Trying to build decent AM transmitter

[Mike Bates]

The other main reason im using a CMOS ocillator, is because the frequency is
rock solid. It NEVER drifts, at least not noticble to me.

im thinking the problem lies in the carrier wave, because the CMOS is
supposed to put out a 5vp-p drive to the mosfet. Problem is, i think that
5vp-p isnt enough to drive the mostfet to its max output. i tried using a
buffer amplifier between the mostfet and the CMOS ocillator. it didnt change
anything. Tried using a fet buffer stage with another irf510, and that made
it worse. (probably not matched/tuned)

 

[Paul Burridge]

The other main reason im using a CMOS ocillator, is because the frequency is
rock solid. It NEVER drifts, at least not noticble to me.

im thinking the problem lies in the carrier wave, because the CMOS is
supposed to put out a 5vp-p drive to the mosfet. Problem is, i think that
5vp-p isnt enough to drive the mostfet to its max output. i tried using a
buffer amplifier between the mostfet and the CMOS ocillator. it didnt change
anything. Tried using a fet buffer stage with another irf510, and that made
it worse. (probably not matched/tuned)

I've seen the two 'donor' diagrams you posted but would like to see a
diagram of the final, modified circuit you've come up with in its
entirety. How did you got about selecting the irf510? What makes you
think it a suitable mosfet for this application? There are TTL drive
level mosfets out there that would fully switch on from your 5v input
signal, but they might well be lacking in other necessary
characteristics. Just because you've breached V_threshold with your
CMOS input drive signal doesn't mean you're going to get a decent
output. You may well need more voltage *and* current drive. What's the
input capacitance of that FET? I thought you said you'd tried a
driver, anyway.
Forget about Paintshop; download a free copy of LTSpice from
www.lineaer.com and use it to assemble a circuit diagram (the symbols
are already created for you so you just choose the right ones and join
'em up. Then use the Tools menu to copy the diagram to bitmap in
Paintshop; convert to .gif and post the complete schematic to your
website. At least then there's a fighting chance of nailing the
problem!

 

[Joerg]

Hi Frithiof Andreas,

HDTV took so long to roll out that people got used to the Internet and those
people are now unhappy about the slow, liniear, push-format 'offered' by the
TV media; i.e. *why* should *I* adjust my schedule to when the *broadcaster*
deigns appropriate? *why* should I sit through a boooring news item?

In the US people use TiVo to get around that. It lets you hand-pick the
content and avoid much of the commercials. But AFAIK it is only possible
on a paid subscription basis. No free lunch here.

Not to mention that most of the programmes are crap anyway: a deluge of
quizz shows, advertising and movies - that were bad even a decade ago - the
kind of swill upon which better picture and sound quality is entirely
wasted.

That's why we went to the video store last week and rented "Once upon a
time in the West", to see it for the tenth time or so ;-)

HDTV is dead - some sort of Internet-based distribution will be the cable
TeeVee of the future.

I think you may be right about that. But it will require the roll-out of
fibers into the homes for enough bandwidth. Then you have your HDTV
again, just in a different delivery mode.

I remember when an analog form of HDTV was touted big time in Europe and
some Scandinavians bought these extremely expensive sets, only to
discover a short time afterwards that the programming started to vanish.

Regards, Joerg

 

[Yzordderex]

Is this thing class E? probably not enough drive to get the fet going
good. Somebody already told you that. Check out monkeypuppet.com

You shouldn't be transmitting illegal tho.

Bad boy

bob

 

[Andrew Holme]

im thinking the problem lies in the carrier wave, because the CMOS is
supposed to put out a 5vp-p drive to the mosfet. Problem is, i think that
5vp-p isnt enough to drive the mostfet to its max output. i tried using a
buffer amplifier between the mostfet and the CMOS ocillator. it didnt change
anything. Tried using a fet buffer stage with another irf510, and that made
it worse. (probably not matched/tuned)

As Paul points out, perhaps you should try a MOSFET intended for logic
level drive.

As the author admits, that Kyle Drake schematic is a very bad circuit:

1. There is no dc gate bias on the MOSFET. I don't know if your
driver is ac or dc coupled. You might try a resistor or choke from
gate to ground. Alternatively, you could bias the MOSFET slightly
into conduction.

