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Old 7th Mar 2008, 6:51 pm   #61
dominicbeesley
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Default Re: AM Micro Transmitter

Hello all,

I've taken some pics to show progress so far. Its now back to using pretty much the same circuit before except that I haven't wound a proper aerial coil with the feedback circuit on it yet so the feedback is coming from the output of the cathode follower.

Attached are some pictures of the chassis I've made up, trapezium traces to follow...
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Old 7th Mar 2008, 7:08 pm   #62
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Default Re: AM Micro Transmitter

Here's the trapezium figures:

1) Driven "hard" at 100Hz
2) Driven "hard" at 1kHz
3) Driven 50% at 100Hz
4) Driven 50% at 1kHz
5) Driven 50% at 10kHz

Sorry for the poor quality, my old digital camera is a bit poor and I'm not allowed to touch SWMBO's fancy digital SLR without supervision any more!

As can be seen the AF stages start to clip/distort well before 100% modulation so my idea is to raise the PA valve above ground somehow. I'll try by increasing the quiescent current a bit to about 15mA and taking the grid leak resistor down to a tap on a voltage divider arrangement in the cathode bias resistor.

I hope that if I raise the quiescent current I'll get a better response from the cathode follower though I'm not really sure of the theory here for valves only the emitter follower and transistors...

I'll try and raise the cathode up to about 40V and then I should be able to get 100% modulation, though to be honest it sounds pretty good and loud on all the sets I've tried already!

Still struggling to get the PA tank circuit to tune with a grounded variable cap. At the moment there's another floating about on the bench at 65V I keep forgetting its there and wondering why I get a tingle from it!

As a side note I'm running my whole setup via an isolating transformer for safety's sake and noticed that its radiating some horribly distorted version of the signal (low level) whats the best way to stop this occurring? Do I need caps across the mains or caps from mains to earth?

Dom
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Old 8th Mar 2008, 8:04 pm   #63
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Default Re: AM Micro Transmitter

Hi Dom, thanks for traces!

The first one shows phase shift between the modulating AF and the RF.

The second one shows almost no phase shift, but there's a fair bit of non-linearity because the sloping side isn't straight.

I see with all of these, the RF output (on the right-hand-side of the screen) does not go to zero. I think you're on the right track with raising the power amp cathode up with a 40V pedestal - you'll potentially be able to achieve 100% modulation then, which is satisfying. Of course, bear in mind that many radios will distort with 100% modulation, due to detector shortcomings!

I can't see why the two RF tuned circuits aren't equivalent. I'm assuming that the HT (or modulator output) is bypassed to 0V with a suitable capacitor - I'd suggest 10 times the tuning capacitor - with short leads? If so, then the only difference should be a slight re-tuning of the PA tank, when you change from one configuration to the other.

Great work, all you are doing!
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Old 8th Mar 2008, 10:11 pm   #64
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Default Re: AM Micro Transmitter

I'm really stumped why the tuning circuit isn't working. Both HT and cathode are bypassed to Ground via decent caps!

The slope on the trace is due to the fact that the X-mod input was connected with a scruffy bit of bell wire trailed across the top of the whole setup causing it to pick up a lot of RF, I've bypassed it with a cap at the Oscilloscope end for better pictures. I'll post more tomorrow hopefully.

Now I'm having trouble with it upsetting my broadband after I fitted an extension in the workship yesterday afternoon! This is even at low signal levels (enough to just pick it up on a sensitive set 6' away). I'm going to try running the extension cord a few turns through a ferrite core. Not sure what else to do?

Dom
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Old 9th Mar 2008, 2:42 am   #65
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Default Re: AM Micro Transmitter

Hello all,

After a rebuild (with shorter wires) that circuit now does work! Can't think why it was so sensitive before. I suspect another nasty ground loop

On the ADSL thing, its not just the transmitter that sends it loopy, if I use my sig-gen into a dummy load or even overload the oscilloscope with an RF signal it drops out! Having looked elsewhere this is not uncommon when running a number of phones (4 old GPO ones + long broadband connection). So I'll move the splitter and router to the master socket and wire all the phones from there. Hopefully that will sort it out.

