Sunday, 19 October 2014

QRP HomeBuilder QRT

Those of you who monitor the QRP-L list already will know that Todd Gale, ve7bpo, has taken down the QRP HomeBuilder website


This excellent resource has been a site of special interest to me over the years as the home of the Funster 40, which motivated my own Funster Plus rig.

Todd's work has always been of the very highest quality, being solid, evidence-based, honest engineering. It has been coloured in recent years by the introduction of some new dimensions of "comedy" or "character" which I don't claim to understand - but that doesn't change the core quality of Todd's offering.

Fortunately - although the website is no more - Todd has generously made an archive of the site available for download as a PDF. I don't know how complete that archive is - but it is there for all to capture, as I have done. Ironically, given that I referred to his excellent library for the Si5351 recently, Jason, nt7s is one of the people hosting the archive here.

Of course, QRP HomeBuilder is now removed from the links at the right hand side of is page - and the space that it has made in 'M0XPD's LINKS' has been occupied by Jason's 'Ripples in the Ether'.

We all owe Todd an enormous debt of gratitude for all he has done at QRP HomeBuilder - but, as Benard Ighner's song says,

Everything must change 
Nothing stays the same 
.... 
Nothing and no one goes unchanged 

Todd hasn't stayed the same - he has started a new initiative: Popcorn QRP , which takes its place in the links.

I look forward to continuing to learn from Todd's experiments.

...-.- de m0xpd

Sunday, 12 October 2014

Si5351 in the BITX

Having played with the little Si5351 device in the Adafruit breakout board last weekend, I took the opportunity to build it into a VFO system and attach it to a receiver...

The new VFO was functionally a re-work of the dual DDS system I put together for my BITX at the tail end of last year, even sharing the same breadboard and Arduino NANO controller I used in early experiments...


On the breadboard you can see (from left to right) the NANO, a rotary encoder for tuning and the Si5351 breakout board. The LCD reveals that the system is running a modified version of my dual DDS code, which led to the SPRAT article with Pete Juliano, n6qw, "RF Generation for Superhets". This code generates both "BFO" and "VFO" signals - thereby making use of two-of-three of the available oscillators on the Si5351.

The system worked FB - so I hooked it up to my experimental "Breadboard BITX" receiver for a test. Again, everything worked perfectly (once I had set up the code's implementation of the BFO to the correct frequency for the particular 10 MHz IF filter I had mounted at the time).

However, the implementation of the "VFO" system on its own separate solder-less breadboard, seen in the photo above, was all a bit clumsy - so I exploited the simplicity and small physical size offered by the new Si5351 signal generator and tidied things up...


Above you see a complete BITX receiver (that's to say - only the parts required to operate as a receiver are present but all of them are fully bi-directional) on a single plug-in breadboard. Antenna input at "top left", speaker output at the bottom.

An annotated picture will save a thousand further words of description ("BA" = Bidirectional Amplifier)...


As you see, I made a new plug-in tuning module, hosting a rotary encoder. What is not so clear from the picture is that on each side of the tuning knob are push buttons that drive the remainder of the user interface (moving the "cursor" to select tuning rate, navigating the menu structure, etc).

The system seems to work every bit as well as before (as far as I can judge from a few minutes subjective listening in rather noisy band conditions on 40m) with the exception that there is a rather strident "noise" when it is tuned to 7.143 MHz the origin of which I can't explain. Also, I haven't yet done anything to optimise the level of the input to the amplifiers (and / or adjust their gain) I provided for the mixers I just stuck the signals from the Adafruit board straight in and saw what happened. Whilst the system is already working well, I suspect things can only get better.

There will be a release of the "Double DDS" code for the Si5351 - but I'm not yet ready for that. When it is suitable for public disclosure, you'll be the first to know.

This little Si5351 certainly is living up to expectations - I want more.

...-.- de m0xpd


Sunday, 5 October 2014

Si5351

I've been wanting to get my hands on an Si5351 for months, excited by the prospect of three programmable oscillators in a tiny, inexpensive package. Well - today, it happened...

I got a couple of the nice little breakout boards from Her Ladyship, Limor Fried


As is usual for Adafruit products, this board is supported by an excellent tutorial and some software.

The tutorial support includes everything you need to know to get running with the Si5351 in an Arduino context (although the board can be used with other systems too) and the software is in the form of an Arduino sketch.

I had a MEGA lying on the bench - so I hooked my new Si5351 up to that (it needs just power and the two wires of an I2C interface)...


I downloaded the Adafruit library and example program and soon had the little chip squirting out square waves...


The Si5351A3 has three (independent) outputs and I only have a two channel 'scope here at the moment - so here are three channels captured on the ripoff logic analyser I purchased from Banggood the other day...


