On Jan 31, 12:31 pm, Cecil Moore wrote:
K7ITM wrote:
To me, having a
linear power scale is a big advantage, because then you can reasonably
accurately figure SWR without having to worry about temperature
compensation of the detectors.

Can you define what you mean by linear? Straight line?
Since we can only measure voltage and current, in order
to obtain a linear power scale from a linear meter, it
is necessary to supply some pre-display computing
ability (microcomputer).
--
73, Cecil http://www.w5dxp.com

See earlier posting in this thread. See various Avago ap notes, such
as AN 969. A diode detector run at low input provides an output DC
voltage that's a constant times the square of the input RF voltage.
If the input voltage is, or is assumed to be, at some constant
resistive load impedance, the DC output is linear with RF power
input. The proportionality is temperature dependent, but if two
detectors are constructed the same and run at the same temperature,
and run in the signal level region where that relationship holds, then
the ratio of the output DC voltages is a very good approximation of
the ratio of the input RF power levels, and thus is useful for finding
the SWR if the detectors are attached to the forward and reverse ports
of a good directional coupler. Top end of the useful "linear power"
range using an HSMS-2850 single diode detector is about 10mV DC
output. If you can measure the DC accurately down to 1uV (a bit
tough, given thermal emfs, but possible), that gives you about a
10000:1 power range, or 100:1 RF input voltage range -- or about
1.02:1 SWR. Chances are very good that a home-built coupler won't be
accurately enough matched to 50+j0 ohms to worry about anything that
low anyway, even if you had a reason to care about it.

Cheers,
Tom

K7ITM wrote:
See earlier posting in this thread.

Thanks Tom, when I said "linear power scale", I meant
e.g. a meter reading where 2000 watts is full scale
and 1000 watts is half scale. I have seen such meters
but not without a digital or analog computer on the
front end.
--
73, Cecil http://www.w5dxp.com

K7ITM wrote in news:9e844e58-a673-4ec0-9a0b-ec15f8cc8f30
@c4g2000hsg.googlegroups.com:

On Jan 31, 12:31 pm, Cecil Moore wrote:
K7ITM wrote:
To me, having a
linear power scale is a big advantage, because then you can
reasonably
accurately figure SWR without having to worry about temperature
compensation of the detectors.

Can you define what you mean by linear? Straight line?
Since we can only measure voltage and current, in order
to obtain a linear power scale from a linear meter, it
is necessary to supply some pre-display computing
ability (microcomputer).
--
73, Cecil http://www.w5dxp.com

See earlier posting in this thread. See various Avago ap notes, such
as AN 969. A diode detector run at low input provides an output DC
voltage that's a constant times the square of the input RF voltage.
If the input voltage is, or is assumed to be, at some constant
resistive load impedance, the DC output is linear with RF power
input. The proportionality is temperature dependent, but if two
detectors are constructed the same and run at the same temperature,
and run in the signal level region where that relationship holds, then
the ratio of the output DC voltages is a very good approximation of
the ratio of the input RF power levels, and thus is useful for finding
the SWR if the detectors are attached to the forward and reverse ports
of a good directional coupler. Top end of the useful "linear power"
range using an HSMS-2850 single diode detector is about 10mV DC
output. If you can measure the DC accurately down to 1uV (a bit
tough, given thermal emfs, but possible), that gives you about a
10000:1 power range, or 100:1 RF input voltage range -- or about
1.02:1 SWR. Chances are very good that a home-built coupler won't be
accurately enough matched to 50+j0 ohms to worry about anything that
low anyway, even if you had a reason to care about it.

Cheers,
Tom


Tom,

This is further from Suzy's needs, but...

Operation of a diode detector in the square law region isn't out of the
question, but it takes some serious gain to drive a meter. There are some
good chopper stabilised op amps out there that have uV offset levels and
single supply rail and input to below the negative rail eg LTC1050.

Another alternative is the AD8307AN log amps for a linear dBW scale. You
could even use one on FWD and REF detectors and difference the outputs in
an op amp for a direct indicating VSWR or RL scale. I have thought of
getting one of these chips and seeing whether its response is fast enough
to drive a PEP amplifier for SSB telephony.

Back to Suzy's problem...