I'm now trying to work out how I caclulate a rough number of turns to put on my PA load coil to give a 75ohm output. But I have to admit I'm stumped.

I'm hoping to have both the Oscillator coil and PA coil the same with about 340uH tuned by a 550pf cap in parallel with a 100pf cap (this gives coverage from about 300kHz to 1200kHz). These are made by winding 120turns of 0.4mm wire onto a 1.6" former (waste pipe).

The reason for the strange frequencies above is so I can inject direct into IF stages when setting up other sets.

Any help in deciding the number of turns to put on the PA coil and whether to have a seperate winding for the feedback or to use the same winding as for the 75R take off?

I've just rewound my osc coil and it no longer oscs - time for sleep I think!

Cheers

Dom
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Old 9th Mar 2008, 7:31 am   #66
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Default Re: AM Micro Transmitter

Dom,

This is where you need to know the anode current and voltage of the "pa" valve - you can then calculate the operating impedance, and from that you can calculate the ratios required for transforming this down to 75r.

Saying that, it is just as easy to use 50r, as the slight mismatch should be tunable with your tank circuit.

Cheers
Sean
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Old 9th Mar 2008, 9:52 am   #67
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Default Re: AM Micro Transmitter

Quote:
Originally Posted by dominicbeesley View Post
Hello all,


One thing I can't work out quite is why the two circuits in the attached pictures don't behave the same at RF. Am I missing something. I want to use the second (rightmost) as the load of the PA valve as I've already got a dual gang variable capacitor, one gang of which I'm using for the oscillator section.

Dom
One ciruit is parallel resonant and the other is series resonant.

With series resonance the current is max and in phase with the impressed voltage and the impedance is a minimum.The voltage across the reactances can be very high , much higher than the supply voltage

With parallel resononce the current is @a minimum as the impedance is a maximum, in this case the circulating current in the tuied circuit can be much higher than the supply current.

Regards
Mike
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Old 9th Mar 2008, 3:40 pm   #68
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Default Re: AM Micro Transmitter

Thanks Sean,

I'm not too bothered about matching it down exactly just something close.

Are these the DC currents/voltages (which at present are 3mA/65V or are we talking AC currents, I've no idea how to measure those using just a scope / AVO 8! should I just stick in a 1R resistor and measure the voltage across it?)

Thanks MichaelR,

I thought the were both parallel tuned and the Thevenin equivalen circuit (at RF) was the same as the attached sketch, though I must admin to feeling my way now. I've now had it working with a grounded cap and it appears to tune to a DC current low.

More questions:

Also attached is a sketch of the layout of my house. I'd like the transmitter to live in the computer room (contains all my records and computers for making the program content!) and run little aerials in each of the highlighted rooms.

How do I best run this? Am I best making a Tee configuration one going up one going down and terminate both ends? In which case should I parallel connect them or series connect them or do I need to make up some complicated transformer. Also How should I best take off power for each room. Ideally I'd like to step it up to a high voltage in each room using a hand wound coil feeding a single ended aerial but how to tap into the feed without losing all the power?

The plan diagram shows the rough idea. Each aerial having something like the last diagram attached.

Would this work? I've no idea. I'd like a strong enough signal in each room. But a strong enough signal to cover the entire house from basement to attic without radiating too far horizontally.

All this started so I could listen to music in the cellar. I'm sure 10m of speaker cable would have been cheaper but not nearly so much fun!

Dom

PS: Once again thanks for everyone's continued interest all comments are making this a very fun project. I've learnt more about electronics this past few weeks than years of formal education!
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Old 9th Mar 2008, 4:20 pm   #69
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Default Re: AM Micro Transmitter

Quote:
Originally Posted by dominicbeesley View Post
Thanks Sean,

I'm not too bothered about matching it down exactly just something close.