It works - but I have to say the Adafruit library is hardly "ready to use".

Fortunately, Jason, nt7s, has come to our rescue with a much smoother library which is also available for download from a GitHub repository.

I tried that code with the MEGA / Adafruit breakout combo and found it much easier to generate Ham-friendly RF.

Here's a 14 MHz signal...


which was set up very quickly and cleanly, as you can see below - all thanks to Jason's efficient library...


This is the way I'll be moving forward with the Si5351 - thanks Jason.

I wonder if I can drive it with the Arduino DUE.

...-.- de m0xpd

Wednesday, 24 September 2014

Hats off to India

Having saluted China's achievements in space exploration it is only fair that we should also recognize another nation's entry into the elite club...

India today successfully placed an object into orbit around Mars - here's a graphic from the front page of the rightly proud Indian Space Research Organisation...


There is something extra-special about inter-planetary exploration that commands additional kudos.

Now we've demonstrated an inter-planetary capacity, perhaps we can inspire solutions to problems within arm's reach.

Well done India - hats off to you!

...-.- de m0xpd

Saturday, 6 September 2014

Automated IF Filter Measurement

Readers will have seen how I have used an Arduino to generate sweeps of the Intermediate Frequency of a receiver, allowing me to visualize the response of a crystal filter in situ. Well now - with the recent work on peak voltage detection associated with the "Power" meter - I've found myself in a position to automate more of the process...


An m0xpd DDS shield generates a sweep of the IF range of interest, in controllable frequency increments (currently I'm using 50 Hz steps). Its output is fed to a broadband amplifier (actually one of the plug-in BITX amplifiers), which drives the crystal IF filter under test (in this case, it is the little 12 MHz plug-in module described previously). The output of the filter is fed to a second amplifier, which is terminated by 50 Ohms.

The immediate intention is that this copies the arrangement in the BITX receiver - as I want to be able to make in-situ measurements of the IF filter response in a BITX (for reasons that will become clear over the coming months). Surrounding the filter by the bi-directional amplifiers and the 50 Ohm termination (which copies the loading presented by the pi pad in front of the second mixer)  matches the conditions in the BITX. Later, this might develop into a more general "test bed" for measurements on any crystal filter.

The voltage developed over the 50 Ohm termination (actually realised by my here rather over-specified new dummy load) is sensed by my "power meter". The result is sent by serial link to a PC, for each frequency measured in the sweep. The results are sent in tab-delimited form, with the frequency sent first, then the peak voltage measured at that frequency (represented by the ADC code - although any other format is possible - and easy!).

Here's the whole shooting match in the flesh...


As you can see on the 'scope trace in the top right hand corner of the image right at the top of this post, the output of the second BITX amplifier is actually pushed into asymmetric non-linearity at this signal level, with its negative peak level significantly larger in magnitude than its positive.

This is exactly what's expected of this simple amplifier, as confirmed by this little LT SPICE model...


Of course, as (bad) luck would have it, your humble servant's peak voltage meter reads the POSITIVE peak voltage - I was rather worried that I'd lost a lot of dynamic range and that the system wouldn't work - but I pressed on...

The serial data from the Arduino can be read into any terminal program (PuTTY, Realterm, etc) - but I used the Serial Monitor feature within the Arduino environment for simplicity.

Here you can see (an extract of) the data read into the Serial Monitor, which is then copied into Excel (or your favourite spreadheet) for plotting...


The ADC code is the raw number read from the analog to digital converter (proportional to the peak voltage), which I've converted first into voltage, then into dB re 1 V ("dBV").

A plot of the magnitude frequency response of the IF filter is easy to produce...


As a further test of the system's flexibility, I switched my attention to an 8MHz IF filter I'd been using previously for CW applications and plugged it into the "Filter under Test" position...


I changed the start and end points of the frequency sweep in my code and ran a new test...


I knew this was a nice, clean filter - but I didn't know quite how flat it was!

This test takes about ten seconds to run. With a prepared "template" in Excel, it takes about another 10 seconds to copy the data from the Serial Monitor window into Excel and produce the graph!

The bare bones of my code are seen below - they're quite enough to communicate (as "pseudo-code") the intention of what I'm doing to anybody who wants to follow.

The code runs a sweep of frequencies (variable f), starting at fStart and running to fEnd, with an increment fStep. There's a delay of T milliseconds between each increment (on top of all the other tasks). The code uses my DDS library (of course).



Measuring the IF response of my SSB rigs in situ is now easily done, in less than a minute.

...-.- de m0xpd

Saturday, 30 August 2014

Dummy Load

My recent games with RF peak voltage measurement left me wanting another dummy load...


It isn't that I haven't got enough dummy loads already - in fact there seem to be quite a lot of them here at m0xpd...


