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

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

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