Are these the DC currents/voltages (which at present are 3mA/65V or are we talking AC currents, I've no idea how to measure those using just a scope / AVO 8! should I just stick in a 1R resistor and measure the voltage across it?)

Thanks MichaelR,

I thought the were both parallel tuned and the Thevenin equivalen circuit (at RF) was the same as the attached sketch, though I must admin to feeling my way now. I've now had it working with a grounded cap and it appears to tune to a DC current low.

More questions:

Also attached is a sketch of the layout of my house. I'd like the transmitter to live in the computer room (contains all my records and computers for making the program content!) and run little aerials in each of the highlighted rooms.

How do I best run this?
Hi Dom,

Now that you have added the capacitor you essentialy now have a parallel resonant circuit, prior to that your tuning problem I was suggesting was due to the fact that you did not.

At resonance the value of the circulating current in the parallel L/C will be dependent upon the Q of the resonant circuit.

This is Q= R/( sqrt(L/C)).....

For a series tuned resonance - Q= (1/R)(sqrt(L/C))

If you ran a lossy coax cable from the top of the house and had it almost dangling down the centre ( hidden or routed of course within the fabric of the house ...) and adjusted the power output to give you reception in every room ( and obviously not beyond) this would work. The advantage to using lossy feeder is that it absorbs the rf power and you would not have to worry about matching it too much.

Mike
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Old 12th Mar 2008, 8:03 pm   #70
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Default Re: AM Micro Transmitter

I've been thinking about the tank circuit, mode of resonance, etc. Certainly, it is parallel resonant whether forms A or B below are used. To be series resonant, you'd need the topology in C.

Now, how to calculate values! I would attack it as follows: assume the capacitor value is given and can't easily be changed, because Dominic doesn't want to get another variable capacitor. That means that the inductance is then determined by frequency (340uH - he's done that bit). Next, the load, which is 75 ohms, must be matched to the tank, and finally the drive to the valve adjusted so that the anode voltage drops to near zero each RF cycle.

The tank circuit gives a 'flywheel' effect, with only a small part of energy absorbed by the load each cycle. If too much is absorbed, there will be more distortion of the RF sine-wave, radiating harmonics. But, if the flywheel effect is excessive, the RF amplitude won't change in response to modulation. So, assuming the highest AF modulating frequency is 10kHz (more than broadcast AM!), and the RF is 1MHz, the required bandwidth is 20kHz. Then, the Q of the tank can be calculated:

Q = 1MHz / 20kHz = 50.

This is a reasonably standard formula which you'd use to calulate the Q of receiver tuned circuits for a 3db drop at the outer bandwidth limits.

Next, the load must be matched. At 1MHz, the tuned circuit coil and capacitor will have a reactance of 2 x pi x 10^6 x 340 x 10^-6 or 2136 ohms. So, the effective parallel resistance must be Q times this, or 107 kilohms. Assuming that the coil, capacitor, and anode resistance of the valve all offer negligible losses, the load must offer this resistance to the tuned circuit.

Since the load is 75 ohms (Dominic's choice), it is 1,424 times too low and must therefore be coupled via a turns ratio of square root 1: 1424, or 1:37. And, knowing how many turns are on the anode coil, just divide this by 37 to give the turns required to couple to a 75 ohm load.

These calculations are crude and ignore lots of things (such as less-than-100% coupling to the output coil), but it is where I would start. With a 75 ohm dummy resistive load, it should be possible to optimise the pa stage, leaving the next challenge of developing small room-local radiating aerials...
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Old 12th Mar 2008, 10:35 pm   #71
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Default Re: AM Micro Transmitter

Thanks Peter,

That's given me real food for thought. I really had no idea where to start guessing the number of turns on the secondary.

I've started to do roughly what you've outlined above - with mixed results.

I've wound a secondary on my tank coil of 30 turns (120 turns for primary) and used this to feed the 75R.

