Hey Bob,

I am feeding a signal from my RF signal generator to the circuit on my
workbench. I mention harmonics because the sinewave goes from nice and
clean to "blurry" and looking "smeared" across the screen of my O-scope.
I may *be* overloading it, but I thought that would result in clipping of
the waveform. I have the signal generator set to attenuate the signal
severely, and *thought* that would prevent overloading. Maybe not...

Back to work on it some more, and try to make sure I am not overloading
the device.

Thanks,

Dave

Hi again Dave,
It is also possible that the circuit is oscillating. I have seen amps are
ok with no input signal but will break into oscillation when a certain
freq/amplitude input signal is applied (and vice versa). How did you build
this thing? Circuit board, protoboard, dead bug.....???

Bob

"Bob Liesenfeld" wrote in message
...
Hey Bob,

I am feeding a signal from my RF signal generator to the circuit on my
workbench. I mention harmonics because the sinewave goes from nice and
clean to "blurry" and looking "smeared" across the screen of my O-scope.
I may *be* overloading it, but I thought that would result in clipping of
the waveform. I have the signal generator set to attenuate the signal
severely, and *thought* that would prevent overloading. Maybe not...

Back to work on it some more, and try to make sure I am not overloading
the device.

Thanks,

Dave

Hi again Dave,
It is also possible that the circuit is oscillating. I have seen amps
are ok with no input signal but will break into oscillation when a certain
freq/amplitude input signal is applied (and vice versa). How did you
build this thing? Circuit board, protoboard, dead bug.....???

Bob


Hey Bob, thanks for your interest.

I built it on a piece of perfboard with point to point wiring, clustering
the active components and their biasing resistors fairly closely. I don't
*think* it is oscillating, although to wouldn't swear to anything just now.

I have the signal attenuated as much as possible, to the point where it
doesn't even cause a wiggle on the scope with my X10 probes and the scope
set to 50mV/div. With the tuner set to resonate, it shows up as about 30 or
35 mV (5 MHz) going into the circuit. The output is 70 or 80 mV, but it is
"blurred" and "smeared" as I described. Also, I just realized that the
scope shows *2* traces, each identicle to the other (only using one channel,
and one scope probe.) I can upload pictures of all this if desired,
probably to alt.binaries.schematics.electronic. Don't know how else to
describe what I am seeing. How could one signal become two traces, each
identicle to the other?

Thanks much,

Dave

Wrong resistor value = bad biasing = harmonics? Just a S.W.A.G?

Dave wrote:

"laura halliday" wrote in message
roups.com...


On Apr 7, 7:57 pm, "Uncle Peter" wrote:


"Dave" wrote in message

...



What do you do for resistors if you can't get carbon composition in the
ratings you need? I built a MFJ-1020A out of surplus and scrap before


I can't imagine that the resistors have enough inductance to
make any difference at HF. Plus, the reactive component
is in series with the resistance, which would increase the
impedance of the resistance slightly--in most cases that
would be more of a benefit than a hinderance; since I'd
bet most of the resistors are being used for biasing active
components, or to provide signal isolation..

Does anyone have any evidence that it makes a hill of
beans difference at HF?


Lots of stuff that works should be ample evidence.

There are some numbers in Experimental Methods for
RF Design that suggest such resisters are fine to low UHF.
It's really a non-issue.

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89lg pied a terre..."
ICBM: 49 16.57 N 123 0.24 W - Hospital/Shafte





Hmmm. Okay, well, could the problem be the monolithic ceramic caps I used?
Something is creating a boatload of harmonics, and that's the only other
component besides the transistors themselves.

Thanks much for the input...

Dave





--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

From: Ian White GM3SEK on Sun, 8 Apr 2007
12:03:18 +0100

John A wrote:
"Dave" wrote in message

What do you do for resistors if you can't get carbon composition in the
ratings you need?

Do what the professionals do - use film resistors. Concerns about inductive
effect at HF are greatly exaggerated. There are rarely as many "turns" as
often suggested. ( cf. Radcom Jan 2007, p58, fig 1! )

As the person who wrote that article, I strongly agree.

Hello Ian,

I don't get a chance to read Radio Communications often and didn't
see your article. As a professional in the design end, I'll offer
a few comments:

There is still (needless) confusion in amateurism as to metal
film resistors' "inductance" in comparison to wire-wound
resistors which DO have considerable self-inductance.

