Hi,
I am contemplating a pep reading wattmeter so I can check the output of my HB
amplifier. Consequently, I am putting it right on the output of the amp. The
output of the amp should always see 50 ohms because it will be feeding either a
50 ohm dummy load, or an ATU tuned to 50 ohms. With that in mind, I am simply
using a resistive voltage divider, to get a voltage sample, and squaring it
with an AD633 multiplier. This seems like a simple, cheap way to get watt
info. assuming you will always be working into a near 50 ohm resistive load.
Anyone see any reason why this will not give you a pretty good indication of
your power output? I realize that the load may not always be exactly 50 ohms,
and that there are losses in the ATU.
The reason I have not tried getting a current sample and using the
conventional VI COS Theta with the multiplier is due to the additional
complexity of circuitry. It is also difficult to get accurate current samples
over a wide frequency range. By making everything resistive it somewhat takes
the frequency dependency out of the problem. Thanks.
73 Gary N4AST

This should be fine if you're not after a great deal of accuracy.

I think the biggest problem you'll have is insuring that the load really
is 50 ohms and resistive, and that your divider is doing what you think.
It doesn't take very much inductance at all at HF to produce a reactance
that's a good sized fraction of 50 ohms. Also, if you're tapping into a
transmission line, it can be extremely difficult to maintain a 50 ohm
environment at the measurement point. I don't really know what level of
accuracy you can easily achieve, but I do see the possibility for some
fairly large errors to creep in without being obvious. I'd recommend
trying to calibrate against some known standard if at all possible, to
get an idea of what kind of accuracy you're achieving.

Roy Lewallen, W7EL

JGBOYLES wrote:
Hi,
I am contemplating a pep reading wattmeter so I can check the output of my HB
amplifier. Consequently, I am putting it right on the output of the amp. The
output of the amp should always see 50 ohms because it will be feeding either a
50 ohm dummy load, or an ATU tuned to 50 ohms. With that in mind, I am simply
using a resistive voltage divider, to get a voltage sample, and squaring it
with an AD633 multiplier. This seems like a simple, cheap way to get watt
info. assuming you will always be working into a near 50 ohm resistive load.
Anyone see any reason why this will not give you a pretty good indication of
your power output? I realize that the load may not always be exactly 50 ohms,
and that there are losses in the ATU.
The reason I have not tried getting a current sample and using the
conventional VI COS Theta with the multiplier is due to the additional
complexity of circuitry. It is also difficult to get accurate current samples
over a wide frequency range. By making everything resistive it somewhat takes
the frequency dependency out of the problem. Thanks.
73 Gary N4AST

In article ,
says...

Hi,
I am contemplating a pep reading wattmeter so I can check the output of my
HB
amplifier. Consequently, I am putting it right on the output of the amp. The
output of the amp should always see 50 ohms because it will be feeding either
a
50 ohm dummy load, or an ATU tuned to 50 ohms. With that in mind, I am simply
using a resistive voltage divider, to get a voltage sample, and squaring it
with an AD633 multiplier. This seems like a simple, cheap way to get watt
info. assuming you will always be working into a near 50 ohm resistive load.
Anyone see any reason why this will not give you a pretty good indication of
your power output? I realize that the load may not always be exactly 50 ohms,
and that there are losses in the ATU.
The reason I have not tried getting a current sample and using the
conventional VI COS Theta with the multiplier is due to the additional
complexity of circuitry. It is also difficult to get accurate current samples
over a wide frequency range. By making everything resistive it somewhat takes
the frequency dependency out of the problem. Thanks.
73 Gary N4AST

Resistive dividers don't work worth a crap at rf, unless you use
non-inductive resistors, and then you must consider that your input
to the squaring circuit will have some capacitance to ground, who's
ability to screw up your higher frequency readings will be astronomical.
Why the squaring circuit?

Bottom line is this, all the resistive ladders I have played with
were worthless unless they contained capacitive compensation, which
can be found only by trial and error. I prefer a 'T' connection with
a good quality 'coaxial pad' of 10 db, then I feed it to my Tek 2215A
scope. It's deadly accurate from D.C. to at least 30mhz. More accurate
than any of my inline wattmeters, with the exception of the 'BIRD', and
that is only good across the spectrum of the slug I use.

The problem with a plain resistive divider is that the detector (or, in
the case of a scope probe, the scope input) invariably has an associated
shunt C. Without compensation, the lower arm of the divider gets less
voltage than it should, due to that shunt C. If the capacitive reactance
is high compared to the resistance of the lower divider arm at the
highest frequency of interest, then you can probably get by without
shunt capacitors. But otherwise, you need to make a compensated
attenuator by putting C across at least the upper divider arm.

The rule for making a compensated divider is that the time constants of
the top and bottom should be equal. That is, R1C1 = R2C2 where R1 and C1
are the parallel R and C at the bottom of the divider, and R2 and C2 at
the top.

