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Old 26th Apr 2010, 4:20 pm   #21
HamishBoxer
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

Should have been more accurate,it was the inductors that i was looking at too.

David
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Old 26th Apr 2010, 5:34 pm   #22
kalee20
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Could i ask where most of the bits came from?
Preset capacitors - one salvaged from goodness knows where, one bought from Bi-Pak 25+ years ago.

Other capacitors - mostly Farnell (electrolytics are long-life 105 deg C Rubycon; non-polariseds are Wima polyester and polyproplylene except for C11 which is silvered mica from LCR, surplus part from a minimum order quantity at work; and C14 which is a Rifa polypropylene with a fair current rating to couple out the RF [and NOT one of their epoxy moulded impregnated paper types which I avoid like the plague!]).

Resistors - RS, Farnell, etc. All metal film except for two wirewounds (vitreous Welwyn W21 type).

Potentiometers - preset just sets my auxiliary DC output voltage, Bourns 3386 cermet. Input level is a combined potentiometer/knob made by Sfernice/Vishay. Both from Farnell

Semiconductors - RS, Farnell, etc

Ferrites - mostly Deltron Hawnt, although RS / Farnell sell them. Think they have enamelled wire, too.

Matrix board - RS (the plain stuff seems to be getting rarer. Shame - it's my favourite!)

Box - ebay, less than a fiver I seem to remember.

Connectors - RS, Farnell, Electrovalue (no longer with us ). RS for the two chunky connectors for aerial and earth which are spring-loaded and accept bare ends, really good but they're a bit expensive and moreover not insulated from panel. So, as I used a brass panel (from friendly one-man engraving shop), I had to design an insulating bush for the aerial (made by another friendly engineering shop using Tufnol rod from ebay).

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Most of the components look straight forward to source with the possible exception the coils; where could I get them or will I have to wind them myself?
'Fraid so, though as you can see they are all single-winding jobbies. I'll put up the details in the next few days. Worst thing is the bobbins are a bit fragile so they need supporting well, otherwise pressure of layers of wire can break off the flanges.

Last edited by kalee20; 26th Apr 2010 at 5:41 pm. Reason: Correct spelling
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Old 26th Apr 2010, 8:24 pm   #23
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

Many Thanks for that info.

Regards David
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Old 26th Apr 2010, 10:27 pm   #24
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I'll put up the details in the next few days.
Thanks Peter I shall look forward to that.

Regards

Colin
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Old 29th Apr 2010, 3:30 pm   #25
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

Beautiful!

I like matrix board too - as a kid I did a lot of stuff with it because it was much cheaper than Veroboard, and easier to re-use as well. And, as all my components were "recycled", the short lead lengths were impractical with Veroboard layouts.

I'm intrigued by the negative feedback - did you run into stability issues? I'm just thinking about all the potential phase shifts through the transmitter and demodulator... Also, given that you did it for frequency response reasons, I'm wondering if pre-emphasis might have been easier?

I know very little about RF stuff, so plan to investigate this circuit further - I'm thinking it will be a good project to get my feet wet. The modulation scheme is new to me - every circuit I've seen uses multipliers to achieve AM - did you rule these out on complexity grounds?

BTW, as people are discussing reducing the component count, I reckon you could lose C3, and replace R6, 7, 8 with two resistors. But hardly worth worrying about it - it's not like you're going into production! Personally, I like belt-and-braces design too

All the best,

Mark
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Old 29th Apr 2010, 5:37 pm   #26
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

Well, C3 is there (rather than a direct connection) so that there is 100% DC negative feedback from Q2 collector to Q1 emitter. This gives a very well defined quiescent operating point. But, at AC, a gain of 1 is no good, hence the AC potential divider R7, R6 giving a feedback attenuation of 16, and hence a forward gain of 16.

Incidentally, the operating point of the entire AF circuitry is set by R3, R4, as the stage after the 10kHz filter (Q3 base) is biased from the previous stage via L1, R9 - the base current of Q3 giving a miniscule drop in these components.

