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Old 18th Oct 2018, 2:34 pm   #41
ThePillenwerfer
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Default Re: FM to AM converter - variation on a theme!

I don't know if you intend to fit them and simply haven't put them on the diagram but a 10kΩ resistor between R (ie the right audio output) and R5 and L and R5 would be a good idea.

Simply connecting both channels together often works — I did it for years before I knew better — but things can sound odd and a few devices point-blank won't work like that.
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Old 18th Oct 2018, 2:38 pm   #42
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Default Re: FM to AM converter - variation on a theme!

Yes, the junction of R4/L2 de-coupled, the other de-coupling is up to you, personally I wouldn't bother.

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Old 18th Oct 2018, 7:01 pm   #43
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Default Re: FM to AM converter - variation on a theme!

Thanks Lawrence.

The fewer components the better, consistent with good design, but if I omitted the two caps on the input to R4, (or rather reposition them to the downstream side of R4 at the Junction of R4/L2, instead of adding the two caps I’ve called C9/C10), wouldn't that then be as Joe suggested in post #38 and as I depicted in post #39 by deleting one section of track and adding another?

@ Joe: Thanks for mentioning the resistors for the combined L&R inputs (as per the Mini Mod). Good suggestion - I don't think Derek did it, but it's no hassle. I won't complicate the PCB by trying to add them there - I'll fit them into the centre terminal of the 3 terminal socket and connect the flying leads from the FM module. I don't want to make the PCB any longer than 9.4cms as I want the fixing holes to line up with the module.

I have in mind to make a small bespoke wooden 'box' with rounded ends in which to house the converter, something like that in the sketch below.
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Old 18th Oct 2018, 7:45 pm   #44
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Default Re: FM to AM converter - variation on a theme!

Yes David, please see Post#26 and Post#27, my recommendation/answer circuit wise is in there for all to see.

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Old 30th Nov 2018, 6:30 pm   #45
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Default Re: FM to AM converter - variation on a theme!

Just to update this thread before it goes into hibernation, I tweaked the earlier layout slightly, have built an AM modulator PCB and have obtained one of the little Chinese FM modules of the type that Derek - 'Coopzone' used. The modulator works fine, though I had to change C4 & C5 to 220pF to lower the frequency slightly away from an interfering station. The modulator comes out on 950 kHz (315 Metres).

One slight peculiarity is that the sine wave output has a slight kink in it (crossover distortion?).

We're not aiming for hi-fi 'audiophoolery' perfection with something as basic as this, but I'd be interested in any thoughts that anyone might have as to the possible cause.

Pic 1 shows the modulator circuit, update 19 Nov.
Pic 2 shows the updated layout.
Pic 3 shows the built modulator PCB.
Pic 4 is the positive and negative artwork for the PCB if it's of any use to anyone.
Pic 5 shows the slight distortion of the sine wave output of the modulator board.

The little digital FM unit comes with no instruction as to how to use it, and some may argue that it's so simple that it ought not to require instructions. When I press 'scan', it brings up 'P1' scans, and when it finds a station, I assumed it would stop on that station, and maybe you then have to hold the 'scan' button down to store the station as programme #1. However, in a second or two, it simply moves to the next available station, then in another second or two, the next, and the next...so I've yet to discover how to store stations for each programme position. (I think it goers up to 30 programme positions, but of course on most channels there's no station - just 'white noise').

I assume that when I've worked out how to store stations, they can be selected on the remote using the the numerical buttons.

When I've got it working, I'll have to decide how to house it. (Surely not yet another comb-jointed box!).

Incidentally, the FM module has what appears to be a rudimentary antenna printed on the PCB.
There's also a wire to which to connect and external antenna. I guess a 40cm length of wire as with the original converter would be adequate.

Hope that's of interest.
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Old 30th Nov 2018, 7:43 pm   #46
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Default Re: FM to AM converter - variation on a theme!

Quote:
Originally Posted by David G4EBT View Post
Incidentally, the FM module has what appears to be a rudimentary antenna printed on the PCB.
That'll be for the Bluetooth.
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Old 30th Nov 2018, 8:03 pm   #47
David G4EBT
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Default Re: FM to AM converter - variation on a theme!

Thanks for reading the thread Joe and for your comment.

