Saturday, 29 August 2015

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

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