Stability... yes, with more overall feedback (R41 lower value), I did oscillate. But 12db was easily attainable - just unnecessary. I had an hour or two of fun with a square-wave input source, tweaking R41 (and also R38) and watching the for overshoot, damped oscillations, critical damping on Q15 collector. You'll notice that the time constants in the RF filter following the demodulator R37/C25, and R38/C26 are different, this stops the phase changing rapidly over a small band of frequencies within this 2-stage filter.

Pre-emphasis would have been a bit easier, only not so much fun. Plus, of course, negative feedback does give a reduction in distortion (not that there was much anyway), and it also stabilises the characteristics against changes in the overall gain of the whole thing, which could change if I alter my aerial etc.

As for modulation, yes, a multiplier would work, followed by linear amplification. But it is a bit more complicated; linear amplifiers are less energy-efficient (not that this was a consideration); and I wanted to use high-level modulation like a 'big' transmitter!
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Old 29th Apr 2010, 6:24 pm   #27
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

Audio is my specialism and I do a lot with discrete designs too, so I appreciate C3's (and C5) function - my initial thinking was that as the rail is 15V, a bit of DC imprecision might not be a problem. But, perhaps I should have asked about the size of the signal at Q5's emitter, and also the relationship between V and mod depth. Ignoring NFB (and finite Ao in each stage), there's nearly 50dB of AF gain there - meaning that 20mVRMS from the pot will make Q5 emitter swing 14Vpk-pk. Looking at the over-mod detect, it looks like the LED lights when Q5's emitter goes lower than around 0.7V. (But the quiescent point is around 8.4 volts, so isn't there a danger of the positive half-cycle might be clipping before over-mod is detected? Presumably it's slightly lower - things don't always follow the theory!). Anyway, yes, if you're encountering such large signals at the output of the two amps, then C3 and C5 save a lot of extra complications, like op-amps or other such heresy

Sounds like playing with the demod NFB was highly educational! I was never much good at control theory, but have got a long way with an experimental approach. By coincidence, I'm having dinner this evening with someone who understands RF and big transmitters, so will try to find out a bit more about this modulation scheme...

Anyway, thanks again. Lots of good ideas in here, and I've got a growing collection of AM-only radios that will need feeding for many years into the future

All the best,

Mark
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Old 30th Apr 2010, 3:28 pm   #28
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

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But the quiescent point is around 8.4 volts, so isn't there a danger of the positive half-cycle might be clipping before over-mod is detected? Presumably it's slightly lower - things don't always follow the theory
Thanks for comments and interest, Mark.

The quiescent point is around 7.5V and with a sinewave input gives symmetric clipping as it approaches overload. So, there's no danger of the positive half-cycle clipping before the negative.

The reason that the quiescent point is 7.5 not 8.4V is, the emitter current of Q1 flows through R7 (and similarly Q3 and R13), so each stage's DC feedback drops the quiescent output voltage by a small amount to give the requisite DC voltage across the feedback resistors.

Hope you had a good dinner and chat about modulation! This stage is straightforward if you bear in mind that the Q13/D4 pair is either 'on' or 'off', with no linear region at all. Then, all voltages are proportional to the supply voltage at the top of L4, which varies at an audio frequency rate.
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Old 30th Apr 2010, 4:09 pm   #29
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

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The reason that the quiescent point is 7.5 not 8.4V is, the emitter current of Q1 flows through R7 (and similarly Q3 and R13)
Ah yes, depending HFE, etc, there's anything up to 0.5V across R7 and perhaps slightly more across R13. I initially ignored the collector currents as they were in the microamp range. Good; I was sure you'd have spotted any problems


Quote:
Hope you had a good dinner and chat about modulation! This stage is straightforward if you bear in mind that the Q13/D4 pair is either 'on' or 'off', with no linear region at all. Then, all voltages are proportional to the supply voltage at the top of L4, which varies at an audio frequency rate.
Yes, very nice, thanks!