While there IS some self-inductance in metal film resistors
(due to laser-trimming and patterns of film on the usually
ceramic substrate), it is difficult as @#$%!!! to measure and
easier (but still grudge work) to model as a conductive strip
spiral-wound on the same physical dimensions. For nearly all
amateur applications up to and including 6m, that won't be
noticeable. With some caveats, of course.

Self-inductance of metal-film resistors will vary depending on
the manufacturer and their methods. So will construction which
adds varying self-capacitance from the end-caps (metal) holding
the wire leads. Self-capacitance is easier to measure on a Q-
Meter but is seldom over a half a pFd. That results in an
equivalent of a resistor in series with self-inductance, the
whole in parallel with self-capacitance. The effect on a
circuit depends on WHERE it is placed in the circuit. I've
found that carbon-composition resistors - in general - have a
slightly higher self-capacitance...but that depends on who
made them and what internal structures were involved (has to
be broken and observed if no X-Ray machine is handy).

As a dummy load consisting of many smaller resistors in series-
parallel, one can estimate the total capacitance and inductance
based on individual resistor models arranged in whatever
combination is planned. Offhand, I'd say that rarely does
that affect the dummy load's VSWR beyond 1.3 at 6m. In
arranging a series-parallel combination, there will probably
be more effect from whatever conductors' shape are in doing
the interconnects...less so if on a PCB, probably more if by
wires. A good rule-of-thumb is simply "make all connections
as short as possible, consistent with allowing air flow to
dissipate heat."

The only place to get paranoid about effects of self-inductance
and self-capacitance is in metrology. Metrology NEEDS to have
a minimum of each and to have accurate resistance values at
the rated frequencies. Everyday dummy loads for amateur radio
are far from lab-quality metrological stuff and don't need to
be in that precision range.

73, Len AF6AY

On Apr 8, 9:23 am, "Dave" wrote:
"Bob Liesenfeld" wrote in message

...



Hey Bob,

I am feeding a signal from my RF signal generator to the circuit on my
workbench. I mention harmonics because the sinewave goes from nice and
clean to "blurry" and looking "smeared" across the screen of my O-scope.
I may *be* overloading it, but I thought that would result in clipping of
the waveform. I have the signal generator set to attenuate the signal
severely, and *thought* that would prevent overloading. Maybe not...

Back to work on it some more, and try to make sure I am not overloading
the device.

Thanks,

Dave

Hi again Dave,
It is also possible that the circuit is oscillating. I have seen amps
are ok with no input signal but will break into oscillation when a certain
freq/amplitude input signal is applied (and vice versa). How did you
build this thing? Circuit board, protoboard, dead bug.....???

Bob

Hey Bob, thanks for your interest.

I built it on a piece of perfboard with point to point wiring, clustering
the active components and their biasing resistors fairly closely. I don't
*think* it is oscillating, although to wouldn't swear to anything just now.

I have the signal attenuated as much as possible, to the point where it
doesn't even cause a wiggle on the scope with my X10 probes and the scope
set to 50mV/div. With the tuner set to resonate, it shows up as about 30 or
35 mV (5 MHz) going into the circuit. The output is 70 or 80 mV, but it is
"blurred" and "smeared" as I described. Also, I just realized that the
scope shows *2* traces, each identicle to the other (only using one channel,
and one scope probe.) I can upload pictures of all this if desired,
probably to alt.binaries.schematics.electronic. Don't know how else to
describe what I am seeing. How could one signal become two traces, each
identicle to the other?

Thanks much,

Dave

A picture would help. This sounds to me like it _could_ be just the
scope. Seeing two traces (obviously not really identical or they
would look like a single trace...maybe you're seeing two copies of the
same thing, one shifted in time from the other?) tells me that the
scope may be triggering or syncing twice per cycle. What kind of
scope? Do you have full control over the triggering? If so, try
varying the trigger level.

If you see a sine wave that's clipped (flat-topped) on the top and/or
bottom, that's an overload of the active devices in the circuit,
assuming you fed your circuit a good sinewave to begin with. If you
see a trace that is widened out, it may well be oscillations on top of
your desired signal.

There are lots of things you can do to determine what the problem
really is. I'd guess it's unlikely that it's the resistors. I've
used 2W metal-oxide spiral film resistors out to 450MHz for a dummy
load, and the load had a very low reflection coefficient out to 2
meters, and quite usable at 450MHz, per an HP network analyzer.

In fact, carbon composition may NOT actually be better! They may show
less inductance, but greater variation in resistance, than film-type.