As you posit above this lead to worse distortion and a roll off of high frequencies, whilst delivering about 10-15V pp (1/4W) into the 75R load.

I soldered a 75R resitance to the end of a roll of 75ohm crappy B&Q TV coax with hardly hardly any braid. On its own this barely radiates enough to pick up on my most sensitive valve set (Pye P76F) unless the roll of coax is placed on top of the set. I tried putting the "signal" on the outside / inside of the coax with little effect.

However a couple of feet of wire run from the coax along the floor is enough to flood a room to a reasonable level, without noticeably upsetting the tuning of the tank circuit (and falls off quickly enough to not upset the neighbours or the ADSL).

I'll give your ideas a go tonight if I get a chance. 120/37 will give me about 3 1/2 turns so I'm guessing this will give me a lot fewer volts across the resistance but hopefully a better response?

The other trouble I'm having now with the higher inductances in the osciallator and tank coils is that the oscialltor "goes out" if its tuned too far off peak or bang on peak. I've no idea why this is, I've measured round the oscillator and heavily decoupled its power supply it all looks rock solid, the only feedback path that I can see is that the grid of the PA valve is heavily loading the oscillator with a (miller??) capacitance? Sorry this just a guess based on too little knowledge!

Dom
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Old 13th Mar 2008, 12:10 am   #72
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Default Re: AM Micro Transmitter

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Originally Posted by kalee20 View Post
The tank circuit gives a 'flywheel' effect, with only a small part of energy absorbed by the load each cycle. If too much is absorbed, there will be more distortion of the RF sine-wave, radiating harmonics. But, if the flywheel effect is excessive, the RF amplitude won't change in response to modulation. So, assuming the highest AF modulating frequency is 10kHz (more than broadcast AM!), and the RF is 1MHz, the required bandwidth is 20kHz. Then, the Q of the tank can be calculated:

Q = 1MHz / 20kHz = 50.

This is a reasonably standard formula which you'd use to calulate the Q of receiver tuned circuits for a 3db drop at the outer bandwidth limits.

Next, the load must be matched. At 1MHz, the tuned circuit coil and capacitor will have a reactance of 2 x pi x 10^6 x 340 x 10^-6 or 2136 ohms. So, the effective parallel resistance must be Q times this, or 107 kilohms. Assuming that the coil, capacitor, and anode resistance of the valve all offer negligible losses, the load must offer this resistance to the tuned circuit.

...
Hi Pete,

Surely at resonance in a parallel tuned circuit the effective reactance is zero as the inductive and capacitive reactances cancel each other .

Mike
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Old 13th Mar 2008, 11:16 am   #73
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Hi Mike,

In a parallel tuned circuit, at resonance any external circuit sees infinite reactance (not zero, that's a series circuit). So, if there a load resistance also in parallel, at resonance, the external circuit just sees this resistance.

However, if the amplitude tries to rise, the external circuit not only has to drive the load, but it also has to put energy into the tank ('winding up the flywheel'). Another way of looking at it is, a rising amplitude is no longer a steady sine wave, so all arguments assuming resonance conditions are off! And similarly, if the external amplitude tries to fall, then the tank circuit releases some of its energy.

The energy retained in the tank is dependent on the value of the (equal) reactances, and the amplitude of oscillation. And, in an LCR circuit, with all elements in parallel, the resonant frequency is 1 / (2 x pi x SQRT(L x C)), and the Q is R / SQRT (L/C). You'll notice that the term SQRT(L/C) turns out to be the reactance of the capacitor or the coil, at the resonant frequency.

If you have a small amount of resistance in SERIES with anything, as is usually the case when considering coil wire resistance, life is a lot more complicated - but let's not go there yet!
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Old 13th Mar 2008, 11:32 am   #74
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Default Re: AM Micro Transmitter

Hi Pete,

Many thanks for the excellent explanation , it is clear to me now.I must say that I do seem to get a better understanding when things are explained in terms of energy

I think I can see how we could be moving into a tricky area here with the design, ideally this ratio of L/C of the tuned circuit is going to be important.