2. There is no output filter / matching network. The antenna is
connected directly to the drain. It will put out a lot of harmonics.
Are you using the same arrangement? Do you have an ATU?

3. There is no RF decoupling (try >100n to ground) at the junction of
the modulating transformer and the drain choke.

The amount of power sucked out of the circuit by the antenna depends
on the load impedance "seen" by the drain. No amount of fiddling will
help if there is a bad match. If the load is not purely resistive,
the voltage and current waveforms at the drain will be out of phase,
the current maxima will not coincide with the voltage minima, and the
efficiency will be poor.

Can you post a schematic of your circuit and a photo of your layout?
What construction method are you using? Prototyping boards are no
good for RF. You need to prototype this "ugly" style on top of a
copper ground plane.

Watch out for feedback from the PA to the oscillator. This can cause
very wierd things to happen. Your transmitter may not be "working"
the way you think! You should be able to smoothly adjust the output
by varying the drive. If oscillation suddenly stops or starts,
something is wrong.

 

[Frithiof Andreas Jensen]

I think you may be right about that. But it will require the roll-out of
fibers into the homes for enough bandwidth. Then you have your HDTV
again, just in a different delivery mode.

Rumour has it that the 'vanilla' ADSL we have here in the DK can actually
run at 8 Mbit/s - the requirement presently being that one knows a techie
with the right access at the dominant telecoms provider. Who shall not be
named

The other rumour is that they want/need 12 Mbit/s om ADSL before they commit
to providing TV on it - in reality it is probably the 'Brownshirts' from
RIAA that are preventing any operator from enabling the downloading of DVD's
legally on players with storage, which would alleviate the BW problems
somewhat.

 

[Joerg]

Hi Frithiof,

Rumour has it that the 'vanilla' ADSL we have here in the DK can actually
run at 8 Mbit/s - the requirement presently being that one knows a techie
with the right access at the dominant telecoms provider. Who shall not be
named

Yesterday I talked with an engineer who is currently trying to squeeze a
whole more than that through twisted pair.

The other rumour is that they want/need 12 Mbit/s om ADSL before they commit
to providing TV on it - in reality it is probably the 'Brownshirts' from
RIAA that are preventing any operator from enabling the downloading of DVD's
legally on players with storage, which would alleviate the BW problems
somewhat.

Oh, they'll cave in some day. After all, you can already record
digitally. Over here in the US the most widespread system is probably
TiVo. From there to being able to record a DVD is just one small step.
But why do it if you can rent movies for two to three Dollars from the
local rental place? With a coupon it drops to one Dollar. Considering
that people won't want to watch movies more than a couple times or so
any illegal recording just isn't worth the hassle to them, I'd think.
Except for the bootleggers but they do it anyway, no matter where the
source comes from.

Regards, Joerg

 

[SioL]

Hi Frithiof,

Yesterday I talked with an engineer who is currently trying to squeeze a whole
more than that through twisted pair.

Actually you need about 4-6Mbit/s for TV. Plus 1-2Mbit/s for Internet, usually.
I have it in my living room. I already had ADSL so upgrade to tv was logical and
cheaper step since my new apartment did not have cable.

Unfortunately my ADSL line is a bit far from the central so with some programs
I get a bit of "blocking" once in a while.

Another negative aspect is that you can only watch one channel at a time (only
one
TV) or it would indeed take 12Mbit/s + whatever internet needs.

SioL

 

[Joerg]

Hi SioL,

Actually you need about 4-6Mbit/s for TV. Plus 1-2Mbit/s for Internet, usually.
I have it in my living room. I already had ADSL so upgrade to tv was logical and
cheaper step since my new apartment did not have cable.

Interesting. Where do you get the TV channels from? The fastest I have
seen were channels like Bloomberg at 250kbit/sec. Maybe a bit more
nowadays but not much.

Another negative aspect is that you can only watch one channel at a time (only
one
TV) or it would indeed take 12Mbit/s + whatever internet needs.

But watching more than one channel at a time usally fries a human brain
quite quickly, doesn't it. Unless you want to create a sports bar in
your basement.

