Noise floor on AM band Vs. micropower AM modulator?

[Al (astral highway)]

Hi folks,

I am planning my own design of a few tens of milliwatt output modulator for use with some of my vintage mains sets on medium wave, probably around 1.8Mhz as the dials go up to 2Mhz (on quite a few older sets ) in some cases and there is a little lower noise floor up there.

However, noise floor there certainly is - from walwarts, SMPS, the usual suspects. It is at a significant level. It shows as 3-4 out of 5 on the signal strength meter of a high-quality 1980s tuner, for example, pretty much across the whole AM band.

So my question is, what range is my modulator going to need to operate to overcome this noise floor (roughly) or what are others' experience of getting their modulator to obliterate this noise floor for satisfactory reception. Thank you.

Secondly, can anyone post a simple RF buffer with a gain of around unity please?

Secondly, can anyone post a simple RF buffer with a gain of around unity please?

Modern OP-AMPs will easily do the job.

...of getting their modulator to obliterate this noise floor for satisfactory reception

Yes, SMPS noise is a big problem, I feed my SSTRAN into my aerial distribution system https://www.vintage-radio.net/forum/...ad.php?t=73477 , the main aerial is a Welbrook loop at the end of the garden, little noise there. The SSTRAN does work quite well in the kitchen (where it and it's aerial are), outside this SMPS noise dominates, and that is with 100mW of power available.

[Al (astral highway)]

Ahah, great idea to use an op amp and I like your ingenious solution to SMPS noise...I will see how I can adapt this, cheers!

[Radio Wrangler]

The antenna on your pantry machine is going to be far short of resonance so its efficiency will be very poor, and it'll be a lousy match, so MM won't have been radiating much of that 100mW capability.

There are loads of video amplifiers wih plenty of output, there are driver devices for ADSL lines which will do the job too. MC1496 with a bit of DC offset as a full carrier modulator, or maybe an NE602

Lateral thinking..... Modify an SMPS wall wart to radiate more effectively and more stably, instead of a DC output lead, it has an audio input lead which modulates it. Very stealthy and a homage to WWII spy sets.

David

[Al (astral highway)]

Hi David, posting on the hoof so I can't use the edit quote function ---but thank you , these are brilliant �� ideas! I particularly like the challenge of working with the system by adapting a native SMPS! That is really worth pursuing -- have you tried it yet? Also I hadn't thought of the video amplifier route. Thank you ...

I will build a pantry job anyway as I have the parts including a nice crystal and I'm curious and it's an each way to get back into things after my op., certainly by two or three weeks' time . I was thinking of also fitting a 1.8Mhz bandpass filter and loop antenna to the receiver (comment? ) to improve things as it will be set to this frequency and also to use a tuned aerial on the modulator to address your VSWR point...

[Radio Wrangler]

I've used the Analog Devices AD811 quite a lot as a precision amplifier at up to 21.4 MHz, and they'll output quite a potent signal.

No I've not tried the wall wart idea, but it's just so seductively subversive. Once the idea struck, I couldn't just sit on it.

David

[G6Tanuki]

Alternatively, look at the ways old "Wireless intercoms" and baby-alarms used to couple their signal into the mains. Quite a lot of these used a carrier in the 80-150KHz range - push it a bit higher and you could tune it on the bottom end of Long Wave.

[julie_m]

A transistor with a resistor in the collector will draw a current which varies with the signal applied to its base, and should modulate the SMPS switching frequency and its harmonics. If you're lucky, one or more will fall within your set's coverage .....

[kalee20]

That'll give you FM, though. It's AM which is wanted!

[Radio Wrangler]

Slope demod? But the distortion will be horrible.

David

[julie_m]

It depends on the frequency stability of the SMPS oscillator. If it was highly stable irrespective of the load on the secondary side, the modulation would be AM (as more or less energy that might otherwise be radiated gets swallowed up by the load in sympathy with the modulating signal). If the frequency is easily pulled, then the modulation will be some combination of AM and FM as the modulating signal will interfere with both the frequency and strength.

I have my doubts about those things being designed for frequency stability, though .....

[G8HQP Dave]

You said

and should modulate the SMPS switching frequency and its harmonics

That is FM.

Maybe you meant to say that it will modulate the signal at the SMPS switching frequency? That would be AM.

[Al (astral highway)]

Anyone with a spectrum analyser up for breaking down the main frequencies in SPMS hash, please ? Hello

[G6Tanuki]

It'll vary between different manufacturers/designs of SMPS. So trying to spot what is the overall 'signature' of SMPS hash will be about as sensible as trying to correlate speed or number-of-occupants with the colour of cars on a road.

[Al (astral highway)]

Originally Posted by G6Tanuki

... trying to spot what is the overall 'signature' of SMPS hash will be about as sensible as trying to correlate speed or number-of-occupants with the colour of cars on a road.

Pink cars go slower on motorways; the police need them for speed-ticket 'snooker' and their rarity makes them vulnerable.

Your analogy, which implicitly ridicules my proposition (I'm not in the least offended, just saying did you realise how it comes across as a bit condescending, intentionally or not?) doesn't seem very easy to map back to my question . You may know a lot more about what 'cars' (frequencies?) are on this road (band) than others do. So I have no idea if we are looking at say 3,5th harmonics of a 60 KHz typical fundamental or 100kHz typical or whatever. I do know that Panasonic used to use around 15kHz in their microwave oven inverters. You probably do have a better idea and it would help to indicate the range of typical fundamental frequencies in SMPS! Thank you

[russell_w_b]

I've come across 61-odd kHz: ELV dichroic luminaire PSU and 46kHz: fluorescent lamp electronic control gear. I haven't looked in to wall-warty stuff.

I've not had an analyser on any of them, just a 'scope, and it's evident that controlled bursts of LF are used to regulate the load, so it won't be a continuous 'shash' you'll see. The attached link shows a quick o-scope measurement I did on the dichroic luminaire transformer. Perhaps more investigation is needed as they play hell all the way up the LF and MF band.

http://www.vintage-radio.net/forum/s...1&postcount=42

[Al (astral highway)]

Ahah! That's it! The fundamental you show is 4V peak to peak. So even the 25th harmonic will be around 0.15V at source which is huge. I can imagine this propogating way out and jamming microvolt (with good aerial , ATU) an millivolt radio signals on medium wave, which is my bugbear...

Thanks, Russell , very illuminating 👍

[russell_w_b]

Originally Posted by astral highway

Ahah! That's it! The fundamental you show is 4V peak to peak.

It's 40V. I forgot to say - I was using a x10 probe! It's actually looking across the output. This SMPSU wasn't rectified and is meant to drive a 12V lamp. I'll need to get hold of a 50W lamp and observe how the W/F changes with the increased load.

[Al (astral highway)]

Originally Posted by russell_w_b

It's 40V. I forgot to say - I was using a x10 probe! It's actually looking across the output. ...

Woah! That's some signal to contend with! No wonder AM is swamped!
Please post your dummy load results, that will be very telling!

Thanks!👍