Let's take another look at AM

[WEBB-TECH]

Hi Webb-Tech

Mustn't grumble! Getting old, but still enjoying life!

Cheers!

I know the feeling mate. As long as you are well I'm happy

[stretchyman]

Hi there Peeps, OK Managed to find this forum via some vague word search and cant actually find the thread I wanted to post in (M.W. was mentioned?). Anyways this thread looks appropriate.

OK, I sell an A.M. TX on FleaBay and altho' primarily aimed at U.S. 6925KHz pirates it will work on ANY frequency upto 10MHz.

As per most radio related activities I see lots of misinformation and over complication.

The circuit I sell is simple and efficient, Class E with a Class D modulator (+Mod Tranny) and sell a PWM circuit if you can use 24V or more.

20W carrier (100W pep) and the RF circuit is no bigger than your thumb and I sell a kit (cheap) or built and tested (more £). Yes it's Xtal controlled which keeps it simple and Yes (as mentioned on here) Xtals are made by 'Quartslab', don't bother with anyone else.

I can build (design) anything but not interested in VHF FM only SW & MW as they're not watched at all.

Let me know if you're interested or need any more info, I'm happy to give a decent cost reduction if ordered direct and via this forum or HFU.

ALL the Best Buds

Stretchy.

Oh it's eBay Item 131595757554.

[Albert H]

eBay Item 131595757554.

Excellent! I presume that you give the details of the turns required and the capacitor changes needed in the output filter for various frequencies, too?

Is your crystal oscillator generating a sinewave or is it square and modified later? I ask because if it's square, it's trivial to replace the rock with a simple CMOS synthesiser and make the thing more frequency-agile whilst using standard, cheap, off-the-shelf crystals. This would be cheaper (and more convenient) than getting a specially cut crystal.

If it's a sinewave, that's a bit trickier (but not much) - there are then two approaches - a digitally generated pseudo-sinewave or a phase-locked Colpitts, Hartley or Vackar oscillator.

Also - if you want absolute frequency precision - you can phase-lock to WWV at 10MHz or BBC Radio 4 long wave (198.000000000000 kHz), which is derived from the caesium clock at the National Physical Laboratory. I use a 198 kHz reference in my latest medium wave synthesisers!

Incidentally, if you did a ready-to run, boxed SW unit, complete with a basic audio processor built-in and an aerial matcher, you'd be on to a real winner in the USA!

[stretchyman]

Hi Al, thanks for the prompt response!

Use to posting on HFU and having to wait a day (time diff in U.S.) for a reply!

OK, Yes sure I have Cap and Turns details for any freq.

Xtals produce a sinewave but then the driver produces a squarewave, so either is fine.

I actually produce a 64CH PIC driven DDS PCB but have never sold any so have saved them for a much higher power (200W carrier) design I'm working on.

I'm not interested in absolute frequency precision, Xtal is fine.

I've sold close to a 100 in the U.S. already and I'm not interested in selling a boxed unit as it would quadruple the cost, far too much work, so sell a kit or a built and tested system.

May change my mind on this one as some folk have far to much money and absolutely no idea (which is fine) but I'm TRYING to inform and educate as best I can and hopefully keep the hobby alive.

Have a PWM PCB only an inch square and will mod 100W carrier to 400W pep but as no-one understands PWM so have sold less than 5! has Audio BPF and optional compressor too SSM 2167.

BR

Stretchy.

[Albert H]

One AM box I built some while ago used a SMPSU IC driving a FET as the pulse width modulator. The SMPSU circuit would run very nicely at the carrier frequency! With a PIC, the SMPSU IC, two FETs and a reference rock, it was probably the simplest 10W carrier / 50W peak rig I've ever seen!

Do you know the "three chip synthesiser" trick? A 4060 as the reference oscillator and divider to 9 (or 10) kHz, a 4046 as the phase comparator and VCO, and a 40103 binary down-counter which can be programmed with an 8-way DIP switch. The whole thing can be tiny (if you use SM parts, it can be VERY tiny!) and is completely frequency agile. Obviously you get squarewaves out of the VCO, but your driver won't mind that!

I've also been working on an "ampliphase" set-up, using two 4046 PLLs oscillating 135° apart with no mod, and then leading and lagging (in opposite directions) as the audio is applied. I used two (nominally) 20W Class D PAs and a bridging circuit with capacitors to ground and inductors to the antenna. With a bit of envelope feedback, it sounds amazing, and runs completely cold!

[stretchyman]

Hi Al, Yes your 'simple' design sounds interesting!

Yep, seen every design out there and the 3 chip synth is somewhat outdone by the $5 AD9850 boards from China. I sell a PCB with pre prog'd PIC to talk to these, 64 CH's.

The Ampliphase thinghy sounds cool and that's the 'modern' way to do A.M. but in software.

Hoping that folk will start to use M.W. more as it's wide open now, needs some ingenuity antenna wise tho'!

Str.

[Albert H]

The best improvised aerial I've ever used for medium wave is the "Tower Block Sloper". I found that it was possible to get a 25Ω aerial match without much effort, so for testing loaded the rig into a paralleled pair of 50Ω loads, and made it match 25Ω instead of the usual 50Ω. This had the advantage that the output impedance conversion wasn't too far (12Ω up to 25Ω as I recall) which kept the efficiency high.

[Premier-Carousel]

Hi Albert,

I'd certainly be interested to see your AM design

Regards
Stuie

[Albert H]

OK Stuie. I'll put some circuits up here for people to play with. AM isn't as layout-critical as FM, and the parts can be much cheaper!

[hoffy]

Hi Albert, nice to see you back here. I'd be interested to see the am stuff too.
Cheers
A