Thursday, 11 February 2016

Switching Bilateral Amp

The January number of QRP-ARCI's QRP Quarterly just landed here at m0xpd...


It contains the usual mix of neat stuff - all presented in a superbly produced, large-format journal, between full-color (it's American!) covers.

This issue is marred only by the inclusion of an article in which I've described an idea about how to electronically switch a bidirectional amplifier using FETs, rather than the usual relays, as shown below...


The QQ article goes on to use a pair of the amplifiers, sharing a single direction controller, either side of a parallel pair of IF filters to form a new 'Parallel IF Board' for Ben, kk6fut and Pete, n6qw's 'LBS' transceiver project.

Here's the initial 'ugly' development version of that system, in which the shared direction controller is using the SMD FETs you can see mounted on the green SOIC carrier close to the centre of the image...



The 'complementary' FETs used in the construction example in QQ are leaded components, to match the single-sided, through-hole PCB design I've presented there: 2N7000 / ZVP3306 (although if you are willing to use surface mount devices, there's a wider range of options available for the P-channel device).

If you're not a member of QRP-ARCI, I recommend it - despite the occasional dodgy article from yours truly!

...-.- de m0xpd


Sunday, 31 January 2016

IDE 1.6.7 and Progress

I have this afternoon been paying obeisance to the great deity 'progress' (also known as IDE 1.6.7).

In so doing, I've discovered what others who have trod the path before me have noticed - namely that the LiquidCrystal_I2C library (in its original form) does not work with this IDE as it used to with earlier 'editions', such as the nice, homely 1.5.5-r2, with which I'm still working on my main machine.

Lots of my example sketches (VFOs, rigs &c) use an alphanumeric LCD display on an I2C interface, so it hurts me and (more importantly) my readers!

Well - there is an easy fix which seems to work...

Simply download and use a modified version of the library from here.

By way of proof, here's the Kanga_VFO_3 code, compiled from IDE 1.6.7, running on a UNO and a 16*4 LCD, just as the first test...


There are no controls or even a DDS module, as I was just interested in getting the display working. Eagle-eyed viewers will notice that the hardware is resting on carpet - rather than the usual backdrop of my bench. That's because I'd downloaded IDE 1.6.7 onto a PC I'm using as a media centre behind my TV in the sitting room - I'm not risking my main PC yet HI HI.

Satisfied that things looked promising, I re-programmed the "tour" Kanga VFO demonstrator, which runs on an Arduino MEGA (variety is the spice of life, after all)...


OK - so now I've cracked the errors - I just need to get rid of the warnings. That, however, is just a matter of vanity, which can wait.

...-.- de m0xpd

Power Play II

This weekend I continued the Power Play which started back in 2014, so as to extend recent interest in RF probing and voltage detection.


Readers of long-standing may recall how I described a variation on a theme first expounded by Eamon "Ed" Skelton, ei9gq, in RadCom, concerning a little compensated peak RF voltage detector. Unlike Ed's, my version was hooked up to the ubiquitous open-source electronics platform, to offer advantages in user interface and calculation of derived units - not unlike the approach recently described by Tex, g1tex, in Practical Wireless, for his RF Power Meter (or, indeed, g4fon's original digital power meter).

I hinted back in the original article in 2014 that my unit had some additional features - now it is time to get them proven and documented.

Most important among those features is an enhanced power-handling range, which allows the unit to accept RF signals of up to 50V pk-pk (which corresponds to 25W into 50 Ohms). This is managed by simple external circuitry, with no 'switching' (of the sort which Tex uses). Of course, I don't care about running up to 25W (of which, more below) but the power handling is important to qualify the unit as a complete 'QRP' power meter, where the term 'QRP' might imply up to 10W and some kind of headroom is required.

Here's the prototype, set up on the bench for testing...


Input RF (here shown at QRP levels) enters the system stage left, to meet a couple of 100R resistors, playing the role of a dummy load.

The compensated detector is pretty much as described back in August 2014 (which itself is pretty much as described by Ed) but, grafted into this is both i) means to protect the analog channels of the Arduino against over-voltage and ii) a second, "high power" channel, which comes into play when the input signal climbs to vulgar, QRO power levels.

