - it fills a hole that was in the original enclosure that I was too lazy to fill! Last grey knob on the right is the audio volume control.

The system works well - first impression is that putting it in the box has made it work much better than before.

But perhaps I'm biased!

...-.- de m0xpd

Monday, 14 July 2014

Raspberry Jam for Tea?

It is now 18 months since last I played with my Raspberry Pi and irritated some folk over at Hackaday (I must remember: "There is no such thing as bad publicity").

I've become so used to playing with all things Arduino that I'd forgotten all about fruity single-board-computers. Even to the extent that I was surprised by today's announcement from the folks over at the Raspberry Pi foundation...

A new product is launched: The Raspberry Pi B+

Of several interesting features this new product offers over the standard B model, most important to me is the significantly expanded GPIO interface!

This speaks of the Raspberry Pi foundation making a strong move to encourage and support PHYSICAL COMPUTING, which I applaud (and which might make the Raspberry Pi of practical use to me once again).

This is an interesting contrast to the strategy we see our cari amici over at Arduino playing. They seem determined to mark their newer offerings by more and more abstraction and complexity.

Interesting times - I might try another pot of Raspberry Jam.

 ...-.- de m0xpd

Sunday, 13 July 2014

New BITx Mic PreAmp - for CW !

My new rig (that is to say, the rig that was new in November last year and that has been the subject of continual development) has been sprawled all over the bench for too long, making a general nuisance of itself. Well - finally, I've found a suitable enclosure - thanks Paul!

Unfortunately, before it can be neatly sealed away inside a box, there's one issue I need to sort out...

Although this is a sideband rig, intended primarily for voice, I've been using it to send CW - as readers of the G-QRP club's SPRAT magazine will have seen in the current issue.

My article "Working CW from an SSB Phone Rig" explains how I'm exploiting the flexibility of my coupled, digitally controlled VFO and BFO to use the transceiver's IF filter to turn an audio frequency square-wave (generated by the Arduino) into pure CW at RF, without disturbing any of the rest of the system's functionality.

The article also explains that I'm "jury-rigging" the phone transceiver to work CW. That currently entails hooking a mini crocodile clip onto the microphone preamp whenever I want to work CW - hardly practical if the whole thing is inside a box!


You can see the clip in the middle of the photo above (the line labelled "CW Audio"). The other white croc clip (by which the Arduino can switch the rig into transmit mode if I'm sending CW through one of the several logical "OR-ed" electronic "PTT" lines I incorporated in my build) and the yellow sidetone wire can stay in place - so they are no threat to boxing up the system.

The trouble is, hooking up the CW Audio connection loads the microphone signal path, such that the rig doesn't work as a phone rig with the croc clip in place. It is easy to see why with reference to the schematic...

Here's my current microphone pre-amp stage - the first stage is a general improvisation on the contents of the junk box to achieve an electret microphone pre-amp with around 40dB of gain, whilst the second stage (within the dashed red line) is Farhan's original input stage...


You can see where the croc clip goes from the label in the schematic (actually the label lies - it is easier to clip onto the collector of the first stage - the other side of the capacitor from the point shown in the schematic - that's where it is hooked to in the photo above). The problem is that the impedance presented by the clip (and the Arduino output pin at the other end of the wire) is rather low - such that any signal from the first stage is shunted by the clip and the overall operation of the microphone preamp is destroyed by the presence of the clip..

I decided to build a new microphone stage which would allow continuous connection to my CW signal source without any loading effects.

Here's my new schematic - it uses an op-amp for the first stage to make an inverting summing amplifier - a "mixer" (as the audio frequency community would call it)...


and here's the new (right) and the old (left) microphone preamp stages...


The new microphone stage - with its additional "line audio" input - is now installed on the main board, where it works perfectly. I even found an old 741 op-amp to run in it, just for "nostalgia's" sake...


While I was about it, I also took the frequency divider circuit (which forms Figure 5 of my SPRAT article)...


from its rather temporary incarnation on a little solder-less breadboard...


to something a little more permanent...


Now all the connections can stay in place all the time.

Working CW is just a matter of pressing on the key or the paddle and working voice is just a matter of pushing the PTT button on the fist mic. Just as it should be.

We're ready to box it all up.

I wonder if a quart will fit into a pint pot.

 ...-.- de m0xpd

Saturday, 28 June 2014

Repairing my Walkabout mk II

Last weekend in the North of England was graced by glorious sunshine and radio rallies on both Saturday and Sunday - perfect conditions to take drives in the country...

Saturday saw the XYL and I take the short run to Bickershaw, for South Lancs Amateur Radio Club's Summer Rally at the Bickershaw Labour Club.

