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Old 15th Mar 2008, 4:42 pm   #81
kalee20
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Default Re: AM Micro Transmitter

Hi Dom and Mike,

First, if you are using a short length (and at 750kHz this would be less than 50 metres), you can forget about it being a transmission line, and think of it as being a capacitance. If it is 'lossy' coax, then it will be a capacitor with a resistor in series. You won't need to worry about matching to it - I'm assuming you haven't yet wires it around your house, Dom.

Coax cable is lossy due to the series resistance of the conductors, and the leakage due to the insulation not being perfect. So, power is lost as heat. If the ratio of these two loss resistances is correct (and the value of the ratio is related to the value of capacitance per metre and inductance per metre), the cable will have a constant impedance with frequency - even though what you get out is less than you put in. And, if you have an infinitely long length, you can measure this impedance with an AVO at the available end, no matter what the remote end is conected to!

When people talk about 'leaky feeders' they mean that there's an additional loss due to the cable radiating some of its signal. I doubt whether any sensible-sized coax would radiate anything at 750kHz, but in post 68 above, Dom's already got the idea of a scheme to ensure a bit of local radiation in each of his rooms.

Now, as to practice, Dom I'd certainly remove your dummy load when it's time to try radiating. You need your wire aerial from the centre of the coax, and a good earth (or a similar length wire) from the coax outer. And, you'll need a lot more voltage! The turns ratio down from the tank is only to match down to your coax (in anticipation of your full-house-length). At the remote end, you need to transform back up again, exactly as you've thought in your post 68.

Mike, I'm not convinced that matching for maximum power transfer from the tank circuit is the way to go. I'd certainly agree with you that the higher the Q of the loaded tank, the more power is lost in the tank itself due to coil losses. But, in principle, you can get around that by making a better coil (which will increase the Q of the loaded circuit), then increasing the coupling to the aerial (which will bring Q back down again). And now, you've got more power in the aerial! If you match for maximum power transfer with a given tank, then without doing any sums I'd predict that for maximum power transfer you'd have half the power dissipated in the tank, and half in the aerial. Not the best for efficiency. (After all, very few audio amplifiers are matched for maximum power transfer - most are either matched with a lower resistance, such as pentode output, or with a higher resistance, such as triodes or emitter-followers.)

The reason that maximum power transfer is often a red herring, is that there is the extra degree of freedom in that the drive is (in principle) adjustable. If you slug the output too much, you just increase the drive. And the limiting factors are distortion-causing mechanisms such as bottoming the valve (or even letting the anode swing negative, as Dom's reported).

Both of you - you are challanging my thoughts and I'm really appreciative!
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Old 19th Mar 2008, 12:09 pm   #82
dominicbeesley
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Default Re: AM Micro Transmitter

Hello all,

attached is the latest schematic, I've added an EM87 magic eye for show! It's a bit underrun and dim at 115V but works ok. Just have to work out how to power its filament, might have to add an extra winding on the mains Tx

I'd now like to raise the cathode and grid leak about 20V above ground, I was going to use a simple resistor / capacitor arrangement but this is now no good as tuning the tank circuit has a big effect on the standing current.

So my other options are:
- rectify the heaters (~50V rms) and use some of that as a bias voltage.
- zener diodes I've got plenty of these!
- A string of LEDs
- A Potential divider from the HT line suitably decoupled
- any other ideas?

I'm guessing a PD would have to pass a lot of current (and dissipate a lot of heat!)

If using zeners is it the peak current or the standing current that matters when chosing parts? They'd be bypassed by a big capacitor I've got plenty that'll stand the 2-15mA standing current but the peak current is pretty high!

Dom
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Old 19th Mar 2008, 8:25 pm   #83
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Default Re: AM Micro Transmitter

Hi Dom,

Gosh, it's growing isn't it??!

As the standing current varies, a resistor / capacitor combination is no good as the resistor value will need to be altered too often while you adjust other things.

A potential divider will probably waste too much power.

The Zener diode idea is best (apart from more PN junctions infecting the circuit). I'd rate the zeners for peak current, then you know that whatever you do during development, you won't fry them. As an alternative to having the diode raise the cathode/grid-leak voltage of the final RF amplifier, you could instead connect the diode between the cathode of the cathode follower and the RF choke, thus dropping 20V from the modulator output.
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Old 20th Mar 2008, 12:22 pm   #84
dominicbeesley
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Default Re: AM Micro Transmitter

I tried some zeners last night, first at 22V which should have given 100% modulation but didn't quite then at 31V which still didn't! I think probably as the cathode follower gets close to bottoming it stops trying as hard, I'll try a smaller load resistor tonight!