I say 'comes into play' - that's not strictly true; it is in play at all times. The Arduino decides whether to believe the low-power channel or the high power channel result, depending on the data they are providing. In that way, no switching of the external circuitry is required, simplifying both the circuit and the code which accompanies it.

Of course, your humble servant doesn't really have much in the way of vulgar sources of RF at QRO power (as sign of his humility HI HI). Again - that's not strictly true - because there's the old FT-101ZD sitting here next to me as I type - but let's turn a blind eye to that.

Fortunately, it is a useful property of the type of peak detector used in this circuit that it may be 'calibrated' by a d.c. voltage - so that is what I did in this case.

For the third time in this post, that's not strictly true either, as the d.c. currents in the detector diode and the 'reference'  diode will not be in the same ratio during so-called calibration from a d.c. voltage as the ratio of currents in application - so this is only an approximate calibration. But is is way better than nothing and certainly good enough for the rough-and-ready style in which I approach my games in radio-frequency-land.

I looked for a nice source of controllable, high d.c. voltages and - guess where I found it?

Yes - the nice old HT power supply I found at the Red Rose Rally a fortnight ago. Ironic that a power supply purchased for tube projects should come in handy for dainty micro-controller applications!

So - here's the system, working with a wide range of inputs...


The top two were generated by real RF, flowing into a real 50 Ohm load.

The bottom one was d.c. - with no 50 Ohms resistor connected (to save on my electricity bill). But the same 'measurement' system was used, with no change to the hard- or soft-ware configuration. This is yielding a wider useful display dynamic range than (e.g.) the excellent Sandford Wattmeter - but it does have the rather significant disadvantage of needing a power supply (such as a battery)!

More details of "how it is done" to be revealed at a certain gig in May, after which there will more publication - somewhere.

The remainder of 'playtime' this weekend has been spent downloading IDE version 1.6.7 for the 'open-source electronics platform' and confronting some of the less-than-attractive consequences. I now have to spend a lot of time making what was once entirely worthy and acceptable into an offering which is fit to be submitted to the new compiler without it spitting back a series of warnings.

They tell me this is progress. I am not convinced.

...-.- de m0xpd

Sunday, 17 January 2016

Bench PSU for Tube Projects

Just back from the West Manchester Club's Winter "Red Rose" Rally, which is held at a leisure centre only a few miles from my home. Unusually, today's Rally lived up to its "Winter" billing, as there was a sprinkling of snow on the ground to greet visitors.

I bumped into some friends from Warrington and found a nice old Farnell E350 power supply - just right for tube projects...


It gives metered, variable HT up to 350V (in seven switched ranges) and a range of filament supplies. This is a useful addition to the m0xpd range of HT supplies - particularly as it is both metered and variable.

The only issue obvious at time of purchase was the ding on the corner of the case...


A little gentle attention in the 'body shop' soon made the case look a little more presentable...


Just needs a lick of paint (don't hold your breath).

I hadn't noticed at the time of purchase a more serious problem - the voltage selector on the rear panel is broken, exposing the internal conductor, which carries mains line voltage...


This is currently addressed using what Roger used to call "insulting tape" - until somebody can come up with a replacement for the nice plug-in, insulated link (not that I will ever wish to change the voltage setting).

Now for a test...

Fortunately, I hadn't got round to yesterday's threat / promise to dismantle the single-tube regenerative receiver. So - there on the bench was a convenient tube project just waiting to be powered.

When I used it before, I had a Farnell L30B on the filament and a Farnell L30BT with its outputs running in series for the HT. Now, the new purchase could power the whole shebang...


It even prompted me to experiment a little more - I can get better performance with higher HT levels than the 40V previously reported. This afternoon I was running around 70V.

This is a nice new (that's to say, old) toy. All for £20. Bargain.

...-.- de m0xpd

Saturday, 16 January 2016

Single Tube Regen

OM Jeff, from Traverse City, MI, wrote the other day, asking if I would share the circuit diagram for the single-tube regen receiver I described just before the holidays. I told Jeff I'd be happy to share - but reminded him that I was just riffing on an idea for my own entertainment and suggested he might do better to look elsewhere for more reliable designs. Jeff found lots of other options - but he left me thinking...

It seemed like a good idea to document what I had done with the regen - not with the implication that it is either novel or even especially worthy. Simply for the record - my record.

So, here's the schematic of my little receiver...