Regards, Joerg

 

[Jim Thompson]

Hi SioL,

Interesting. Where do you get the TV channels from? The fastest I have
seen were channels like Bloomberg at 250kbit/sec. Maybe a bit more
nowadays but not much.

But watching more than one channel at a time usally fries a human brain
quite quickly, doesn't it. Unless you want to create a sports bar in
your basement.

Regards, Joerg

It's rare that I and my wife are watching the same channel (in the
daytime)... I watch Fox News (actually just listening... the TV is
behind me when I'm at the computer), and the wife watches soaps and
game shows.

When the grand kids are visiting it is possible to have six different
channels on view. (Sets in kitchen, 3 bedrooms, great-room and my
office.)

So TV over twisted pair would be useless to us.

...Jim Thompson

 

[SioL]

Hi SioL,

Interesting. Where do you get the TV channels from? The fastest I have seen
were channels like Bloomberg at 250kbit/sec. Maybe a bit more nowadays but not
much.

We have that service in place around here about 1 year now. Its just like a
cable provider,
except it provides signal via IP. There are about 130 channels, mostly the same
as available
on cable, with a few extras.

But watching more than one channel at a time usally fries a human brain quite
quickly, doesn't it. Unless you want to create a sports bar in your basement.

Joerg, nowadays you usually have a TV in the living room and another one in
bedroom
or other room.
Or you want to VCR one program while you watch another one. Or if you have PIP
TV
set with multiple tuners you can't take advantage of this either.

But so far this is not really a problem for me.

Siol

 

[Joerg]

Hi SioL,

We have that service in place around here about 1 year now. Its just like a
cable provider,
except it provides signal via IP. There are about 130 channels, mostly the same
as available
on cable, with a few extras.

Ok, so it is a special service in your area. This means it's not
something coming off the web. Oh, well, then we'll just have to wait
until this technology comes to our neck of the woods.

Joerg, nowadays you usually have a TV in the living room and another one in
bedroom
or other room.
Or you want to VCR one program while you watch another one. Or if you have PIP
TV
set with multiple tuners you can't take advantage of this either.

Nah, there is just a lone 25" set in the living room, mostly in the OFF
position. I did wire up the house with a home-run network for
everything, including TV. Actually TV has two lines each, the aux line
is for feedback where I use a modulator to be able to watch a video on
the deck (our 'summer living room'). Our neighbors heckle us all the
time because we don't even have a DVD player. Thing is, the only movies
we watch are so old that they don't come on DVD. Neither does cable
carry much of these, so all we've got is ye olde aerial.

Regards, Joerg

 

[Joerg]

Hi Jim,

When the grand kids are visiting it is possible to have six different
channels on view. (Sets in kitchen, 3 bedrooms, great-room and my
office.)

We don't have kids, but if we did or if there are kids visiting there
won't be any TV. Whenever I get a glimpse of what is on cable at
neighbors I am glad we don't spend those $50 or so. Not worth it. Except
during what you call 'gimme week' at PBS but that also comes in terrestrial.

So TV over twisted pair would be useless to us.

There is a company (Key Eye Communications) out here that attempts to
run more than an order of magnitude faster over CAT-5 and CAT-6. Short
runs only, like a 150ft or so to the street box but that would allow all
your visitors to watch a different channel if they wanted to. I believe
their goal is 10Gbit/sec over copper. On the local level that could blow
fiber out of the water.

Regards, Joerg

 

[Ken Smith]

There is a company (Key Eye Communications) out here that attempts to
run more than an order of magnitude faster over CAT-5 and CAT-6. Short
runs only, like a 150ft or so to the street box but that would allow all
your visitors to watch a different channel if they wanted to. I believe
their goal is 10Gbit/sec over copper. On the local level that could blow
fiber out of the water.

You don't really need that much bandwidth if you assume some good
compression is used. Assuming we compress down to the information
contained in a network program we should be able to work out the average
number of bits needed. Then we can just divide by 3600 to get the data
rate. I'll make a start on the estimate for you:

There are a total of 4 basic plots which when combined with the various
subplots will take about 8 bits to transfer. So thats 1 byte per show.