The instrument downstream of the sampler is not so much the issue as
building and calibrating a sampler when you have no test gear.

Suzy, if you see a Revex W560 going on VKHAM for $100 or so, it is a good
buy. It has HF to 70cm (two independent couplers, ie four coax
connectors), and works pretty well.

For a dummy load, the market was flooded with terminations from 25W to
about 60W that had been scrapped from AMPS base station equipment, and
they were sold at hamfests for $20 or so, you may find them if you look
around.

Owen

Owen Duffy wrote:
Operation of a diode detector in the square law region isn't out of the
question, but it takes some serious gain to drive a meter. There are some
good chopper stabilised op amps out there that have uV offset levels and
single supply rail and input to below the negative rail eg LTC1050.

If the diode is DC-biased and the bias subtracted out
during calibration, doesn't that improve the low power
accuracy considerably?
--
73, Cecil http://www.w5dxp.com

"Richard Clark" wrote in message
...
On Fri, 1 Feb 2008 06:12:14 +1100, "Suzy" not@valid wrote:

I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

This is called residual SWR in a reflectometer. You load it with a
known good load, and what SWR you find (or what is exhibited by the
two meters) inhabits the reflectometer itself. Then you flip it over
and apply your source into the goesoutta with the known load on the
comesinna. You then proceed to reduce the residual SWR in both
directions.

Finding a good load is another matter, and I reported one (a precision
RF resistor) with specific characteristics here last week. Consult
the thread "RF Power Resistors from Caddock."

All of $10-$20 to accomplish.

73's
Richard Clark, KB7QHC

I'm in Australia!

"K7ITM" wrote in message
...
On Jan 31, 11:12 am, "Suzy" not@valid wrote:
Hello Tom

My responses**

"K7ITM" wrote in message
...
OK, I gave this some thought last night. I see a couple problems...

Though you could use 1mA meter movements, that puts you at a detected
power level high enough that the meter scale won't be linear in power,
assuming Schottky or germanium diode detectors.

**Pardon my ignorance, but isn't it just a case of using an op amp or
whatever to suit whatever meter movement I have? ANyway, I have now
sourced
a 100 microamp meter (MU65) with a 3.9K resistance (sounds strange as the
1
mA one has a 210 ohm resistance.

Ah, OK. I had assumed you wanted to use just the meters, with no
amplifier. With amplifiers, the 1mA meters will be fine. But we need
op amps that have very low offset voltage and drift--I would prefer to
set the meter full scale to correspond to around a millivolt or two of
detected DC. I suppose if you have a way to zero the offset and it
doesn't drift, that'll be OK. Then we need to make sure the amplifier
is reasonably immune to 450MHz signals floating around... This is all
"do-able" but there are some details you'll have to pay attention to.
I'd suggest using a couple of the RF power detector ICs available from
Analog Devices or Linear Technology, but we're back to surface mount
stuff again at that point.


BTW, I'n not bothered about the linear issue. I will be having two meters
to
show forward and relected powers simultaneously. I don't want to clculate
actual SWR.

Well, yes, but wouldn't you want to know whether "0.1" on the meter
represented 1/10 the power of "1.0" on the meter, rather than 1/100 of
the power?? If you don't pay a little attention to the level of RF
the detector is actually detecting, you're liable to have that
problem.


...
I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

What do you mean "no" test gear? You'll have the directional coupler
with meters itself, and a power source. The only other thing you need
is a 50 ohm load to put on the coupler output. Is it not worth having
at least a load you can trust? With a known good load, you feed some
power through the coupler and adjust for zero indicated return; turn
the coupler around and make sure the other port also reads zero. With
an open or short load you should get equal readings on the forward and
reverse meters. (I suppose you need two couplers and a good load to
insure that the through line of the coupler is also the same impedance
as the load...) I have a good network analyzer on my bench at work,
but without a calibration load to test and calibrate it with, I don't
know how good its readings really are.

Cheers,
Tom

Having been told so by two of you, looks like I'll have to trade a rolling
pin for a "known good load". Trouble is, there's probably no source here in
Australia.

