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

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.

Don't forget, an S-meter is a power or wattmeter. It indicates signal
STRENGTH. S9 = 50 pico-watts.
----
Reg.

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.

Don't forget, an S-meter is a power or wattmeter. It indicates signal
STRENGTH. S9 = 50 pico-watts.
----
Reg.


Yeah thats for that tip. But the question is how do you actually build a
piece of electronics that measures signal levels at the receiver
terminals. Or is it reading agc voltage just as accurate. The problem
the method you described is that it will vary from band to band.

I just wonder how field strength meters are designed, especially the new
solid state ones. Spectrum analyzers too have a pretty flat response. I
want to build this ability into my homebrew receiver much the same way
a selective level meter works. Maybe someone has done it.

Will

"Will" wrote
I just wonder how field strength meters are designed, especially the
new
solid state ones. Spectrum analyzers too have a pretty flat
response. I
want to build this ability into my homebrew receiver much the same
way
a selective level meter works. Maybe someone has done it.

===========================================

The accuracy of S-meters varies from one band to another because the
receiver input impedance varies from one band to another. Even from
one end of a band to the other, especially if the input stage is
tuned.

Also, the stage-gain of an RF stage can vary very much from one band
to another.

To ensure a consistent gain it is necessary to convert to the
intermediate frequency (IF) at the very first stage of a receiver.
Preferably using a high-level, balanced diode-bridge, modulator.

The IF stages can be assumed to have a constant, wide-band gain,
except, of course, for the automatic gain control (AGC) action.

Then ensure that the receiver input impedance is constant (usually 50
ohms) over the whole HF range of the receiver. Remember the S-meter
is a power or watt-meter.

It so happens that with dual-gate FET's, and 3 or 4 IF amplifier
stages, a meter which responds to AGC volts will fairly accurately
indicate S-units and dB above S9 in a linear fashion.

However, below about S4 the linearity of S-meter readings begins to
fail. And you will have to make and calibrate your own meter scale.
There's no way of avoiding it! That is unless you can mentally
visualise what the scale ought to look like as you use it.

Accurate commercial field strength meters use antennas dedicated to
the job, conjugate-matched to the receiver, are insensitive to weak
signals, are narrow band and are very expensive.

To obtain an S-meter scale to be proud of, you will have to use pen
and ink with a signal generator and 100-dB stepped attenuator.
Attempts to calibrate the scale with clever, highly complicated
electronics will get you nowhere in a long time. And will cost you
more than the remainder of the receiver.

Sorry to be so despondent.
----
Reg, G4FGQ

Reg Edwards wrote:
"Will" wrote
I just wonder how field strength meters are designed, especially the
new
solid state ones. Spectrum analyzers too have a pretty flat
response. I
want to build this ability into my homebrew receiver much the same
way
a selective level meter works. Maybe someone has done it.

===========================================

The accuracy of S-meters varies from one band to another because the
receiver input impedance varies from one band to another. Even from
one end of a band to the other, especially if the input stage is
tuned.

Also, the stage-gain of an RF stage can vary very much from one band
to another.

To ensure a consistent gain it is necessary to convert to the
intermediate frequency (IF) at the very first stage of a receiver.
Preferably using a high-level, balanced diode-bridge, modulator.

The IF stages can be assumed to have a constant, wide-band gain,
except, of course, for the automatic gain control (AGC) action.

Then ensure that the receiver input impedance is constant (usually 50
ohms) over the whole HF range of the receiver. Remember the S-meter
is a power or watt-meter.

It so happens that with dual-gate FET's, and 3 or 4 IF amplifier
stages, a meter which responds to AGC volts will fairly accurately
indicate S-units and dB above S9 in a linear fashion.

However, below about S4 the linearity of S-meter readings begins to
fail. And you will have to make and calibrate your own meter scale.
There's no way of avoiding it! That is unless you can mentally
visualise what the scale ought to look like as you use it.

