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Hi-Q RF filters, anyone?

P

Paul Burridge

Jan 1, 1970
0
Hello Paul,
you are dragging the anchor a bit. That was months ago! :)

Have a look here
http://www.ife.tugraz.at/datashts/nsc/h7912.pdf

You'll have to forgive me here, John, as i'm battling with a duff
display on the computer I'm using at the moment and I was only able to
glimpse the data for a few seconds, but this chip doesn't seem to be
suitable for 40Mhz FM, does it? I'm sure your general solution to the
problem is an excellent one, though, if a suitable chip can be found.
 
J

John R. Strohm

Jan 1, 1970
0
Paul Burridge said:
John, what in your experience causes this 'servo-twitching'? I've
observed it myself at close hand many times. The last time it
happened, we cured it by isolating the die-cast box the rx was mounted
in from the chassis. I still can't figure out why this worked, as I'd
have thought grounding it *ought* to solve the problem. But in this
instance, grounding it *created* the problem and isolating it solved
it! Sometimes when I see those servos behaving like they've got a mind
of their own it almost makes me believe in the supernatural.

R/C servos are EXTREMELY sensitive to trash on the power supply leads.
Grounding the box probably coupled some trash into the ground lead to the
servo.

I don't know what you have in your system, but I'd start by investing in a
few small 250 uF (nominal) capacitors and put them directly across the servo
supply leads. Use Y-harnesses to hook them in if you don't feel like
modifying a cable.
 
J

John Crighton

Jan 1, 1970
0
John, what in your experience causes this 'servo-twitching'?
In that incident with the rich guy, he was a few hundred yards
away from his model and my transmitter, only 20 feet away. even
though it was on a different channel upset his signal. Therefore
his servos twitched and his model crashed.
I've observed it myself at close hand many times.

Waving a transmitter with a fully extended antenna beside
the model will cause a bit of servo twitching. The signal
is too strong. That is why you see people doing close
range testing with the antenna retracted.
The last time it
happened, we cured it by isolating the die-cast box the rx was mounted
in from the chassis. I still can't figure out why this worked, as I'd
have thought grounding it *ought* to solve the problem.

I would have thought that grounding the RX to the box would
be good practice also.
Not having a circuit to guide you, maybe you were grounding
a spot on the PCB that should not be grounded.
But in this
instance, grounding it *created* the problem and isolating it solved
it! Sometimes when I see those servos behaving like they've got a mind
of their own it almost makes me believe in the supernatural.

You got some good advice from John R Strohm in regards
to fitting decoupling capacitors across the servo supply leads.

I am assuming that your R/C gear is operating from its
own battery supply not the main driving motor supply.
Using the same battery for the radio and main driving
motors could cause servo twitching.

Pity there isn't one of these fighting robot clubs
here in Sydney. I would like to have a go.

Paul you will have to put some pictures up
somewhere so we can have a look.

Regards,
John Crighton
Sydney
 
J

John Crighton

Jan 1, 1970
0
You'll have to forgive me here, John, as i'm battling with a duff
display on the computer I'm using at the moment and I was only able to
glimpse the data for a few seconds, but this chip doesn't seem to be
suitable for 40Mhz FM, does it? I'm sure your general solution to the
problem is an excellent one, though, if a suitable chip can be found.
--

Hello Paul,
under Features on page 1 it says 50KHz to 72 MHz
but not FM. I forgot about that. My mistake.
OK we'll keep looking for something that suits your
FM transmitter.

Or find, beg, borrow, "buy used," or build an ordinary
AM R/C transmitter. Ten quid should get you an old
style metal cased Futaba transmitter.
A model shop gave me the circuit diagram decades
ago of an old JR brand transmitter that I bought off
them in dud condition, cheap. Keep looking. That
model shop/sanwa agency are a bit mean that is why
I said dump their product.
Stick with brands that will offer support and circuits.

There is plenty of info with that LM1872 chip,
that is what makes it attractive to use.

