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

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

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