On Tue, 23 May 2006 12:16:27 +0100, "Reg Edwards"
wrote:

Replace the meter scale with a white card and calibrate it yourself
with pen and ink. You will also need a signal generator and an
attenuator.

If you derive the S-meter reading from the a.g.c. control voltage, be
prepared to perform the calibration at various temperatures, since the
gain of simple IF-strips vary with temperature (and thus need a
different amount of a.g.c.), unless the gain is controlled by passive
components only.

With various front end filter for various frequency bands and
attenuation depending of the frequency, you would have to perform the
calibration on several frequencies.

Instead of a pen and ink, it would be more practical to use a micro
controller with sufficient non-volatile storage for the conversion
tables and use analog inputs for a.g.c. voltage and temperature and
some kind of input for the frequency being received. With these inputs
and the conversion table, a voltage proportional of the log of the
antenna terminal power could be generated.

Paul OH3LWR

"Paul Keinanen" wrote in message
...
On Tue, 23 May 2006 12:16:27 +0100, "Reg Edwards"
wrote:

Replace the meter scale with a white card and calibrate it yourself
with pen and ink. You will also need a signal generator and an
attenuator.

If you derive the S-meter reading from the a.g.c. control voltage,
be
prepared to perform the calibration at various temperatures, since
the
gain of simple IF-strips vary with temperature (and thus need a
different amount of a.g.c.), unless the gain is controlled by
passive
components only.

With various front end filter for various frequency bands and
attenuation depending of the frequency, you would have to perform
the
calibration on several frequencies.

Instead of a pen and ink, it would be more practical to use a micro
controller with sufficient non-volatile storage for the conversion
tables and use analog inputs for a.g.c. voltage and temperature and
some kind of input for the frequency being received. With these
inputs
and the conversion table, a voltage proportional of the log of the
antenna terminal power could be generated.

Paul OH3LWR
===========================================

I don't doubt that the problem is capable of being solved.

Just throw enough electronic hardware and software at it.

The cost can be estimated at 100,000 Euros.

The size and weight of the receiver will be quadrupled.

No amateur would be able to afford buying one.

American tax-payers would prevent NASA from doing the research and
development work. Even Bush would object.

As for me and most other amateurs, I'm quite happy with an S-meter
reading which puts signal strength reports in the right ball-park.
Very strong, very weak, or indifferent.
----
Reg.

On Tue, 23 May 2006 16:26:43 +0100, "Reg Edwards"
wrote:


"Paul Keinanen" wrote in message
.. .
On Tue, 23 May 2006 12:16:27 +0100, "Reg Edwards"
wrote:

Replace the meter scale with a white card and calibrate it yourself
with pen and ink. You will also need a signal generator and an
attenuator.

If you derive the S-meter reading from the a.g.c. control voltage, be
prepared to perform the calibration at various temperatures, since the
gain of simple IF-strips vary with temperature (and thus need a
different amount of a.g.c.), unless the gain is controlled by passive
components only.

With various front end filter for various frequency bands and
attenuation depending of the frequency, you would have to perform the
calibration on several frequencies.

Instead of a pen and ink, it would be more practical to use a micro
controller with sufficient non-volatile storage for the conversion
tables and use analog inputs for a.g.c. voltage and temperature and
some kind of input for the frequency being received. With these inputs
and the conversion table, a voltage proportional of the log of the
antenna terminal power could be generated.

Paul OH3LWR
===========================================

I don't doubt that the problem is capable of being solved.
Just throw enough electronic hardware and software at it.
The cost can be estimated at 100,000 Euros.
The size and weight of the receiver will be quadrupled.
No amateur would be able to afford buying one.
American tax-payers would prevent NASA from doing the research and
development work. Even Bush would object.

While NASA might have needed such sums in the 1960's, the
functionality suggested above should be doable these days with any
micro controller with a few kilobytes of non-volatile storage.

While the frequency, power level and temperature calibration points
might require a three dimensional array, in practice, it should be
acceptable to get a few single dimensional vectors for each amateur
band. One vector for a specific band might contain the front end
frequency response for 5-10 frequencies within and around a band.

Getting the a.g.c. setting for different power levels for a fixed
station at three different temperatures (say +10, +20 and +30 C) or
five for a mobile station (with additional values for, say -50 and +70
C) should be enough. A few kilobytes should be more than enough to get
the sample points for interpolation.

As for me and most other amateurs, I'm quite happy with an S-meter
reading which puts signal strength reports in the right ball-park.
Very strong, very weak, or indifferent.

Exchanging RS(T) reports is so stupid, so I try to avoid it.

The only meaningful exchange would be to exchange the field strength
(V/m) or the power density (W/m²) in order to study the propagation
conditions. Even if we could reliably measure the antenna terminal
voltage (e.g. in dBuV) or power (dBm), we still would have to measure
the antenna efficiency and capture area to get any meaningful
readings.

Paul OH3LWR

Will take a look at URL:
http://www.ac6v.com/sunit.htm


Maybe the National Semiconductor NE604 IF amplifier IC would be worth
looking into. Claims say it provides an accurate signal strength logarithmic
output that closely tracks the input signal level over a wide dynamic range
that could possibly be used for driving an S meter circuit.

--
CL -- I doubt, therefore I might be !


"Will" wrote in message
...
I was wondering how commercial equipment go about calibrating the S meter
and whats the best way of building a calibrated S meter that would be
reliable as a professional field strength meter?

Since most S meters just read AGC voltage and it would be easy to build
some interface with a CPU to read voltages and calibrate this way. The
question is this, is a S meter calibrated this way actually reading in a
accurate way whats occurring at the antennas terminal. Since most antenna
inputs are not 50 ohms J0, how would you design a system like a spectrum
analyzer that measures signal voltage at the antenna terminal?. I would be
interested in some ideas, i am homebrewing a shortwave receiver however i
wanted a calibrated S meter in Dbuv, Dbm, S units and Millivolts. I also
want to use a calibrated antenna for a Antenna factor input to have a
meaningful long term view of propagation signal strength.

Thanks
Will

"Caveat Lector" wrote in message
news:64Gcg.177511$bm6.76448@fed1read04...
Will take a look at URL:
http://www.ac6v.com/sunit.htm


Maybe the National Semiconductor NE604 IF amplifier IC would be worth
looking into. Claims say it provides an accurate signal strength
logarithmic
output that closely tracks the input signal level over a wide dynamic
range
that could possibly be used for driving an S meter circuit.

--
CL -- I doubt, therefore I might be !


This is ok, but the one thing is that any Signal strength IC like the 604
has a constant slope and the S-Meter will be rather compressed in the 0-9
s-units range compared to the over S9 range (obviously fixable with meter
scale). I don't recall the 604's dynamic range. You could make a two slope
correction above S9, but I'd have to do some scratch paper work to figure
out the circuit [ probably as simple as a resister and diode in the right
place]. Or use two of them and change the gain into one to get the
different slope. I don't recall the 604's dynamic range. I have some
SLx16...oops can't remember the numbers (? 1316?... 1613? ) , that are
cascaded for large dynamic range log amps.

Just some ideas.

73, Steve, K9DCI