Have Fun,
John Crighton
Sydney
 
J

John Crighton

Jan 1, 1970
0
This also looks a possibility:

http://w1.859.telia.com/~u85920178/use/rc-prop.htm

Might be a contender for the next 'bot, possibly?
--

Hello Paul,
yes it does. That is a great site. Harry gives you
the circuit, the artwork and tells you how he makes
his printed circuit boards.
Harry explains that he has managed to use that
little AM Rx on FM also.
I think you plenty of info now Paul.
Have Fun,
John Crighton
Sydney
 
H

Hans Summers

Jan 1, 1970
0
John Crighton said:
Hello Hans,
what brand/make was that R/C set that you had?

It was a Futaba, I don't recall which one. Very old and battered but worked
great. I've been away from aero-modelling for 9 years or so and don't know
what's new now - but at that time, the latest control sets were dual
conversion (avoiding that image problem) and PCM (Pulse Code Modulation)
compared to the old PPM like mine (Pulse Position Modulation). It always
seemed to me (perhaps somewhat subjectively) that the PCM tranceivers, which
were supposed to be more reliable, were in fact more prone to interference.
In the presence of interference the flyer seemed to have no control,
presumably because the incoming code was scrambled and the receiver couldn't
make head or tail of it. With the old fashioned PPM sets it seemed that
often even if interference was causing your servos to twitch a little you
still had some degree of control over it, often enough to turn it round and
bring it in closer whereupon full control would return and permit a hasty
landing.
You have reminded me of a silly incident at my model
flying club decades ago. One of the wealthier club
members was having all sorts of trouble with his
model, engine and radio gear so he flung heaps of
money at a ready made, ready to fly model with an
expensive four stroke engine, and a new expensive
all singing and dancing radio control set. First day
out with the new model and everything is going well
for him, he was doing stunts all over the sky.

My models were el cheapo sticks and tissue construction,
I couldn't afford nice covering film/material. I was
more interested in home built radio control gear. Radio
assisted free flight was more my style.
When the fuel ran out in my models I didn't mind
walking a few hundred yards if necessary to retrieve
my model. So I am off for a long walk with my transmitter
still switched on as one would. (The receiver gets
switched off first then the transmitter.)

The rich guy is stunting around and decides to buzz
me at low level about 20 feet above the ground.
His model flies over me and then nose dives into
the ground near by. I could here his servos twitching
away as I walked past the wreckage.
After I retrieved my model and switched off my
transmitter, I stopped by the little gathering at the
wreck site. The rich guy was operating his servos OK
and scratching his head. " I spend thousands on
my model and that bastard John Crighton comes
here every weekend with 50 dollars worth of homebuilt
junk and flies. It just isn't fair." "Moan...grumble..moan."
I didn't try to explain that his receiver got swamped. His
mates put the crash down to pilot error at low level, and
that was that.

Ha Ha, similar story here. I was a student at the time, had very little
money. Serves that rich fellow right for flying so low near you, sounds
dangerous. I used to have a Hi-Boy 4-channel trainer with 0.40 cu inch
2-stroke OS engine. When it came to me it had spent at least a decade in a
damp garage and required an almost complete fuselage rebuild to clear the
rot. Quite likely therefore that by the time I finally flew it, the original
dimensions weren't adhered to any longer. A particular weakness seemed to be
the nosewheel which was endlessly breaking up during my bad landings.
Solution to that one was just to remove it and fly the thing as a
taildragger.

The OS40 engine worked a treat, perhaps slightly overpowering the model: I
used to be able to take off and fly vertically immediately like the jet
fighters at air shows.

Had SO many crashes with that plane, and came to love it - just glued it
back together every time. In the end it was probably more glue than balsa
wood. A number of times I had to carry the pieces home in a plastic shopping
bag. By the following Saturday it was glued back together and generally
caused jaws at the flying field to drop. No-one could believe anything with
that much glue in it and such bad aerodynamics could actually fly. The wings
(balsa sheet covering polystyrene) had snapped 4 times and been repaired by
application of a fibre-glass bandage, adding significant weight of course.
It actually flew quite well too, all the aerobatics no problem and I
particularly used to like slow low level inverted fly bys at 10 ft or less.