In addition, if the resistors are used to bias transistors, and not as
RF loads, a little self-inductance is not necessarily a bad thing. At
first I thought your circuit was an RF bridge or some such, but
apparently that's not the case. I do think you've barked up the wrong
tree, and just need to find the right one. Leave no tree unbarked??

Cheers,
Tom

"K7ITM" wrote in message
oups.com...
On Apr 8, 9:23 am, "Dave" wrote:
"Bob Liesenfeld" wrote in message

...



Hey Bob,

I am feeding a signal from my RF signal generator to the circuit on my
workbench. I mention harmonics because the sinewave goes from nice
and
clean to "blurry" and looking "smeared" across the screen of my
O-scope.
I may *be* overloading it, but I thought that would result in clipping
of
the waveform. I have the signal generator set to attenuate the signal
severely, and *thought* that would prevent overloading. Maybe not...

Back to work on it some more, and try to make sure I am not
overloading
the device.

Thanks,

Dave

Hi again Dave,
It is also possible that the circuit is oscillating. I have seen amps
are ok with no input signal but will break into oscillation when a
certain
freq/amplitude input signal is applied (and vice versa). How did you
build this thing? Circuit board, protoboard, dead bug.....???

Bob

Hey Bob, thanks for your interest.

I built it on a piece of perfboard with point to point wiring, clustering
the active components and their biasing resistors fairly closely. I
don't
*think* it is oscillating, although to wouldn't swear to anything just
now.

I have the signal attenuated as much as possible, to the point where it
doesn't even cause a wiggle on the scope with my X10 probes and the scope
set to 50mV/div. With the tuner set to resonate, it shows up as about 30
or
35 mV (5 MHz) going into the circuit. The output is 70 or 80 mV, but it
is
"blurred" and "smeared" as I described. Also, I just realized that the
scope shows *2* traces, each identicle to the other (only using one
channel,
and one scope probe.) I can upload pictures of all this if desired,
probably to alt.binaries.schematics.electronic. Don't know how else to
describe what I am seeing. How could one signal become two traces, each
identicle to the other?

Thanks much,

Dave

A picture would help. This sounds to me like it _could_ be just the
scope. Seeing two traces (obviously not really identical or they
would look like a single trace...maybe you're seeing two copies of the
same thing, one shifted in time from the other?) tells me that the
scope may be triggering or syncing twice per cycle. What kind of
scope? Do you have full control over the triggering? If so, try
varying the trigger level.

If you see a sine wave that's clipped (flat-topped) on the top and/or
bottom, that's an overload of the active devices in the circuit,
assuming you fed your circuit a good sinewave to begin with. If you
see a trace that is widened out, it may well be oscillations on top of
your desired signal.

There are lots of things you can do to determine what the problem
really is. I'd guess it's unlikely that it's the resistors. I've
used 2W metal-oxide spiral film resistors out to 450MHz for a dummy
load, and the load had a very low reflection coefficient out to 2
meters, and quite usable at 450MHz, per an HP network analyzer.

In fact, carbon composition may NOT actually be better! They may show
less inductance, but greater variation in resistance, than film-type.

In addition, if the resistors are used to bias transistors, and not as
RF loads, a little self-inductance is not necessarily a bad thing. At
first I thought your circuit was an RF bridge or some such, but
apparently that's not the case. I do think you've barked up the wrong
tree, and just need to find the right one. Leave no tree unbarked??

Cheers,
Tom


Hey Tom,

I'll try to upload a jpeg of the scope trace, and possibly the schematic and
setup producing it. Would love to solve this mystery. There was a time when
I had this active antenna working wonderfully. Now it's not much use, and I
can't figure out why.

Regards,
Dave

"Dave" wrote in message
...

"K7ITM" wrote in message
oups.com...
On Apr 8, 9:23 am, "Dave" wrote:
"Bob Liesenfeld" wrote in message

...



Hey Bob,

I am feeding a signal from my RF signal generator to the circuit on
my
workbench. I mention harmonics because the sinewave goes from nice
and
clean to "blurry" and looking "smeared" across the screen of my
O-scope.
I may *be* overloading it, but I thought that would result in
clipping of
the waveform. I have the signal generator set to attenuate the
signal
severely, and *thought* that would prevent overloading. Maybe not...

Back to work on it some more, and try to make sure I am not
overloading
the device.