I didn't find much to enthrall me at the event - but I was tempted by a scanner antenna, which looked as if the telescopic section could be just the thing to repair my Walkabout mk II's top section, recently damaged on Aldeburgh beach...


I paid £3 for the donor part - which seemed fair for the complete antenna, even though it was a bit steep for a cheapskate like me who was intending to take it home and tear it to pieces!

On getting back home and stripping off the heat shrink at the base, I confirmed that the telescopic section was indeed exactly the 48 inch length that I needed to replace the broken section of the Walkabout - seen here below the donor scanner antenna...


I removed the loading coil and BNC from the donor and decided I would try to recover the brass base from the broken Walkabout part to save the female 3/8 inch 24 TPI thread...


Unfortunately, in unscrewing the antenna from this base component, the threaded stud from the antenna sheared off and could not be removed - so I drilled it out and tapped the over-sized hole 2BA.

I then made a brass coupling piece with a 2BA male thread at one end and an M2.5 female thread at the other to accept the new telescopic whip from the donor...


A final covering with heat shrink and the new antenna top section is complete - differing only from the original by being slightly less stealthy (in its bright chrome finish) and a little longer when fully extended (which won't hurt tuning at all)...


Not a bad result for £3 - for anybody else wishing to follow in these footsteps, I note that 48 inch telescopic antennas can be had on eBay from Far East sellers for around £3, including shipping.

I haven't had any QSOs on my new antenna - but I have confirmed it tunes up on 40m and the FT817 gives exactly the same indicated SWR and Power Output as before - result!

Sunday offered the LAMFEST Rally at the Elsecar Heritage Centre, so a blast over the backbone to Yorkshire was indicated.

Now this is what I call a Rally location - combining Radio and Railways AND some entertainment for the XYL (gift shops, antiques, vintage clothing shops, cafes etc) all on one site.

We were met by views of a little ex-Coal Board 0-6-0 Saddle Tank plying her trade in front of the car park as soon as we arrived...


I got myself a Sealed Lead Acid brick to enjoy longer visits to future beaches with the repaired antenna. There was even a COMPLETE Walkabout mk II on offer for £25 - a good deal for somebody!

A great weekend.

 ...-.- de m0xpd

Thursday, 19 June 2014

Walkabout on the beach

I was fortunate to spend last week on the beach at Aldeburgh, Suffolk. I took my FT817 and Walkabout II Antenna, hoping to work some /p.

Well - I have to report a pretty poor show - I logged only one contact (with Richard, g0iln, in Bexhill, who also was running an FT817, but into a "real" antenna). I was only trying to work G stations - mainly FISTS calls. Perhaps I would have done better to turn my attention to the continent, where I've had better luck with the same portable equipment before.

Nil desperandum  -  band conditions were the least of my problems...

Packing away one day I accidentally bent the telescopic section of the antenna, which promptly snapped...


I thought that would put an end to the week's activities on 40m (the Walkabout II needs the full length of the telescopic section to tune up on 40 metres) but the good folks at MCT Electrical on the High Street kitted me up with an off-cut of garden wire which I could stuff inside the broken antenna and I was back on air.


All their kind efforts were in vain for, try as I might, I could not raise a further contact. I reverted to a different interpretation of "Walkabout", which turned out to be infinitely more fruitful...

Aldeburgh's beach is graced by two conspicuous "Lookout" buildings, once operated by two rival companies (known, as I discovered in the interesting little museum in the Moot Hall, as the "Up-Towners" and the "Down-Towners").

The Southern Lookout is now owned by Caroline Wiseman...


This ecclesiform curio, with its west tower and its east opening to the dawn, lives up to its billing of a "tiny temple of inspiration for artists, poets, writers, performers and thinkers".

It drew me in when I wandered by, because it was playing host last week to a collaboration entitled "The Awe of the Other"...



The collaboration was between Pauline and Matthew Bickerton and Thomas Garnon. Most of their work soars high above the scope of this mundane blog. But Matthew's practice embraces some technologies of real interest and significance to us...

The "Otherness" implied in the "Awe of the Other" title refers (in part) to impromptu conversations between members of the public who choose to sit on two larger-than-life chairs placed some 100 metres apart on the beach, facing the horizon (seen in the images above). Snippets of these conversations have been recorded and assembled into a work which was exhibited on Saturday, 14th June, in the Lookout. Part of this "performance" was delivered through a modified radio.

Matthew took an old domestic radio and fitted a custom media player, which he built around an Arduino-compatible system. The host radio and the new internal media player is seen here...