I presume this series resistance factor is going to be related to the Q when transformed into a shunt resistance.

Resonance is not so straightforward is it ?

Mike
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Old 13th Mar 2008, 1:34 pm   #75
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Default Re: AM Micro Transmitter

Hello,

I continued with the experiments last night, the lower number of turns seemed to reduce power a lot but didn't improve quality. All experiments are carried out at about 750kHz

So I tried using my older lower inductance coils ~100uH. These don't give as much power at the PA valve and now I can tune to resonance again. So I could try out coupling up the coax:

With about 6 turns on a 200 turn coil seems to give higher Q but quite low power about 20 turns on 200 turn coil delivers about 6V pp to the 75R load (60mW), but the Q is lower and tuning the tank has less effect (not a bad thing? higher bandwidth?)

6V p-p is really weedy and even with wire taken off the centre of the coax and draped over the set gives a very week signal with a lot of background noise breaking through. 60mW most of which is being eaten by the terminator isn't enough power?

The smaller coils also give a lot less harmonics but I think that was because with the bigger tank before the PA valve was actually producing a sine wave bigger than 2*HT! swinging about 5V below ground! So this was probably a bit distorted!

[I'd still like to keep the bigger coils to also be able to inject signals at IF frequency for testing sets, I'd got a couple of unrestored deaf ones that I brought to life like this, so now I know its the front-end/mixer that's broken!]

I think if I could get the oscilator to stop playing up with the bigger coils I'd be more or less there. Perhaps some compromise in between.

Still no idea why loading the PA valve puts such a strain on the oscillator?!? There's evidnence of this with the smaller coils but not enough to put the oscialltor out. As I tune the tank coil to resonance the oscillator amplitude goes down.

Would it be worthwhile using my spare triode as a buffer between the oscillator and the PA stage, it's not doing anything at the moment but I was hoping to use it to make a better first AF stage. Or is there something I could do to the oscillator to make it a bit more robust?

I did some calculations, based on Peter's excellent explanation above: not sure how relevant they are but including them for completeness

Code:
L           100
C           500
F           750
Pri Turns   200
Pri React  4712.38898

Sec Turns   Ratio   refl. Imp     Q     Band (kHz)
  3         0.015    333333      70         10
  6         0.03      83333      18         42
 10         0.05      30000       6        118
 20         0.1        7500       1.6      471
 40         0.2        1875       0.4     1884
100         0.5         300       0.06   11780
Dom

Last edited by Kat Manton; 13th Mar 2008 at 1:58 pm. Reason: Fixed table (I hope...)
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Old 13th Mar 2008, 4:51 pm   #76
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Default Re: AM Micro Transmitter

Try using this....

http://wireless.org.uk/g4fgq/pi_tank.exe

I use this a lot, and it works well

Sean
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Old 14th Mar 2008, 12:49 pm   #77
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Default Re: AM Micro Transmitter

I can understand the smaller coils giving lass harmonics - smaller L, bigger C, the LC tank circuit will be 'stiffer' and harder to drive with anything less than a good sine wave.

Can't comment on why the PA is affecting the oscillator. It sounds like there's a feedback mechanism there, from PA anode to grid. Putting a buffer stage in will help to isolate the oscillator, but that sounds like attacking the symptoms, not the disease.

As for output power, I'd have thought that 60mW should produce a really strong local signal - but only if it is radiated. Using 75 ohm coax with a 75 ohm resistor at the far end, you'll get an excellent match but there's no opportunity to radiate any power. It will all end up in the resistor!

To radiate, you need to replace the resistor with a half-wave dipole - which for 750kHz is massive! So, I'd instead replace with a few metres of vertical wire to the centre conductor, taking coax outer to a good earth connection. This will look like a capacitor at 750kHz, so it will need a parallel inductor to tune out the capacitive element. When this is done, there will be a residual resistive element - probably most of it being the resistance of the earth connection and the vertical wire resistance, but there will be a residual resistance which represents power radiated.