Happy days ;-)

Hans
http://www.HansSummers.com
 
M

Mike Andrews

Jan 1, 1970
0
Had SO many crashes with that plane, and came to love it - just glued it
back together every time. In the end it was probably more glue than balsa
wood. A number of times I had to carry the pieces home in a plastic shopping
bag. By the following Saturday it was glued back together and generally
caused jaws at the flying field to drop. No-one could believe anything with
that much glue in it and such bad aerodynamics could actually fly. The wings
(balsa sheet covering polystyrene) had snapped 4 times and been repaired by
application of a fibre-glass bandage, adding significant weight of course.
It actually flew quite well too, all the aerobatics no problem and I
particularly used to like slow low level inverted fly bys at 10 ft or less.

From RFC 1925: "(3) With sufficient thrust, pigs fly just fine. However,
this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly
overhead."
 
T

Tom Bruhns

Jan 1, 1970
0
Watson A.Name - 'Watt Sun' said:
How about a helical resonator. They're smaller than a cavity, maybe
not as high Q, but still higher than lumped constant tuned circuit.

I think that's a popular misconception. The resonator Q is
essentially the same as the Q of the same part used as a shielded
inductor, and the shield actually lowers the Q from what it is with an
inductor in free air (so long as it's not large enough to radiate
significantly).
They're tunable, but I'm not sure how much.

They're certainly easily tunable over a few percent, if you need
that...

But the problem as originally stated implies a filter of fairly high
order and low in-band attenuation, which in turn implies resonators of
very high unloaded Q. 20kHz bandwidth at 40MHz in a single tank is a
loaded Q of 2000, and to keep attenuation low, the unloaded resonator
Q should be perhaps 5 times that much. It would be worse for a
multi-pole filter. All this tells me it's silly to even think of an
LC filter. Add to that the extreme difficulty of getting a set of
resonators to tune together. (To get Qu=10000 in a coaxial resonator
at 40MHz would take an air-dielectric line nearly half a meter in
diameter! Just plain silly.)

I'd opt for a front end with very high dynamic range (esp. low
third-order intermod products), into a good IF filter, etc., and a
communications protocol that optimized whatever performance measure I
needed. Talk to the people who build RF communications sytems that go
on aircraft carriers. Or talk with hams who design receivers with
third order intercepts up in the +50dBm region and higher. By the
way, you may do well by putting an ATTENUATOR on the front end, if
interference (distortion products), and not desired-signal-strength,
is the problem. Distortion products will go down faster than the
inserted attenuation.

Cheers,
Tom
 
P

Paul Burridge

Jan 1, 1970
0
On 29 Jul 2003 17:23:42 GMT, [email protected] (Michael Black)
wrote:

[comments snipped]

Thanks, Michael. I'm archiving all these lengthier posts on the
subject for future reference. All this info is valuable to me and will
be thorougly digested at some point.
 
J

John Crighton

Jan 1, 1970
0
Hi John,

I've already got an AM tx and rx set-up, but am loathed to use it due
to the much increased risk of interference from the speed controllers,
motors, etc., which as you will know, is far more likely with an AM
system. Keep thinking!
--

Hello Paul,
I would try out your old AM set for sure. You never know
your luck. If the servo connections are compatible it will
only take a short time. What brand/model?

I agree with you that the AM set has an increased risk
of interference from your own motors but you said some
time ago that your robot works fine at your place but at
the venue with other competitors and their transmitters
around, the interference is bad for you with your present
Sanwa FM set.

Even if you do not fit the AM set to the robot, take the
working AM set in a box to the venue and see if the
servos misbehave in that electrical noisy environment.