Thanks,

Dave

Hi again Dave,
It is also possible that the circuit is oscillating. I have seen
amps
are ok with no input signal but will break into oscillation when a
certain
freq/amplitude input signal is applied (and vice versa). How did you
build this thing? Circuit board, protoboard, dead bug.....???

Bob

Hey Bob, thanks for your interest.

I built it on a piece of perfboard with point to point wiring,
clustering
the active components and their biasing resistors fairly closely. I
don't
*think* it is oscillating, although to wouldn't swear to anything just
now.

I have the signal attenuated as much as possible, to the point where it
doesn't even cause a wiggle on the scope with my X10 probes and the
scope
set to 50mV/div. With the tuner set to resonate, it shows up as about
30 or
35 mV (5 MHz) going into the circuit. The output is 70 or 80 mV, but it
is
"blurred" and "smeared" as I described. Also, I just realized that the
scope shows *2* traces, each identicle to the other (only using one
channel,
and one scope probe.) I can upload pictures of all this if desired,
probably to alt.binaries.schematics.electronic. Don't know how else to
describe what I am seeing. How could one signal become two traces, each
identicle to the other?

Thanks much,

Dave

A picture would help. This sounds to me like it _could_ be just the
scope. Seeing two traces (obviously not really identical or they
would look like a single trace...maybe you're seeing two copies of the
same thing, one shifted in time from the other?) tells me that the
scope may be triggering or syncing twice per cycle. What kind of
scope? Do you have full control over the triggering? If so, try
varying the trigger level.

If you see a sine wave that's clipped (flat-topped) on the top and/or
bottom, that's an overload of the active devices in the circuit,
assuming you fed your circuit a good sinewave to begin with. If you
see a trace that is widened out, it may well be oscillations on top of
your desired signal.

There are lots of things you can do to determine what the problem
really is. I'd guess it's unlikely that it's the resistors. I've
used 2W metal-oxide spiral film resistors out to 450MHz for a dummy
load, and the load had a very low reflection coefficient out to 2
meters, and quite usable at 450MHz, per an HP network analyzer.

In fact, carbon composition may NOT actually be better! They may show
less inductance, but greater variation in resistance, than film-type.

In addition, if the resistors are used to bias transistors, and not as
RF loads, a little self-inductance is not necessarily a bad thing. At
first I thought your circuit was an RF bridge or some such, but
apparently that's not the case. I do think you've barked up the wrong
tree, and just need to find the right one. Leave no tree unbarked??

Cheers,
Tom


Hey Tom,

I'll try to upload a jpeg of the scope trace, and possibly the schematic
and setup producing it. Would love to solve this mystery. There was a
time when I had this active antenna working wonderfully. Now it's not
much use, and I can't figure out why.

Regards,
Dave



Just posted jpegs of the scopetrace/schematic/device under test to
alt.binaries.schematics.electronic. Please let me know if I can provide
anything else for this discussion. Thanks to all...

Dave

I'm off to fiddle with the triggering level, which I meant to do earlier.

AF6AY wrote:

Hello Ian,

I don't get a chance to read Radio Communications often and didn't
see your article. As a professional in the design end, I'll offer
a few comments:


Hello Len

I beg your pardon, that was a different article by someone else. The one
that I wrote, on the same subject, was in Radcom for July 2005.

Even so, both articles were saying the same thing - and you have written
almost exactly the same again below! Nothing uncanny about that, of
course: we are all looking at the same resistors, and noticing the same
things.


There is still (needless) confusion in amateurism as to metal
film resistors' "inductance" in comparison to wire-wound
resistors which DO have considerable self-inductance.

It seems to be part of amateur radio's eternal quest to reduce the whole
of RF engineering to a series of simple one-liners, like "carbon
composition = non-inductive = good" and "spiral = inductive = bad".

The vital part that gets squeezed out is always "by how much"?


While there IS some self-inductance in metal film resistors
(due to laser-trimming and patterns of film on the usually
ceramic substrate), it is difficult as @#$%!!! to measure and
easier (but still grudge work) to model as a conductive strip
spiral-wound on the same physical dimensions.

I was able to make some measurements of (R+jX) using the N2PK VNA, with
a test jig made from PC board and copper foil, and some care in the
choice of open, short and 50R calibration standards. Sweeping from 60kHz
to 60MHz gave almost constant R, and a "good straight line" of X against
frequency. This implied that a simple series L-R model would be valid,
and that self-capacitance effects were not significant over the measured
frequency range.