In the photo above, [1] is the classic parallel pair of 100 Ohm resistors, as was used on the original power meter experiment's breadboard last week. Nothing wrong with this, except for the QRPp handling capability of 500mW!


Example [2] is a similar parallel pair of 100 Ohm resistors, only this time they're 2 Watt examples, making a total power handling capacity of 4 Watts. [2] is built on the back of an SO239 socket, making connection to a transmitter easy - it also features a diode/capacitor peak voltage circuit (although this isn't really visible in the photo).

Example [3] uses three parallel 150 Ohm resistors - again of 2 Watt variety - to give 6 Watt total handling. There's a BNC socket, a peak voltage circuit and an LED for RF indication (this is a circuit from the G-QRP Club's Sudden Tx). So much for QRP.

Example [4] is a "real" dummy load, rated at 60W continuous, with the extra frisson of a hazardous substance warning sign - it contains Beryllium oxide.

Whilst on the subject of dummy loads, I thought I'd include 50 Ohm terminators in my photo for good measure, [5] & [6]. I often use these as a dummy "load" for a tiny transmitter in a QRPp context or in general RF experimentation. [5] is a "feed-through" terminator - which you'll need if you're using an oscilloscope to make certain measurements on RF systems (see, e.g. EMRFD, section 7.4) whilst [6] are the sort of BNC terminators used to stop reflections from improperly terminated cables.

There is also - of course - the dummy load in my "Made From Junk" Deluxe Versa Tuner II.

As you see - there's no shortage of dummy loads in the m0xpd shack - but there is a bit of a gap between the 6W and the QRO handling ranges. I wanted something that would span this gap.

Fortunately, a couple of packs of 390 Ohm 2 watt resistors from Ian, g4vap, have been lurking in the junk box for a few years...


Looking back, it seems I acquired them by chance in a "Grab Bag" at a Rally Back in 2011 and they've been waiting for me to get round to making them up into the dummy load for which they were intended. The g4vap resistors, each of 2W, give me 16W handling. More than enough for my low-power applications.

I knocked up a couple of end pieces from scraps of PCB material, copying the size of an S0239 socket...


and soon had the whole thing soldered up and ready to rumble...


I've left the central mounting wire long at this point - I may solder it directly onto an SO239 at some point but - for the moment - I'm using it on the power meter experiment...

I temporarily mounted the new dummy load on the power meter front-end prototype ...


and made some quick measurements - my new meter is able to measure easily up to the 5W outputs of my QRP rigs and more ...


The details of the power meter remain - at this stage - proprietary.

...-.- de m0xpd

Saturday, 23 August 2014

Power play

I've been meaning to make some kind of RF power meter for a while now. Well, today I made a start...


I was inspired (not for the first time) by Eamon "Ed" Skelton, ei9gq, whose monthly "Homebrew" feature in RadCom is - in my opinion - the best regularly published piece on practical radio engineering in the UK.

In the current (September) issue, Ed talks about "RF voltage and power measurement" and I drew heavily on his QRP load and compensated peak volt meter, to come up with a system, the important parts of which follow closely Ed's "words and music"...


As you see from the photo at the head of this post, I've hooked up the compensated detector to an Arduino, which makes a voltage measurement (rather than the moving coil meter / current measurement in Eamon's system). The Arduino runs some trivial code, the more important parts of which are reproduced here...


The "LCD_Display_Power()" function displays the measured peak voltage on the LCD (after doing some simple math to convert the number from the Analog to Digital converter to a voltage - and then to convert to dBm [assuming the measured peak voltage to have been associated with a sinusoidal waveform]), resulting in this sort of display...


I observed the response of the new meter as I applied different amplitude RF signals from my old Tektronix SG503, which sits on the shelf ready for little tasks like this...


I monitored the amplitude on a scope and plotted the RF peak voltage (as "eyeballed" from the 'scope) against the new meter's output...


All pretty encouraging for a first shot - there is a nice linear characteristic, indicating correct operation within the "operating range" of the instrument (if I might abuse that grand word). The bottom end of the dynamic range is pretty much where I expected (from a combination of an LTSpice simulation and consideration of the numerical resolution of the ADC), allowing useful measurements down to -6dBm and indication to below -12dBm. The top end of the measurement above ISN'T the top end of the dynamic range of my new meter - it's the top end of the available outputs from the SG503 source, which only goes up to 2V into 50 Ohms.

As you might understand, there are some "other refinements" in my power meter, not described in the "simple telling of the story" above. I'm working on these at the moment and will publish them when the time is right. Notwithstanding these other ideas, the basic architecture above will present a framework for some interesting test equipment - watch this space.

...-.- de m0xpd