Regards,
John Crighton.
Sydney
 
P

Paul Burridge

Jan 1, 1970
0
Hello Paul,
I would try out your old AM set for sure. You never know
your luck. If the servo connections are compatible it will
only take a short time. What brand/model?

It's a Futaba and it *has* been tried. We used it last time out solely
for weapon actuation and the result was *outrageously* dangerous and
uncontrolable. Far, far worse than FM. You wouldn't have believed it
if you hadn't seen it for yourself. It was pure luck that no one was
killed or seriously injured.
Even if you do not fit the AM set to the robot, take the
working AM set in a box to the venue and see if the
servos misbehave in that electrical noisy environment.

See above. We won't be using that set-up ever again for this
application!
 
J

John Crighton

Jan 1, 1970
0
See above. We won't be using that set-up ever again for this
application!
--

Hello Paul,
I think you are missing my point when you said,
"We won't be using that set-up ever again for this application!"

My point is this. If the radio control set works OK, while just
sitting in its own cardboard box , at the noisy Venue, meaning,
the servos work nice and smooth. If you then install that same
Rx and servo set into your metal box robot and the servos
play up, that is now an " installation problem."
You cannot blame the gear.


Let's try and sort this out with some basic checks.

Using a field strength meter (which is just a simple
crystal set with a large moving coil meter as discussed
months ago, I assume you have made one already), are
both the Sanwa and Futaba transmitters producing
similar output power when compared to a known
good working transmitter? Yes or No?

At your place, are range checks of both the Sanwa
and Futaba R/C sets, on there own, not installed
in anything, over 100 yards . Yes or No?

At your place, are range checks of both the Sanwa and
Futaba R/C sets installed in the robot or metal test box
with "no" drive motors connected still over 100 yards.
Yes or No?

At your place, are range checks of both Sanwa and
Futaba with drive motors being controlled and
running nicely, still over 100 yards. Yes or No?

At the noisy Venue, while doing a range check of less than
30 yards with all other competitors absent or their
transmitters switched off in the Tx compound, do both
your Sanwa and Futaba R/C sets play up?
Yes or no?

At The Venue, do other competitor's radio control
sets play up like yours Yes or No?

At the Venue have you scanned the band with a
simple crystal set type radio or fancy scanner for some
bastard with a transmitter who is determined to give
you, personally, a hard time?

One last thought, I take it that you have sorted your
aerial out so that you do not have a long dangly
piece of wire as an antenna lead-in, inside the
metal robot body from the base of your whip antenna
mounting bracket to the Rx input.
If you do have a long wire lead-in, that is bad as it
will pick up local motor noise very nicely.
If your Rx is a long way away from your antenna base,
use coax for the lead-in as explained here.
http://homepages.which.net/~paul.hills/Radio/Radio.html

If after all that basic stuff has been checked and still no joy
then consider a dual conversion superhet Rx. like this one.
http://www.norcim.fsnet.co.uk/Index.htm#U
You could scratch build from the given circuit and the
description of how it works on that web site.
Or buy the kit from Micron for 32 quid.

If you are still having problems after all that,
sorry Paul, I give up.

Regards,
John Crighton
Sydney
 
A

Active8

Jan 1, 1970
0
Hi chaps,

I've decided to bite the bullet and try to build an RF filter for
40Mhz. This filter will ideally have a very, very sharp characteristic
at one single spot frequency +-20Khz and attenuate the crap out of
anything either side of this. It'll need to be tunable over a range of
say 200Khz. Can anyone give me a steer on what type of arrangement
would be best suited to fit this purpose?
Thanks,

p.

i just read this thread and had a few thoughts. i don't know what kind
of interference you're dealing with, nor do i know what restrictions are
placed on your competitions.

i was thinking about front end overload, also. the types of interference
vary in different countries, but those paging towers and, taxis, etc.
all play hell on a front end. you really need a spectrum analyser to
figure out what you're deaaling with.