Of course - the printed antenna is so small that it must be for a much higher frequency than the FM band.

The double-sided PCB populated with SMDs is a joy to behold - yet another another example of cheap as chips ('scuse the pun!) Chinese electronics.
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Old 30th Nov 2018, 8:59 pm   #48
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Default Re: FM to AM converter - variation on a theme!

Where are you sampling the sine wave, David? Is this the unmodulated RF output from the modulator? I don’t think it’s crossover distortion, which normally appears symmetrically on both rising and falling edges of the waveform. It also looks as if the slope of the rising edge is less steep than the falling edge, signifying considerable distortion.
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Old 30th Nov 2018, 9:28 pm   #49
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Default Re: FM to AM converter - variation on a theme!

Thanks for reading the thread and for your comments Phil.

Yes, it's the unmodulated output at the aerial connection terminal Phil.

I don't think it will be any detriment - just curious as to what might be causing it.
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Old 30th Nov 2018, 11:52 pm   #50
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Default Re: FM to AM converter - variation on a theme!

I can’t offer many suggestions David, other to say that when I built both my MiniMod AM modulators the RF waveform was a visibly purer sine wave. On yours, there’s some asymmetry, plus that little ‘kink’ on the negative-going half-cycle. Perhaps that’s the point on the transistor’s characteristic where it turns ‘on’ or ‘off’ or starts to saturate? I’ve not seen anything like that before.

What it will mean, of course, is that with no filtering the output will contain more harmonic energy than you might wish for. That vaguely sawtooth-like waveform means there will be both even and odd harmonics, which could extend well up into the spectrum. Have you tried listening on the second, third and higher harmonic frequencies? They should be well down on the fundamental. If they’re only weak you should be OK, but if they’re strong you might think about adding a simple low-pass filter. It might be easier to try adjusting the bias on the transistor whilst monitoring the waveform, to see if you can improve it.
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Old 1st Dec 2018, 9:17 am   #51
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Default Re: FM to AM converter - variation on a theme!

Thanks for your comments Phil.

I was checking the frequency on my counter, and only decided out of curiosity to check the waveform - I'm beginning to wish that I hadn't now!

Basically it's the same circuit as the modulator part of the BVWS FM/AM converter. Simplest thing to do will be to change the transistor, more in hope than expectation.

Yes, the Mini Mods that I've built have had a clean unmodulated output as shown in the pic below.

When I work out how to use the FM module I'll report back on how it sounds.
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Old 1st Dec 2018, 9:53 am   #52
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Default Re: FM to AM converter - variation on a theme!

Hi David,

The more I look at the circuit, I think the Q of the tuned circuit isn’t high enough. The 100pF main tuning capacitor should be more like 500pF to meet the old ‘1.5pF mer metre of wavelength’ rule of thumb, and those small encapsulated chokes have a lower Q than the older more traditional larger components would have. This is pointing to losses in the tank circuit which could be causing the problem with the wave shape. I agree, it should look more like that from the MiniMod in your last screenshot.

I can’t remember the component values for the MiniMod tank circuit but it would be worthwhile looking them up.
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Old 1st Dec 2018, 11:17 am   #53
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Default Re: FM to AM converter - variation on a theme!

Thanks again Phil for your helpful comments - I'll change the 100pF cap (C6) and the transistor and see what that does.

(Of course, I could just as well have fed the output of the FM module into the Mini-Mod!)

Ho hum.
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Old 1st Dec 2018, 11:51 am   #54
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Default Re: FM to AM converter - variation on a theme!

Just an observation, so far as I can make out the output from the minimod is via a tuned circuit.

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Old 1st Dec 2018, 2:10 pm   #55
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Default Re: FM to AM converter - variation on a theme!

Aagh! Don’t change the cap without changing the 1mH inductor, or you’ll be broadcasting down on Long Wave...!
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Old 2nd Dec 2018, 1:15 am   #56
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Default Re: FM to AM converter - variation on a theme!

David, I'm very sorry, I was talking rubbish - I dashed off a reply not having properly studied the circuit. Of course it's L1 and C5 that form the tuned circuit, not L2/C6. I still reckon C5 needs to be about 500pF though, which would improve the LC ratio by making L1 something like 50uH.