This kind of divider has a theoretically flat frequency response, that
is, the voltage at the tap is constant assuming that the voltage applied
to the top of the divider is constant. But there's the rub. The
impedance of the divider is *not* constant with frequency -- because of
the C, it decreases as frequency increases. So depending on the circuit
it's put across, above some frequency the divider will begin disturbing
the circuit being measured due to the divider's too-low impedance.
Consequently, the design procedure is usually to first minimize the
detector or load C. This allows you to use the smallest possible C in
the top part of the divider, resulting in minimized loading of the
circuit to be measured. In general, you don't want to add physical C to
the lower part of the divider, unless the load (detector) C is variable,
unpredictable, or nonlinear, which requires it to be swamped by a known,
fixed, good-quality C.

Even such a simple circuit isn't trivial, if very accurate division and
wide frequency response (or fast and low-aberration time response) are
required. "Hook", "soak", and other nonlinear effects are well-known to
designers of oscilloscope front ends, as are the tricks and skills
necessary to avoid them. But these problems are much worse for high
impedance dividers than ones designed to operate in a low impedance
environment like 50 ohms.

Roy Lewallen, W7EL

gudmundur wrote:

Resistive dividers don't work worth a crap at rf, unless you use
non-inductive resistors, and then you must consider that your input
to the squaring circuit will have some capacitance to ground, who's
ability to screw up your higher frequency readings will be astronomical.
Why the squaring circuit?

Bottom line is this, all the resistive ladders I have played with
were worthless unless they contained capacitive compensation, which
can be found only by trial and error. I prefer a 'T' connection with
a good quality 'coaxial pad' of 10 db, then I feed it to my Tek 2215A
scope. It's deadly accurate from D.C. to at least 30mhz. More accurate
than any of my inline wattmeters, with the exception of the 'BIRD', and
that is only good across the spectrum of the slug I use.

Why do it the hard way. Use a bidirectional coupler made for the frequency
range and power level of the amp such as the Bird 4266(1500 Watts). Then
you can measure power, SWR or hook up your counter, oscope, or spectrum
analyzer etc. without fear of destroying them. It gives 30 dB isolation from
the line and is flat in frequency response ( plus/minus 0.5 dB or less 1.5
to 40 Mhz measured here.)

"Roy Lewallen" wrote in message
...
The problem with a plain resistive divider is that the detector (or, in
the case of a scope probe, the scope input) invariably has an associated
shunt C. Without compensation, the lower arm of the divider gets less
voltage than it should, due to that shunt C. If the capacitive reactance
is high compared to the resistance of the lower divider arm at the highest
frequency of interest, then you can probably get by without shunt
capacitors. But otherwise, you need to make a compensated attenuator by
putting C across at least the upper divider arm.

The rule for making a compensated divider is that the time constants of
the top and bottom should be equal. That is, R1C1 = R2C2 where R1 and C1
are the parallel R and C at the bottom of the divider, and R2 and C2 at
the top.

This kind of divider has a theoretically flat frequency response, that is,
the voltage at the tap is constant assuming that the voltage applied to
the top of the divider is constant. But there's the rub. The impedance of
the divider is *not* constant with frequency -- because of the C, it
decreases as frequency increases. So depending on the circuit it's put
across, above some frequency the divider will begin disturbing the circuit
being measured due to the divider's too-low impedance. Consequently, the
design procedure is usually to first minimize the detector or load C. This
allows you to use the smallest possible C in the top part of the divider,
resulting in minimized loading of the circuit to be measured. In general,
you don't want to add physical C to the lower part of the divider, unless
the load (detector) C is variable, unpredictable, or nonlinear, which
requires it to be swamped by a known, fixed, good-quality C.

Even such a simple circuit isn't trivial, if very accurate division and
wide frequency response (or fast and low-aberration time response) are
required. "Hook", "soak", and other nonlinear effects are well-known to
designers of oscilloscope front ends, as are the tricks and skills
necessary to avoid them. But these problems are much worse for high
impedance dividers than ones designed to operate in a low impedance
environment like 50 ohms.

Roy Lewallen, W7EL

gudmundur wrote:

Resistive dividers don't work worth a crap at rf, unless you use
non-inductive resistors, and then you must consider that your input
to the squaring circuit will have some capacitance to ground, who's
ability to screw up your higher frequency readings will be astronomical.
Why the squaring circuit?

Bottom line is this, all the resistive ladders I have played with
were worthless unless they contained capacitive compensation, which
can be found only by trial and error. I prefer a 'T' connection with
a good quality 'coaxial pad' of 10 db, then I feed it to my Tek 2215A
scope. It's deadly accurate from D.C. to at least 30mhz. More accurate
than any of my inline wattmeters, with the exception of the 'BIRD', and
that is only good across the spectrum of the slug I use.

I keep telling you, apparently to no avail, all you need is a TLI.

Stop confusing yourselves.
---
Reg

I keep telling you, apparently to no avail, all you need is a TLI.

Hi Reg, What I am attempting is to measure the PEP output power of a homebrew
solid state amplifier. The amp should provide about 20 db power gain. A TLI
will assure me I have a 50 ohm match. That was given in the orginal post.
What I was looking for was a fairly accurate wattmeter, assuming the load was
around 50 ohms. I was wanting to put a voltage divider at the input to my ATU.
I have several replies stating the voltage divider must be frequency
compensated to have a chance of accuracy. I have a surplus 100:1 scope probe I
will take a look, and report back.
73 Gary N4AST