As I said, nothing unusual or novel - just (literally) what came to hand quickly and what worked (just about).

My regen and tuning coils (L1 and L2) were wound on a 35mm diameter former and my tuning coil ended up with 6 turns to get me on the 40m band with the tuning capacitor you see in the photo on the original blog post.

I was running with a power supply of B+ = 40V (or thereabouts) and taking the AF output into the high impedance input of an active speaker (or the line input of a computer) for amplification.

The only other vaguely interesting thing about what I was doing was the little valve-holder I made with a conventional B9A chassis-mounting socket, a pair of M3 standoffs and a scrap of PCB material...


This allowed me to mount the tube (/valve) upright on the "breadboard" base with just a pair of woodscrews.

Now I've documented what it was, I can take the receiver to pieces with a clear conscience and free up a little more space in the shack - to fill up with more gloriously pointless experiments. And perhaps when I'm visiting my friends up in the Thumb of Michigan, ahead of this year's FDIM, I'll try to contact Jeff on 40m and thank him for motivating me to pull together this circuit diagram!

...-.- de m0xpd




Saturday, 9 January 2016

RF Probe

Today I finally got round to building a nice little RF probe kit from Rex, w1rex, the Tuna King over at QRPme.

I took the well-executed PCB which - unusually for Rex - had through hole mounting for the components rather than the 'Limerick' construction that recently I've come to associate with him...


Following the excellent instructions, I whittled away the PCB at the business end, to make the probe easier to probe and poke into awkward places, then added the four components...


Then I just needed to add the probe tip and the connecting wires...


Actually, I tested the whole assembly at this point - but let's pretend I just went confidently ahead and sealed it all up.

I built up a few layers of heat shrink at the tip...


... before covering the whole thing in a protective sheath, which prevented my fingers messing up sensitive measurements by touching the wrong (high impedance) place.



It isn't going to win a beauty prize - but then neither am I.

To check the new little toy in operation, I turned to another gift from Rex (kindly donated at last year's FDIM) - a Dummy Load (which makes a nice addition to my collection)...



I threw some RF at the dummy load and probed about at the top of the resistors, having first connected the new RF probe to an ugly old digital multi-meter of unknown parentage.


As you see from the little inset photo of the 'scope screen, I'd set the RF level to 1V peak. The RF probe was indicating a voltage of 0.656 on the multi-meter but, given the 1V peak on the resistors, it "should" have been reading 0.707106.... V.

The circuit of this RF probe is a shunt design of the form regularly appearing in ARRL handbooks (etc). My most recent copy is the 2009 edition, in which it appears as Fig 25.9 (D) on page 25.9.

The series combination of a 3.9 M and a 220 k resistor as used on this probe gives 4.12 M Ohms.

When this resistor drives a meter WITH A 10 MEG OHM INPUT IMPEDANCE, then the probe will scale the peak voltage detected on its diode to the correct RMS voltage for a SINUSOIDAL input waveform.

This is because of the potential divider action between the meter's input impedance and the source impedance formed by the 4.12 M Ohm resistor:


Obviously - and fortunately - my multi-meter of unknown parentage (actually it came from a Red Rose Winter Rally just down the road at the George Carnall Sports Centre a few years back) must have an input impedance close to 10 M Ohm. I tried it on another random digital multi-meter and it returned 0.696 V. Obviously, these meters have input impedance close enough to 10 Megs to make the approximation useful.

On my Solarton 7050 DVM (with more than 1000 M Ohm specified input impedance) 1.009 V was observed - i.e. the probe became a peak voltage probe!

Neat little toy. Thanks Rex - for everything.

Thinking about it, it must be almost time for the next Red Rose Winter Rally...

Yes - see you next Sunday!

...-.- de m0xpd

Friday, 8 January 2016

Cover Girl

SPRAT 165 landed on the doormat yesterday, with a familiar image on the cover...


It is the 'Nicky' regen, available in kit form from Kanga as 'The Urmston'.

Inside, Colin Turner, g3vtt, offers useful notes about using the radio, which conclude with the words:

"When you have mastered using this little receiver you will have a nice little device for monitoring the band and brushing up your CW. You can then call yourself a real amateur radio operator."

All the emphasis of emboldening, italicization and underlining was Colin's - not mine.

I just agree with him.

...-.- de m0xpd