I'll be interested to hear what happens with a different transistor.
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Old 4th Dec 2018, 11:22 am   #57
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Default Re: FM to AM converter - variation on a theme!

The design concept behind both the Mini-Mod and the FM/AM Converter was to attain an acceptable level of performance whilst minimising complexity.

In that regard, both projects tick all the boxes, have been popular, and deservedly so. However, the mini-mod is less of a compromise, but has a higher component count - 8 resistors, 13 caps, two tuned circuits using adjustable inductors, an IC and two transistors. The modulator section of the FM/AM converter on the other hand, uses only 3 resistors, 7 capacitors, 1 trimmer and one transistor, and two fixed 'resistor style' chokes, originally built on a piece of Veroboard 1" wide x 3.5" long. Despite that, it performs remarkably well and in using the modulator part of the FM/AM converter, I've tried to retain the same concept - 'keep it simple'.

With that in mind, maybe I shouldn't have gone poking around with my 'scope! In thinking more about this, it dawned on me that the original FM/AM converter used only a 3V supply, and by feeding this 'clone' from a 9V 'wallwart' to serve the digital FM module, with R4 to drop the Voltage to the modulator, there was still 7V after R4, and I'd wondered if that was perhaps too high, and causing the kink in the negative going side of the sine wave.

Firstly, as to the supply Voltage, the Digital FM Module has an on-board 5V regulator so will be happy with a 7V supply rather than 9V. Hence, I fed it with 7V, temporarily substituted R4 for a preset pot which I adjusted to drop the Voltage to 3V This indicated that R4 needed to be 2K7, so I fitted that. I also changed the transistor just in case that had a bearing on the waveform 'kink', and on checking the unmodulated output of the modulator, though the kink is still present, I think it's less so.

Below are the 'before' and 'after' traces.

When I've fathomed out how to get the FM module to scan and store stations, I'll give it a go and report back any progress.

Thanks for the interest, advice and assistance.
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Old 4th Dec 2018, 9:53 pm   #58
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Default Re: FM to AM converter - variation on a theme!

Such a low component count is bound to come with compromises! Noting what changes you’ve made, I wonder if feeding the modulator from a high impedance supply (i.e. through the 2.7k dropper resistor) may be contributing to the kink? I’m guessing that the kink represents the point where the transistor turns on, and in Class C there will be a short but significant pulse of current drawn from the supply.

As an experiment, would it be worth trying powering the modulator section from its own separate DC supply, say two AA cells in series, as the original probably did?
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Old 18th Dec 2018, 10:39 am   #59
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Default Re: FM to AM converter - variation on a theme!

I was amazed at the simplicity of this so decided to build it and I too have the little kink in the wave, I used a BC639, the first transistor I had to hand. I am amazed it works at all when you look at the breadboard birds nest
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Old 18th Dec 2018, 1:04 pm   #60
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Default Re: FM to AM converter - variation on a theme!

Quote:
Originally Posted by Phil G4SPZ View Post
Such a low component count is bound to come with compromises! Noting what changes you’ve made, I wonder if feeding the modulator from a high impedance supply (i.e. through the 2.7k dropper resistor) may be contributing to the kink? I’m guessing that the kink represents the point where the transistor turns on, and in Class C there will be a short but significant pulse of current drawn from the supply.

As an experiment, would it be worth trying powering the modulator section from its own separate DC supply, say two AA cells in series, as the original probably did?
I'd overlooked responding to your post Phil, so apologies for that.

Just to clarify, while under test, the modulator only, without the FM digital module was powered up from 2 AA Cells as per the original (but downstream of R4, which is there to reduce the voltage to 3V when powered from a 'wall wart'). Hence, the scope traces in my previous post are based on that.

I've been sidetracked on other projects, (a home-brew guitar pick-up coil winder of all things!), so haven't yet got around to trying the FM module with the modulator board. When I initially tried it I could get no sense out of it, which was rather disappointing, given that on the original FM/AM converter using the RDA5807A chip and the Guy Fernando PIC, I get ten stations at good strength - same as on a normal FM radio at this location.

Well done to Nigelr on getting the modulator to work and discovering that the kink is still there. (I never thought to check the unmodulated output of the original design to see if it was present with that). Have you connected a digital FM module to the modulator Nigel, or at this stage, just built and tested the modulator?
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