The tuning mechanism (which originally drove a ganged variable capacitor) has been retained but modified to drive a potentiometer and so generate a voltage, which is sampled to allow the user to select 1-of-n saved sound files (rather than to tune the radio to a station). 

Matthew's custom media player is seen in close-up here...


It uses a Teensy 3.1 (which itself has a ARM processor - similar to that on the Arduino DUE), sitting atop a Teensy Sound Card (which hosts a MicroSD Card for Mass Storage - just visible in the photo) and a digital power amplifier, seen on the blue evaluation board to the left of the photo.

It turns out that Matthew is no stranger to bringing bespoke technology and art together in exciting new ways, as is exemplified in the Guerilla Dance Project.

 Great to see people making beaches MORE beautiful for a change!

 ...-.- de m0xpd

Thursday, 15 May 2014

Imprints of Redundancy

I am (as a result of a generous Christmas gift from my daughters) a subscriber to Practical Wireless and had been pleased to see some of Don, g3xtt's "holiday snaps" from the DXpedition to the Austral Islands in the June 2014 number.

The whole DXpedition scene isn't really my cup of tea - but it is part of the rich tapestry of our hobby.

I also am a member of the RSGB - and so today I was surprised to see pretty much the same photo used on the front cover of the June number of RadCom as already had graced this month's PW...


There is enough overlap between the TX6G expedition members, the articles' authors and the editorial teams (g3xtt IS the PW editor!) to have spotted and avoided this gaffe.

It is hard enough for any monthly publication to retain sufficiently fresh content to justify its own existence. When the two most important print periodicals on the UK radio scene start treading on each others toes like this, something is seriously wrong.

These two publications can only coexist in my magazine rack if they are strongly differentiated - any more convergence of the sort demonstrated by the repetition and redundancy seen this month will mean they won't be coming through the m0xpd letterbox next year.

I'll go further - with the obvious extrapolation of the above.

These two publications will only coexist if they are strongly differentiated - any more convergence of the sort demonstrated by the repetition and redundancy seen this month will mean they will not survive as respected print publications for much longer.

...-.- de m0xpd

Monday, 5 May 2014

Sweeping the BITX IF

Having brewed up the little scheme for visualising the magnitude response of filters using RF sweeps generated by the Arduino, I decided to take a look at the crystal filter I made for my SSB rig. That rig started from Farhan's famous "BITX" design and, whilst I have added a number of elaborations, it is still very much - and very proudly - a BITX.

The IF filter had been made from Farhan's schematic - but I paid no heed to the accompanying words and music, specifically making no attempt to match or select crystals. Instead, I just took the first four crystals out of the bag and worked with them...


The results were less than perfect - so I made some adjustments "by ear" - you can see evidence of some of these modifications in the capacitors still hanging off the filter in the photo above. It had been sounding and working FB for the past few months - but now I was in a position to measure it...

I started by disconnecting the filter from the surrounding rig and measuring the IF response according to the scheme described in the previous post. This yielded the response shown below...


OK - it might not win any awards - but it is consistent with the acceptable performance my rig has been turning in over the past few months - and consistent with the BFO frequencies I've been using in my Double DDS Code (as described in the current edition of SPRAT)...


[My rig can be used in either LSB or USB mode, according to the band it is working - the two numbers above are the BFO frequencies for LSB and USB operation, respectively.]

However, the filter isn't intended to be used in some artificial test environment - it is intended to be operated in the very specific loading conditions in the middle of my rig - so I really ought to be measuring its performance there. Before I could do that, I decided some housekeeping was in order...

The IF section of the rig was squeezed into the corner of the PCB (as shown by the dashed red box)...


I took a few moments of redistribute things and give myself a little more elbow room, giving the whole IF section a slightly bigger red box to work in (and opening up the whole dashed blue box pregnant with possibilities for future developments)...


Now, it was easy to measure the response of the entire IF section - not only of the filter, but of the amplifiers on either side of it.

The signal from the DDS module was attenuated through a PI pad (actually a cascade of such sections, embodied in my switched attenuators), to both attenuate the signal and ensure a 50 Ohms source impedance and the amplifier after the filter was loaded by the 50 Ohm termination at the 'scope input, as shown by the overall scheme in this graphic...


Readers familiar with the BITX will recognize the segment of the original system under test.

The response is actually similar to that measured with the filter out-of-circuit (albeit with some gain - associated with the rather random combination of the loss from my attenuator and the gain from the two amplifiers)...


This little sweeper actually seems capable of useful work! Perhaps I should use it to try to refine the passband response of the SSB rig's IF filter - then again, life is quite short and what's 2.5 dB between friends?

...-.- de m0xpd