The results are pretty good though I'm getting about 95% modulation but it's pretty ropey sounding by that point (more due to overdriving the AF stages than anything else).

The attached pictures show the trapezium figures at 95% modulation and about 80% modulation driven by a 1kHz sine wave, plus the respective display on the magic eye! There's also a video here: http://www.youtube.com/watch?v=GoQafz2Q8_A of the thing playing a #1 hit from 1970, bonus points if you can name that tune from the silent video!

Next steps:

- devise a switchable circuit to show PA standing current on magic eye for tank tuning as well as modulation
- derive overall AF feedback from demodulating 75R output terminals
- find 6.3V for the EM87 from somewhere!
- somehow get this all in the box!
- fit tank coil to the box
- Design step up coils for each floor.

I briefly tried winding some step-up coils last night but with little success. Any ideas where to start appreciated.

Dom
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Old 20th Mar 2008, 2:11 pm   #85
kalee20
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Hi Dom,

Well, the modulation looks good and linear, even if you're not up to 100% yet.

One thing that bothers me, is the Negative Feed Back loop from the detector diodes goes to the cathode follower cathode, which is a low impedance point (I'm assuming that the circuit diagram in post 82 is correct here). So, the feedback to the cathode of the first triode will be dominated by direct AF feedback from the cathode folloer, with the detected AF having negligible effect. That said, the linearity being good is due to you having it inherently right, rather than being 'corrected' by the NFB.

As for naming that tune, sorry, no! (though I did hear one of a man who could name which Beethoven symphony was on a record just by looking at the grooves, with the label covered up)

Thoughts on step-up coils will follow sometime...
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Old 20th Mar 2008, 2:27 pm   #86
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Default Re: AM Micro Transmitter

Hi Kalee,

Sorry if that bit of the circuit isn't very clear!

The feedback currently goes from the cathode follower to the unbypassed cathode bias resistor of the first AF stage. The diodes clamp and rectify the AF signal into a PPM consisting the magic eye valve.

The feedback circuit is very much a temporary measure, so's I can listen to some nice music while I work! In the final circuit it will go from the coax output through a rectifier, low pass filter and back to the grid of V1. The cathode follower through to magic eye with diodes etc will remain for the simple PPM.

Dom
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Old 5th Apr 2008, 11:47 pm   #87
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Default Re: AM Micro Transmitter

Hello all,

Well, I've finished it off, it now has a circuit pretty much like the one above.

I gave up on the idea of getting the feedback from the 75R RF output as it was really difficult to sort out a dominant pole and keep the audio bandwidth as wide as I wanted.

The LC filter to keep the RF away from the cathode needed to be realtively low (about 100kHz) to keep the circuit working well. This made for a large phase shift and caused instability with any large level of NFB. With the NFB as it is now I can pile on more or less as much as I like, the preamp I've got the setup connected to can produce about 5V pp!

Also having the NFB go to the point it does has the added benefit of lowering the output imepedance of the cathode follower.

It now sounds great and I've just put my back out pulling up carpets to plumb the coax round the house from my computer room to my bedroom and basement workshop.

I'd welcome any more ideas on how best to make aerials before I do some experimenting next week? At the moment I've got a few feet of wire hanging out of the coax sockets but it could do with a bit of stepping up to give the signal a fighting chance against all the switch mode PSUs and ADSL from the phone lines round the house!

I've put a video up on youtube but it hasn't come out very well http://www.youtube.com/watch?v=EgH2w1tuhBo

Thanks for everyone's help, especially MichaelR and Kallee20

Now to try tweaking the IF on one of my 50's sets to give a wider bandwidth...now, then bed!

Cheers

Dom
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Old 6th Apr 2008, 11:33 am   #88
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Default Re: AM Micro Transmitter

This has been a great project to follow Dominic you have good reason to be proud of your "baby", it sounds great.

Mike
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Old 6th Apr 2008, 8:49 pm   #89
kalee20
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Default Re: AM Micro Transmitter

I'll second MichaelR's sentiments. You have a working system, having got there by analysis and on-the-job development. Much more 'real' than Spice simulations!

Peter
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