Accurate commercial field strength meters use antennas dedicated to
the job, conjugate-matched to the receiver, are insensitive to weak
signals, are narrow band and are very expensive.

To obtain an S-meter scale to be proud of, you will have to use pen
and ink with a signal generator and 100-dB stepped attenuator.
Attempts to calibrate the scale with clever, highly complicated
electronics will get you nowhere in a long time. And will cost you
more than the remainder of the receiver.

Sorry to be so despondent.
----
Reg, G4FGQ


Thanks Reg for the summary.

I think the Flex radio and the Winradio have the ability to be
calibrated accurately via a look up table. I always admired shortwave
receivers like the R&S EK 2000 which had a accurate meter calibrated
in the DbUv scale. I think i will just have to abandon my super
accurate S meter project.

Will

To obtain an S-meter scale to be proud of, you will have to use pen
and ink with a signal generator and 100-dB stepped attenuator.
Attempts to calibrate the scale with clever, highly complicated
electronics will get you nowhere in a long time. And will cost you
more than the remainder of the receiver.

Sorry to be so despondent.
----
Reg, G4FGQ
================================
Just an idea ; if one is really 'dead keen' to have an 'accurate ? '
S-meter readout, you could construct an indicator with individual
threshold opamp comparators ( up to 4 in a DIL package) each driving a LED.
When calibrating with a signal gen as a 50 Ohms source ,starting with
S-9 being 50 microvolts being -73dBm (or 5 microvolts being -93dBm for
VHF and higher) the relevant LEDs can then be set separately below that
level with 6dB steps and above S-9 with 10 dB steps by accepting the
quasi-log voltage range generated by the AGC as fed to the traditional
analogue S-meter
A fancy feature would be different colour LEDs showing signal strength
above S-9. I feel that ,while accepting any 'professional'
comments,this would be a practical 'amateur ' (low cost) solution .

Frank GM0CSZ / KN6WH an 'Amateur'

The way that spectrum analyzers are built is typically to use
calibrated attenuators and linear gain stages that have very low
distortion (and similarly low "compression"). Then it becomes a matter
of determining the voltage after amplification. They do NOT use AGC
voltage, or at least not anything like the AGC used in a typical ham
receiver. The way we do it here is to digitize the RF signal and do
some appropriate digital signal processing on it (e.g., FFT) to display
the spectrum and to calculate amplitudes and band powers and the like.
Modern digitizers are very linear indeed and can be used to measure
signal amplitudes over a range in excess of 120dB with relative
accuracy far better than an S meter over most of that range, and still
considerably better even at the bottom end of the range. It doesn't
even take a huge number of bits in the digitization to do it; consider
that a typical delta-sigma ADC is a one-bit converter followed by lots
of processing gain.

The way it can be done "on the cheap" is to use a calibrated attenuator
and a single known signal level. Then you compare your known signal
level with the unknown, adjusting the attenuator to bring your
(typically large) signal down to the same amplitude as the unknown.

For S-meter levels of accuracy, linear non-AGC'd stages feeding one of
the RF power detectors from Analog Devices, Linear Technology or others
will work fine. Most of them have an output voltage proportional to
the log of the input voltage, and so can be calibrated to read dB
linearly on a linear meter scale. If your receiver has a good front
end, it shouldn't need AGC up through the filter following the mixer,
and you could pick off there after the filter to drive the meter
circuit. That seems overkill, but it would get you a _good_ S-meter.
Then you'd have to calibrate out the front-end gain at least per band,
assuming you have at least some front end filtering that doesn't have
the same gain (loss) on each band.

Field strength meters that accurately measure an RF electromagnetic
field are basically spectrum analyzers fed by calibrated antennas.

That may be beyond what you wanted to know or do, but it should give
you a pretty accurate picture of how modern commercial gear actually
does make RF voltage measurements. You could add calibration (for
absolute amplitude accuracy as well as spectral flatness) to all that
as a whole 'nuther topic, though. For example, the amplitude
characteristics of any filters the signal passes through in the
spectrum analyzer must be properly accounted for, as must temperature
drifts in instruments with high accuracy.