K7ITM wrote:
On Jan 31, 12:31 pm, Cecil Moore wrote:

K7ITM wrote:

To me, having a
linear power scale is a big advantage, because then you can reasonably
accurately figure SWR without having to worry about temperature
compensation of the detectors.

Can you define what you mean by linear? Straight line?
Since we can only measure voltage and current, in order
to obtain a linear power scale from a linear meter, it
is necessary to supply some pre-display computing
ability (microcomputer).
--
73, Cecil http://www.w5dxp.com


And what makes one think that a standard meter is linear (unless your
standards are +/- 10% )


See earlier posting in this thread. See various Avago ap notes, such
as AN 969. A diode detector run at low input provides an output DC
voltage that's a constant times the square of the input RF voltage.
If the input voltage is, or is assumed to be, at some constant
resistive load impedance, the DC output is linear with RF power
input.

Not necessarily true for a coupler..


The proportionality is temperature dependent, but if two
detectors are constructed the same and run at the same temperature,
and run in the signal level region where that relationship holds, then
the ratio of the output DC voltages is a very good approximation of
the ratio of the input RF power levels, and thus is useful for finding
the SWR if the detectors are attached to the forward and reverse ports
of a good directional coupler.

where "good" is the operative word

Top end of the useful "linear power"
range using an HSMS-2850 single diode detector is about 10mV DC
output. If you can measure the DC accurately down to 1uV (a bit
tough, given thermal emfs, but possible), that gives you about a
10000:1 power range, or 100:1 RF input voltage range -- or about
1.02:1 SWR. Chances are very good that a home-built coupler won't be
accurately enough matched to 50+j0 ohms to worry about anything that
low anyway, even if you had a reason to care about it.

Which is why "real instruments" have some form of calibration. With
today's technology, there's really no excuse for not putting some form
of calibration into the logic between measurement and display, unless
all you're looking for is the RF equivalent of a battery and test lamp.

Heck, if you MUST use all analog designs and you're at less than 3GHz,
don't fool with diodes, use the less expensive, more sensitive, and more
accurate power measuring chips from Analog Devices.

Example: AD8310, DC-440MHz, 90+dB dynamic range (-91 to +4dBm) linear to
0.4dB, stable over temp(-40 to +85) +/-1 dB

or the 8319, 1MHz to 10GHz, 40dB range, similar accuracy

they also come in dual versions and versions with phase comparators..

These days, there's relatively few applications where a straight diode
detector would be better:
1) Absolute lowest cost in mass manufacturing with relaxed performance
requirements(so you can use a really cheap silicon diode)
(0.10 for a diode vs $3 for a chip is a $30 difference in the retail
list price)

2) very fast rise time requirements
The chips have response times in the 10 nS and slower range. The right
diode in the right mount can get sub nanoseconds.

3) frequencies 10 GHz
The chips don't go there yet.





Cheers,
Tom

"K7ITM" wrote in message
...
On Jan 31, 12:31 pm, Cecil Moore wrote:
K7ITM wrote:
To me, having a
linear power scale is a big advantage, because then you can reasonably
accurately figure SWR without having to worry about temperature
compensation of the detectors.

Can you define what you mean by linear? Straight line?
Since we can only measure voltage and current, in order
to obtain a linear power scale from a linear meter, it
is necessary to supply some pre-display computing
ability (microcomputer).
--
73, Cecil http://www.w5dxp.com

See earlier posting in this thread. See various Avago ap notes, such
as AN 969. A diode detector run at low input provides an output DC
voltage that's a constant times the square of the input RF voltage.
If the input voltage is, or is assumed to be, at some constant
resistive load impedance, the DC output is linear with RF power
input. The proportionality is temperature dependent, but if two
detectors are constructed the same and run at the same temperature,
and run in the signal level region where that relationship holds, then
the ratio of the output DC voltages is a very good approximation of
the ratio of the input RF power levels, and thus is useful for finding
the SWR if the detectors are attached to the forward and reverse ports
of a good directional coupler. Top end of the useful "linear power"
range using an HSMS-2850 single diode detector is about 10mV DC
output. If you can measure the DC accurately down to 1uV (a bit
tough, given thermal emfs, but possible), that gives you about a
10000:1 power range, or 100:1 RF input voltage range -- or about
1.02:1 SWR. Chances are very good that a home-built coupler won't be
accurately enough matched to 50+j0 ohms to worry about anything that
low anyway, even if you had a reason to care about it.