The resistive element will be rather larger than 75 ohms, so you now need a coupling transformer to couple to the 75 ohm coax - or use a coupling winding on the above-mentioned inductor.

To calculate the values is more than I have a feel for at present. So, I'd set up the aerial/earth arrangement, and measure the capacitance (the little Peak Atlas LCR40 is a really good instrument costing much less than £100), crossing my fingers and hoping it won't change much at your 750kHz. Then, I'd work out the inductance to tune it, make it, and fit it in parallel.

Then, I'd drive the setup with 750kHz through a series resistor, using a scope to measure voltage. By adjusting the series resistor, you'll reach a point where the amplitude is half the signal generator output. When you have this (which will be valid only for the one frequency), you know that the series resistance is equal to the RF resistance of the radiator circuit, so you'll now be able to work out turns ratio to match to 75 ohm coax. You can at the same time check for zero phase shift between the signal generator output and the voltage across your radiator circuit, to confirm you've got the right inductance.

It's all lots of fun!
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Old 14th Mar 2008, 1:14 pm   #78
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Default Re: AM Micro Transmitter

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Originally Posted by kalee20 View Post
As for output power, I'd have thought that 60mW should produce a really strong local signal - but only if it is radiated. Using 75 ohm coax with a 75 ohm resistor at the far end, you'll get an excellent match but there's no opportunity to radiate any power. It will all end up in the resistor!

Hi Pete,

if you remove the 75R resistor and have a full Rf reflection on the coax would this not radiate well enough for the purposes here. As the coax is very lossy it may not appear very mismatched anyway from the Rf point of view.

Mike
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Old 14th Mar 2008, 1:39 pm   #79
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Default Re: AM Micro Transmitter

Hello both,

I'll give the ideas a try next week for coupling up to the coax, at the moment I've got a wire aerial from the centre of the terminated coax. I'll try removing the resistance/shorting it.

I got the oscillator back working properly, first with a buffer stage, now without by fiddling with the grid components, it still gets loaded by the PA valve but not as heavily, no idea why what I've done helps!

Back with the bigger coils again gives a bit more power, I've got only 3.5 turns driving the aerial at the moment (120 turns pri.), I'll try it next with about 7 to give more poke and experiment with different loads. Would winding two lots of 4 turns and connecting either in series or in parallel work or would that have unwanted effects?

Then to put back the full loop feedback network....

Dom
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Old 15th Mar 2008, 3:24 am   #80
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Default Re: AM Micro Transmitter

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Originally Posted by dominicbeesley View Post
Hello,

I continued with the experiments last night, the lower number of turns seemed to reduce power a lot but didn't improve quality. All experiments are carried out at about 750kHz

So I tried using my older lower inductance coils ~100uH. These don't give as much power at the PA valve and now I can tune to resonance again. So I could try out coupling up the coax:

With about 6 turns on a 200 turn coil seems to give higher Q but quite low power about 20 turns on 200 turn coil delivers about 6V pp to the 75R load (60mW), but the Q is lower and tuning the tank has less effect (not a bad thing? higher bandwidth?)

6V p-p is really weedy and even with wire taken off the centre of the coax and draped over the set gives a very week signal with a lot of background noise breaking through. 60mW most of which is being eaten by the terminator isn't enough power?




Dom
Hi Dom,

Pete can confirm whether I am reasoning correctly but I think what you are seeing here is the interaction of getting the coupling right for maximum power transfer ( correct loading) and the ratio of the L to C you have chosen. These are all related to the Q at resonance with whatever setup you are running with. The transformed loading is related to Q squared the reactance at resonance.

The higher the Q the higher the current value in the tank circuit at resonance and for the same L/C ratio more power is lost due to losses in the tank circuit.

Mike
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