It's making much more sense now that I know what's going on at Q5 emitter. I'm quite familiar with PWM - if I think of this circuit as being PAM (pulse amplitude modulation), then it makes perfect sense to me. Actually, what little I know about resonant switched-mode power supply design is stirring in the back of my long-term memory - I think there might be some analogies there...

I'm not saying I could work out all the tuned circuits or anything, but I'm now feeling confident about trying this approach sometime soon. Thanks again for sharing

All the best,

Mark
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Old 30th Apr 2010, 4:23 pm   #30
kalee20
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

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Originally Posted by mhennessy View Post
Ignoring NFB (and finite Ao in each stage), there's nearly 50dB of AF gain there - meaning that 20mVRMS from the pot will make Q5 emitter swing 14Vpk-pk
Yes! The gain of the first stage (pot wiper to Q2 collector) is, near as dammit, (R7+R6)/R6, or 16. And the second stage, Q3 base to Q5 emitter, is similarly (R13+R11)/R11, or 15. Total gain is thus 240, (47.6db).

For 14V peak-peak at Q5 emitter, the signal at the pot wiper must therefore be 58.3mV p-p or 20.6mV, as you say. Of course, with the 15V supply, more than 14V is wishful thinking...

The overall feedback reduces gain by half, so 41.2mV is needed - and with the input level pot at quarter travel (it's a linear pot), 165mV gives full swing at Q5 emitter.

With 7.5V quiescent on Q5 emitter, the full 15V swing would give 100% modulation. So 14V would be 93% modulation. (This doesn't take into account the 0.7V drop in the RF diode D4 or the forward drop in Q13, which in practice give a bit of offset). So, achievable % modulation is well into the 90's and as many AM radios might start to give detector tracking distortion, or AC/DC load ratio distortion, at much more than this, I was happy!
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Old 1st May 2010, 7:00 pm   #31
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Thanks Peter I shall look forward to that.
Here are the coil details, as promised. I used Ferroxcube ferrites - had Electrovalue still been with us I would have given the Siemens (Epcos) part numbers instead, as Electrovalue were Siemens distributors. For Ferroxcube, try BFI Optilas, which a Google search will find.

One matter - I've just noticed that L5 on the circuit diagram has the wrong value - it's 137ÁH not 90ÁH.

L1: AF Filter Coil 53mH

Core RM6 (Gapped) Al=630nH e.g. Ferroxcube RM6S/I-3H3-A630
Bobbin RM6 single-section e.g. Ferroxcube CSV-RM6S-1S-6P-G
Clips RM6 (1 pair) e.g. Ferroxcube CLI/P-RM6

Winding: 292 turns 0.18mm Grade 2 enamelled copper wire

L2: Oscillator Coil 23ÁH (for around 783kHz)

Core RM5 (Gapped, with hole for adjuster) Al=160nH e.g. Ferroxcube RM5-3D3-E100/N
Bobbin RM5 single-section e.g. Ferroxcube CSV-RM5-1S-6P-G
Clips RM5 (1 pair) e.g. Ferroxcube CLI/P-RM4-5
Adjuster Ferroxcube ADJ-RM4/RM5-BROWN

Winding: 12 turns in 1 layer 0.355mm Grade 2 enamelled copper wire

L3: RF Tank Coil 22.5ÁH (for around 783kHz)

Core RM6 (Gapped) Al=100nH e.g. Ferroxcube RM6S/I-3F3-A100
Bobbin RM6 single-section e.g. Ferroxcube CSV-RM6S-1S-6P-G
Clips RM6 (1 pair) e.g. Ferroxcube CLI/P-RM6

Winding: 15 turns in 2 layers 0.63mm Grade 2 enamelled copper wire

L4: Modulation Choke 283ÁH

Core RM8 (Gapped) Al=315nH e.g. Ferroxcube RM8-3F3-A315
Bobbin RM6 single-section e.g. Ferroxcube CSV-RM8-1S-8P-G
Clips RM8 (1 pair) e.g. Ferroxcube CLI/P-RM8

