The original "RF Generation for Superhets" scheme (SPRAT 158) featured two digital oscillators under the supervision of a microcontroller, by which the oscillators could be "interlocked" to ensure the dial frequency was correct...
The SPRAT article promised:
"The ability to tune the BFO will be found of great benefit in the alignment phase of the development of a new scratch-built rig (which usually will feature a homebrewed crystal IF filter of uncertain passband). It allows the BFO frequency to be precisely optimised for the filter and leaves the builder totally free to set IF frequency to any value suggested by available crystals etc.."
Promises are all very well, but there comes a point where you have to deliver - otherwise they are seen as nothing more than empty promises. That point had arrived...
My co-conspirator Pete Juliano, n6qw, was struggling setting up another instance of the Parallel IF rig on the "left coast". The greater part of his problem was in setting up the system for some homebrewed IF filters. I felt for him - and realised that I could make things simpler by implementing an ONLINE alignment tool within the code. Pete was excited by the idea, saying that he was looking forward to "getting the radio working properly with BITE (Built In Test Equipment)".
The original approach required that some measurements of IF filter response were made and then the necessary BFO frequencies were entered into the Arduino sketch, which was uploaded to the chip and then tested. This approach is very much OFFLINE.
Instead, I have now added a further menu option (with apologies to all - including Pete - who think Menus are the work of the devil) which allow ONLINE adjustment of the BFO...
You simply turn on the radio and listen to how it sounds - if you want to try a different BFO setting, you enter the new BFO menu, adjust the rotary encoder and the system will change the BFO and the VFO to keep the incoming station in tune.
You can hear the change in real time. This is working ONLINE.
Here's the adjustment window on my Parallel IF rig (where it takes place on Menu 5),,,
As you see, I was on 40m in lower side-band (using the nominal 10 MHz IF).
The cursor indicates that any rotary encoder input will cause a change to the BFO frequency - NOT to the dial frequency (which stays on the display, reminding me that it is held constant as things audibly change).
Unfortunately, I could only arrange for the encoder to advance the BFO in fixed increments (as I don't have many push buttons etc on my rig - the to available push buttons are used to move up and down the menu system and if I tried to use them to change the cursor to another "power of ten" on the BFO frequency, it would move me out of Menu 5). Nil desperandum - I simply set the encoder to change the BFO in 50 Hz steps, which seems to be a useful, practical compromise.
When you're done playing with the BFO adjustment, you just step out of the Menu system (by pressing the rotary encoder's integral pushbutton) and the new value of the BFO frequency is retained.
There's a second BFO frequency for USB operation - if you enter the BFO adjust menu in that mode, it will automatically adjust the appropriate value.
Of course, my Parallel IF rig has two IF paths, each of which can run LSB and USB - so there are four BFO frequencies. Again, the correct BFO is "indexed" according to the mode in which the BFO adjust menu is entered, as in this example, where I was using the nominal 12 MHz CW IF path, again in LSB...
If you find you prefer an adjusted value to the "default" setting that is in the sketch, you can note down the adjusted value and change that default setting "Offline". Next time, the rig will be just as you like it. A useful piece of "built in test equipment" indeed. However, after having this feature in place, I wonder if there might be a place for keeping it with more than the test / development phase in mind.
In QRP applications, particularly in SSB, there are some marginal cases where the ability to change the IF tuning to throw away some wasted power in the low register of the voice might be useful.
I have added the Online Alignment feature to the RF Generator code for the pair of AD9850 modules (Double_DDS_VFO_0v3.ino) and for the Si5351 device (Si5351_VFO_0v3.ino), both of which can be downloaded from this repository. Users will find that the BFO adjustment menu is (in this case) Menu 4 (there being fewer menus than with my Parallel IF rig).
Please send your comments and observations if you experiment with this new code.
As for Pete's feedback on the system - I'll leave it to his own words:
"The BFO trim reminds me a lot of Pass Band Tuning that back in the vacuum tube days was available on just a few radios. So Bravo – now that I have found the BFO frequencies I can go back and adjust the sketch."
Seems he likes it!
If I had done this properly, I should have used the Arduino's EEPROM memory to store the BFO frequencies - but
1) I'm too lazy
2) I'm concerned about inter-operability between UNOs, MEGAs, DUEs ec (with their different processors and incompatible "EEPROM" provision),
3) I'm only trying to get across ideas here, rather than produce polished solutions
4) I really am too lazy!
Talking of doing the job properly, I wonder if you have seen Colin, m1buu's BITX20?
There's a great video of Colin's first QSO on the new-born rig, which Bill, n2cqr has mirrored up on the SolderSmoke blog.
Colin's homebrewed creation is beautiful - putting my own untidy efforts to shame - as you see in the photo below...
Colin's rig is of personal interest because it uses my VFO code, driving an AD9850 module.
Colin tells me that it was Pete (mentioned above) who sent him "an email stating the case for an Arduino / AD9850 VFO instead of the traditional L/C design". Pete gave Colin the code (essentially a version of my Kanga VFO code) and details of the n6qw hardware, a Pro Mini clone and the AD9850 Module, all mounted on a proto board, the wiring of which Colin followed. You can see this "digital" element above the display in the photo of Colin's FB rig.
As you can see from the photo and the concrete proof of the QSO in the video, Colin's rig is a great success - well done!
Colin reminds me that I persuaded him to buy an Adafruit Si5351 breakout board at the Rishworth Convention. Well - that board certainly would give a power saving over the DDS module for your SOTA work, Colin and - who knows, I might even persuade you to try using the digital BFO as well. Not that there's anything wrong with your present crystal oscillator!
...-.- de m0xpd