For nearly all
amateur applications up to and including 6m, that won't be
noticeable. With some caveats, of course.

Exactly so.


Self-inductance of metal-film resistors will vary depending on
the manufacturer and their methods.

Yes, very much so.

All of these tubular metal film resistors are based on a cylindrical rod
of ceramic, with metal end caps connecting to the wire leads. A
continuous layer of resistive film is deposited on the surface of the
ceramic to form a continuous cylinder. A range of resistance values can
then be achieved by cutting away some of the metal film - normally they
remove a narrow spiral of material, leaving behind a broad spiral ribbon
of film.

The resistance value will depend on the width and the total number of
turns in this ribbon. The self-inductance will depend mostly on the
number of turns (along with the other body dimensions, of course). When
the number of turns gets much above about 10, the ribbon becomes quite
narrow, which makes it difficult to keep control over the resistance
tolerance. At that point, the manufacturer will switch to a base
material of higher resistivity, so the next-higher value of resistor
will drop back to having the minimum number of spiral turns; and so the
cycle repeats.

This has two important results:

1. Some values of resistor will have significantly more or less
inductance than others. All the way up the resistance range, from
typically1 ohm to 10Mohm, there will be a series of break-points where
the inductance flips between roughly the maximum and minimum possible
values.

2. These break-points will vary from one manufacturer to another - for a
given standard value of resistance, the inductance could be wildly
different (ask Elecraft about that one!)

For 3W resistors using 3-4 spiral turns, I measured about 4nH. This
correlated fairly well with the value calculated using the usual
inductance formula.

Even in the worst case of about 10 spiral turns, the inductance would
only be about 150nH, which is low enough for most RF applications up to
30MHz. For example, in a 50 ohm resistor, 150nH of series inductance
would increase the VSWR to about 1.75 at 30MHz.

But that is very much a worst case. If VSWR matters at all, you would
use a larger number of higher-value resistors connected in parallel, as
Len describes below. This would divide the effect of the inductance by
the number of resistors used.


So will construction which
adds varying self-capacitance from the end-caps (metal) holding
the wire leads. Self-capacitance is easier to measure on a Q-
Meter but is seldom over a half a pFd. That results in an
equivalent of a resistor in series with self-inductance, the
whole in parallel with self-capacitance. The effect on a
circuit depends on WHERE it is placed in the circuit.

And the frequency, of course.

Agreed with everything else too...

I've
found that carbon-composition resistors - in general - have a
slightly higher self-capacitance...but that depends on who
made them and what internal structures were involved (has to
be broken and observed if no X-Ray machine is handy).

As a dummy load consisting of many smaller resistors in series-
parallel, one can estimate the total capacitance and inductance
based on individual resistor models arranged in whatever
combination is planned. Offhand, I'd say that rarely does
that affect the dummy load's VSWR beyond 1.3 at 6m. In
arranging a series-parallel combination, there will probably
be more effect from whatever conductors' shape are in doing
the interconnects...less so if on a PCB, probably more if by
wires. A good rule-of-thumb is simply "make all connections
as short as possible, consistent with allowing air flow to
dissipate heat."

The only place to get paranoid about effects of self-inductance
and self-capacitance is in metrology. Metrology NEEDS to have
a minimum of each and to have accurate resistance values at
the rated frequencies. Everyday dummy loads for amateur radio
are far from lab-quality metrological stuff and don't need to
be in that precision range.

73, Len AF6AY




--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

All of the discussion about metal film resistors and a pF here or there of
stray capacitance is interesting, but the symptom sounds to me a lot like a
cold solder joint.

...

"John A" wrote in message
...

"Dave" wrote in message
...

What do you do for resistors if you can't get carbon composition in the
ratings you need?

Do what the professionals do - use film resistors. Concerns about
inductive
effect at HF are greatly exaggerated. There are rarely as many "turns" as
often suggested. ( cf. Radcom Jan 2007, p58, fig 1! )


where can I get carbon composition resistors these days?

They are still available. Typically, Farnell offer 220R to 4K7 at 1W -
but
they are quite expensive.


John A

"Ian White GM3SEK" wrote in message
...

I beg your pardon, that was a different article by someone else. The one
that I wrote, on the same subject, was in Radcom for July 2005.


Ahh! I was wondering how to break the news to you, Ian!

BTW, I was making a critical reference to that figure in G3LDO's article. It
is figures like that that reinforce perceptions of significant inductance in
such resistors.

John A