as for those ceramic resonators, they're ok, but in the better
receivers, they're followed by if xfmrs to get rid of the spurious
response of the ceramic filter.

since you are thinking of just starting from scratch, here's what i'd do
if you're not bound by rules.

http://www.aerocomm.com
http://www.radiometrix.com

cost? i dunno. i figure if you can afford one, good. then you can spend
more time on the robotics and weaponry.

if you can use these, the question becomes a matter of whether you can
retrofit one of these in time. i'm not ready to look into using them yet
and therefore haven't gotten into the details. so i don't know what
you'd have to do to get a PWM signal in and out, but since they handle a
higher data rate (1 Mbps) than std R/C PWM, you could bust the PWM
signal up into little "chips" and reconstruct it on the receiving end.
maybe an integrator/LPF would be all you need. on the other hand, maybe
you can just send" the PWM directly. I'm sure the applications engineers
could help.

hope this helps.

mike
 
A

Active8

Jan 1, 1970
0
sorry, i did mean PPM, not PWM

a link was provided earlier. might be this, or it may link to this.

there're links to radios for robot wars. according to this page there
are 2 bands allowed, so maybe those 2.4GHz radios are verbotten. that
sucks! it's a robot competition, not a ham competition.

http://homepages.which.net/~paul.hills/Radio/Radio.html

mike
 
P

Paul Burridge

Jan 1, 1970
0
My point is this. If the radio control set works OK, while just
sitting in its own cardboard box , at the noisy Venue, meaning,
the servos work nice and smooth. If you then install that same
Rx and servo set into your metal box robot and the servos
play up, that is now an " installation problem."
You cannot blame the gear.


Let's try and sort this out with some basic checks.

Using a field strength meter (which is just a simple
crystal set with a large moving coil meter as discussed
months ago, I assume you have made one already), are
both the Sanwa and Futaba transmitters producing
similar output power when compared to a known
good working transmitter? Yes or No?

Thanks again for your input, John.
I only have these two r/c model transmitters to test against
themselves. It would be a remarkable coincidence if *both* were
defective to the same degree. I can, however, confirm that they both
put out almost exactly the same power as each other. I infer from that
that there is no problem with either.
At your place, are range checks of both the Sanwa
and Futaba R/C sets, on there own, not installed
in anything, over 100 yards . Yes or No?
Yes.

At your place, are range checks of both the Sanwa and
Futaba R/C sets installed in the robot or metal test box
with "no" drive motors connected still over 100 yards.
Yes or No?
Yes.

At your place, are range checks of both Sanwa and
Futaba with drive motors being controlled and
running nicely, still over 100 yards. Yes or No?

Ah, well I haven't tried that one. Good suggestion. I'll do so as soon
as I have a colleague handy to assist!
At the noisy Venue, while doing a range check of less than
30 yards with all other competitors absent or their
transmitters switched off in the Tx compound, do both
your Sanwa and Futaba R/C sets play up?
Yes or no?

No one's permitted to carry out such tests at the venue. As soon as
each competitor arrives, all transmitters are immediately impounded
and only released on a book-out/book-in basis under supervision of one
of the organisers' technical bods and only then if no one else is
using that frequency elsewhere in the building at the time.
At The Venue, do other competitor's radio control
sets play up like yours Yes or No?

It's a very common problem indeed, yes.
At the Venue have you scanned the band with a
simple crystal set type radio or fancy scanner for some
bastard with a transmitter who is determined to give
you, personally, a hard time?

It's a pity I sold my last spectrum analyser about the same time as I
started this lark! Big mistake, that. :-( However, I'm given to
understand the studio people have one and keep an eye open for any
such 'irregularities'.

[other valid points noted and snipped]

Things are looking very much better this time than last. I've
discovered a good many possible causes of the problems we had last
time out and have remedied all those I've found. I've also carried out
improvements suggested here by your good self and others. I'm leaving
nothing to chance this time!
If you are still having problems after all that,
sorry Paul, I give up.

So will I.
 
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