Cheers,
Tom

I think I'd follow K7ITM's advice.

I'd split the signal right after the IF filters. Feed one of them into
the rest of the Rx chain, and the other one into one of the Analog devices
logarithmic detector chips.

This is assuming there is no AGC prior to the IF filters.

There are two major choices for the detector chip, AD8307, which is a
power detector, or a true RMS detector (AD8326??)

Feed the output into a A/D coinverter, and then digitally add correction
factors for the front end gain and possible different sensitivity on
different bands.

So, once you have the calibration factors, this wouls be a fairly accurate
iindicator of the input signal. Granted, it is assuming a matched 50 ohm
antenna, and you will get an error due to antenna reactances and so on.
However, it is as precise as an amateur radio system is likely to be able to
get and a heck of a lot more meaningful than most 'S' meters I've seen.


Jim
N6BIU

K7ITM wrote:
The way that spectrum analyzers are built is typically to use
calibrated attenuators and linear gain stages that have very low
distortion (and similarly low "compression"). Then it becomes a matter
of determining the voltage after amplification. They do NOT use AGC
voltage, or at least not anything like the AGC used in a typical ham
receiver. The way we do it here is to digitize the RF signal and do
some appropriate digital signal processing on it (e.g., FFT) to display
the spectrum and to calculate amplitudes and band powers and the like.
Modern digitizers are very linear indeed and can be used to measure
signal amplitudes over a range in excess of 120dB with relative
accuracy far better than an S meter over most of that range, and still
considerably better even at the bottom end of the range. It doesn't
even take a huge number of bits in the digitization to do it; consider
that a typical delta-sigma ADC is a one-bit converter followed by lots
of processing gain.

The way it can be done "on the cheap" is to use a calibrated attenuator
and a single known signal level. Then you compare your known signal
level with the unknown, adjusting the attenuator to bring your
(typically large) signal down to the same amplitude as the unknown.

For S-meter levels of accuracy, linear non-AGC'd stages feeding one of
the RF power detectors from Analog Devices, Linear Technology or others
will work fine. Most of them have an output voltage proportional to
the log of the input voltage, and so can be calibrated to read dB
linearly on a linear meter scale. If your receiver has a good front
end, it shouldn't need AGC up through the filter following the mixer,
and you could pick off there after the filter to drive the meter
circuit. That seems overkill, but it would get you a _good_ S-meter.
Then you'd have to calibrate out the front-end gain at least per band,
assuming you have at least some front end filtering that doesn't have
the same gain (loss) on each band.

Field strength meters that accurately measure an RF electromagnetic
field are basically spectrum analyzers fed by calibrated antennas.

That may be beyond what you wanted to know or do, but it should give
you a pretty accurate picture of how modern commercial gear actually
does make RF voltage measurements. You could add calibration (for
absolute amplitude accuracy as well as spectral flatness) to all that
as a whole 'nuther topic, though. For example, the amplitude
characteristics of any filters the signal passes through in the
spectrum analyzer must be properly accounted for, as must temperature
drifts in instruments with high accuracy.

Cheers,
Tom

Thanks Tom and Jim. The information you provided has given me something
to think about. Even though the task is complex it can be done. I will
experiment with some ideas and see if i can find a sollution.
Regardless how hard it is i think its worthwile pursuing a accurate S
meter.

Will

On Wed, 24 May 2006 15:55:36 +1000, Will wrote:

Regardless how hard it is i think its worthwile pursuing a accurate S
meter.

Will

It will make you very unpopular. People who are used to
getting S9+30dB reports get quite upset when you give
the a 57 :-)

73, Ed. EI9GQ.

--
Linux 2.6.16
Remove 'X' to reply by e-mail.
Yes, my username really is: nospam