Cheers,
Tom

Much too theoretical for me!

Like Cecil's posts in that other longgoing thread, it leaves me glazed!

"Owen Duffy" wrote in message
...
K7ITM wrote in news:9e844e58-a673-4ec0-9a0b-ec15f8cc8f30
@c4g2000hsg.googlegroups.com:

On Jan 31, 12:31 pm, Cecil Moore wrote:
K7ITM wrote:
To me, having a
linear power scale is a big advantage, because then you can
reasonably
accurately figure SWR without having to worry about temperature
compensation of the detectors.

Can you define what you mean by linear? Straight line?
Since we can only measure voltage and current, in order
to obtain a linear power scale from a linear meter, it
is necessary to supply some pre-display computing
ability (microcomputer).
--
73, Cecil http://www.w5dxp.com

See earlier posting in this thread. See various Avago ap notes, such
as AN 969. A diode detector run at low input provides an output DC
voltage that's a constant times the square of the input RF voltage.
If the input voltage is, or is assumed to be, at some constant
resistive load impedance, the DC output is linear with RF power
input. The proportionality is temperature dependent, but if two
detectors are constructed the same and run at the same temperature,
and run in the signal level region where that relationship holds, then
the ratio of the output DC voltages is a very good approximation of
the ratio of the input RF power levels, and thus is useful for finding
the SWR if the detectors are attached to the forward and reverse ports
of a good directional coupler. Top end of the useful "linear power"
range using an HSMS-2850 single diode detector is about 10mV DC
output. If you can measure the DC accurately down to 1uV (a bit
tough, given thermal emfs, but possible), that gives you about a
10000:1 power range, or 100:1 RF input voltage range -- or about
1.02:1 SWR. Chances are very good that a home-built coupler won't be
accurately enough matched to 50+j0 ohms to worry about anything that
low anyway, even if you had a reason to care about it.

Cheers,
Tom


Tom,

This is further from Suzy's needs, but...

Operation of a diode detector in the square law region isn't out of the
question, but it takes some serious gain to drive a meter. There are some
good chopper stabilised op amps out there that have uV offset levels and
single supply rail and input to below the negative rail eg LTC1050.

Another alternative is the AD8307AN log amps for a linear dBW scale. You
could even use one on FWD and REF detectors and difference the outputs in
an op amp for a direct indicating VSWR or RL scale. I have thought of
getting one of these chips and seeing whether its response is fast enough
to drive a PEP amplifier for SSB telephony.

Back to Suzy's problem...

The instrument downstream of the sampler is not so much the issue as
building and calibrating a sampler when you have no test gear.

Suzy, if you see a Revex W560 going on VKHAM for $100 or so, it is a good
buy. It has HF to 70cm (two independent couplers, ie four coax
connectors), and works pretty well.

For a dummy load, the market was flooded with terminations from 25W to
about 60W that had been scrapped from AMPS base station equipment, and
they were sold at hamfests for $20 or so, you may find them if you look
around.

Owen

Thanks Owen. BTW, what type of coax connector? Not PAL surely!

Suzy wrote:
"Richard Clark" wrote in message
...

On Fri, 1 Feb 2008 06:12:14 +1100, "Suzy" not@valid wrote:


I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

This is called residual SWR in a reflectometer. You load it with a
known good load, and what SWR you find (or what is exhibited by the
two meters) inhabits the reflectometer itself. Then you flip it over
and apply your source into the goesoutta with the known load on the
comesinna. You then proceed to reduce the residual SWR in both
directions.

Finding a good load is another matter, and I reported one (a precision
RF resistor) with specific characteristics here last week. Consult
the thread "RF Power Resistors from Caddock."

All of $10-$20 to accomplish.

73's
Richard Clark, KB7QHC


I'm in Australia!

http://www.caddock.com/ has a link to their Australian distributor
(granted, they may have some punitive minimum order).

A begging letter to Caddock for a sample might work?

Or, do the big mailorder companies ship to Australia (the parts are in
the few bucks range.. postage and shipping could be more)