Winding: 30 turns in 4 layers twin 0.5mm (bifilar wound) Grade 2 enamelled copper wire

L5: RF Output Matching Coil 137ÁH (for around 783kHz)

Core RM8 (Gapped) Al=100nH (High frequency low-loss grade) e.g. Ferroxcube RM8-3F3-A100
Bobbin RM6 single-section e.g. Ferroxcube CSV-RM8-1S-8P-G
Clips RM8 (1 pair) e.g. Ferroxcube CLI/P-RM8

Winding: 37 turns in 4 layers twin 0.4mm (bifilar wound) Grade 2 enamelled copper wire

If anybody is seriously interested in replicating my circuit, send me a PM -I may be able to help out with a small quantity of coils. (Though I don't want to step on Ed Dinning's toes here!)
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Old 2nd May 2010, 7:55 am   #32
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

Hi Peter

You have a PM

Regards

Colin.
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Old 3rd May 2010, 12:38 pm   #33
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

Quote:
Actually, what little I know about resonant switched-mode power supply design is stirring in the back of my long-term memory - I think there might be some analogies there..
Class E can be used backwards as a high efficiency synchronous rectifier. The only snag is that, like the transmitter, the tuning will be counter-intuitive - maximum efficiency is not the same as maximum output.
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Old 3rd May 2010, 10:42 pm   #34
kalee20
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Default Re: Build of a Micropower AM 'Pantry' Transmitter

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if I think of this circuit as being PAM (pulse amplitude modulation), then it makes perfect sense to me.
That's it, more or less, in principle! And with with the tuned circuits following, everything is still linear, proportional, hence low-audio-distortion!

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Ah yes, depending HFE, etc, there's anything up to 0.5V across R7 and perhaps slightly more across R13. I initially ignored the collector currents as they were in the microamp range.
Well, although only a 1-off I tried to make the design hfe-insensitive. In fact, most of the collector current in Q1 (and Q3) isn't so much the hfe-dependent base current of Q2 (and Q4), but the current necessary to give the rather more predictable voltage drop across R5 (and R10) sufficient to turn on the PNP transistors.

Really, the value of the feedback resistors should be as low as possible, to minimise the effect of the Q1 (and Q3) emitter currents. The down side is, with low value feedback resistors, the downward swing in the Q2 collector voltage (and Q5 emitter voltage) is limited by how well the pull-down load resistors R8 (and R14) pull the output towards 0V. Fortunately, the signal at Q2 collector is still small; and Q5 emitter also has the pull-down of the RF stage.
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Old 3rd May 2010, 11:02 pm   #35
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I'm going to have to (try to) analyse how you got your NFB working! I never got mine to work that well, demodulating the signal. When I tried as I was struggling to get stability with all the various poles needed to remove the RF from the AF etc...
Yes Dom, the poles are a right pain aren't they, easily upsetting stability.

Doing a rough count, from Q3 emitter, there's one due to the modulation choke L4 (plus one at a much higher frequency, due to C13 with L4). Then the RF tank circuit, considered as RF-envelope-as-output, adds 1 (which is Q-dependent) and the matching network adds another 1. Finally the demodulator's RF filter add a further 2. I wanted to use a single-stage RF filter here, but didn't like the amount of residual RF that this would leave on Q14 source, so in went the second stage...

[Feel free to disagree with the pole count here anybody]

With all these poles, I chickened out of analytical design and went for empirical methods, aiming as much as possible to have one dominant pole (R38 C26), such that by the time the plot of AF frequency rose to the point that other poles kicked in, the loop gain would have rolled off below unity. And because the modulation is linear, I didn't need oodles of feedback to crush distortion, and it worked.

The overall loop gain is highly dependent on accurate aerial tuning (C15) because if this is off tune, the RF output amplitude is low, so feedback is low. Which means the circuit is still stable, but off-tune the feedback virtually disappears!
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