|
Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
|
Thread Tools |
26th Apr 2015, 9:28 am | #1 |
Rest in Peace
Join Date: Jul 2011
Location: Bridgnorth, Shropshire, UK.
Posts: 787
|
MW modulator finished
Well, I've got a Medium Wave modulator working this morning - in time to catch the last twenty minutes of the Serenade Radio Breakfast Show! It is based on Charles Wenzel's design:
http://www.techlib.com/electronics/amxmit.htm I picked up the box at a car boot sale (probably for 50p or less). My, is it tough to drill. At first I thought it was made of epoxy or some hard plastic but it has grain so it must be wood. The dark colour isn't a surface finish either - the wood is dark throughout. I suspect it's genuine Mahogany. Oh well, it's going to good use I've just got to add a lead to deliver the dulcet tones of Serenade to my newly acquired Bush table radio and my future entertainment is assured |
26th Apr 2015, 10:18 am | #2 |
Octode
Join Date: Feb 2006
Location: Ilkeston, Derbyshire, UK.
Posts: 1,397
|
Re: MW modulator finished
That looks really good, Karen. I like your idea of recycling a wooden box. I think that might inspire me to re-house my trusty AM-03 modulator. Much nicer than a plastic project box!
Time to scour those car boot sales... Steve |
26th Apr 2015, 3:06 pm | #3 |
Rest in Peace
Join Date: Jul 2011
Location: Bridgnorth, Shropshire, UK.
Posts: 787
|
Re: MW modulator finished
Hi Steve,
I think the baroque principle applies here: If it aint baroque, don't fix it! Actually, black ABS can look a lot like bakelite at times - maybe you just need to add some feet or a fancy base...? A comment to the moderators: I wasn't sure if this thread should go in this category or 'Homebrew equipment'. Feel free to move if I've made the wrong choice. |
26th Apr 2015, 4:07 pm | #4 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,788
|
Re: MW modulator finished
Useful sized wooden boxes seem to crop up all the time in charity shops.
|
26th Apr 2015, 5:05 pm | #5 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
|
Re: MW modulator finished
That looks like a reasonable circuit. The crystal oscillator is used to almost switch a modulated current source into the output tank, or to bypass the tank. Linearity ought to be reasonable.
Charles Wenzel got going in the 1980s and submitted a very smart frequency multiplier circuit to RF Design magazine's annual design contest. It won first prize, an HP 8590 spectru analyser. He did a couple of other entries in later years and started his own firm doing ultra-low phase noise crystal oscillators. He knows his stuff. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
26th Apr 2015, 5:49 pm | #6 |
Rest in Peace
Join Date: Jul 2011
Location: Bridgnorth, Shropshire, UK.
Posts: 787
|
Re: MW modulator finished
Hello David,
Yes, Charles knows his stuff. It was linearity that guided my choice of his circuit. I've added an opamp which senses the tail current to further improve this. It seems to have paid off. I know he's gone all out for Part 15 in terms of power and range, but I'm using reduced current and I intend to use the absolute minimum of external wire for delivery to my radio. Charles' circuit includes a damping resistor across the output tuned circuit. As well as its intended function of maintaining adequate audio bandwidth, it also ensures that there is no saturation or clipping at the RF output under no load conditions. In other words, you don't have to use Charles' recommended aerial configuration. I think we can get too worked up about radiated power of these little circuits: a couple of feet of wire looks like a few milli Ohms of radiation resistance behind a 5pF capacitor. You have to work quite hard to deliver more than a few pico Watts into that! |
26th Apr 2015, 6:22 pm | #7 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
|
Re: MW modulator finished
Hi Karen,
Yes enclosing the long-tail transistor in an opamp loop ought to help. My day job is designing radio and radar gear for aircraft, so I'm one of the now-dwindled number of people who have to design AM transmitters professionally. The current performance requirements are so strict on harmonics of modulation widening transmissions that we have to use carefully linearised detectors to close a loop around the whole transmitter, including the T/R switch. The aim isn't hifi for the intended recipient, but lack of interference for users of nearby channels. Cheers David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
26th Apr 2015, 6:39 pm | #8 |
Rest in Peace
Join Date: Jul 2011
Location: Bridgnorth, Shropshire, UK.
Posts: 787
|
Re: MW modulator finished
Hi David,
I'm a reformed DSP/microcontroller person. Now that I have the time I'm doing all the analogue/linear/RF things my career wouldn't permit me to do. I have always admired analogue ingenuity of the past and often wished I had been born sooner so that I could have experienced the analogue era. But I know that would have thrown up other obstacles - lack of free university education for a start. Best, Karen |
26th Apr 2015, 8:06 pm | #9 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
|
Re: MW modulator finished
I think I just hit the timing right Pure jam, of course. I've got loads of RF/analogue stuff under my belt and I also got to do a very serious receiver structure to the point of a special RF front end and then doing my own DSP in FPGA, so being able to play on both sides of the fence has its attractions. Did a BSc on a grant, then HP funded the MSc so that worked out OK as well. More jam!
The receiver structure is the Agilent noise figure analyser, which replaced the good old HP8970A. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |