That's why for the HF bands S9 is defined as a receiver input signal of
50 microvolts at 50 Ohms input impedance , translated as -73dBm
For frequencies higher than 30 MHz S9 is defined as 5 microvolts at 50
Ohms input impedance , translated as -93dBm.

The S9 signal strength was first set by Art Collins of Collins Radio
fame during the 1940s and was later accepted by IARU.
This organisation also adopted the 5 microvolt level for a S9 signal for
VHF and higher frequency bands.

Frank GM0CSZ / KN6WH

================================
It can be misleading to think in terms of S9 = 50 uV when one doesn't
know what the receiver input impedance is.
----
Reg, G4FGQ

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'

"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

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

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 :-)
======================================

Why would a serious radio amateur trying hard to use his equipment in
the best way possible be required to be popular ?

Frank GM0CSZ / KN6WH

Highland Ham wrote:

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

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

Why would a serious radio amateur trying hard to use his equipment in
the best way possible be required to be popular ?

Frank GM0CSZ / KN6WH

Zoom!

wrote:
On Tue, 23 May 2006 12:48:18 -0400, (Fred McKenzie)
wrote:

In article ,
wrote:

Two caveats, rarely are RX input impedences 50 ohms and there is no
guarentee that the antenna is 50 ohms.
Allison-

Such is life. However you can assume that both are 50 Ohms and use a
resistive pad between the signal generator and the receiver when
calibrating. When making a reading, use a pad between the unknown signal
source and the receiver.

Without a sophisticated system capable of determining impedances while
making measurements, this may be the best you can do.

Fred

Yes, you can do all that and more easily. Often it means little. I
rarely use my comms as measurement recievers as I have dedicated
equipment for that. For those the available devices from Analog
devices are excellent.

For general on the air work S9 meaning whatever is mostly
convenience in aiming the antennas and even then peak is
the criteria. However I've used at least one radio where the
S-meter was so sluggish on slow AGC and so twitchy on fast
AGC that using for aiming antennas was annoying.

I'd also said for most radios I've encountered the meter even if S9
was a calibrated point the the interval between points was at best
wildly inaccurate. For that case the calibated point is at best moot
and for any measurements the ide will be to keep a fixed point as
reference and use attenuators to assign value above or below that
point.


Allison

I found the N6NB method using a VU meter with the AGC off very useful.
However using a receiver with the AGC off is unbearable. I would have
thought that a radio like the Ten Tec Orion could have easily
implemented a calibrated S meter. However without hams demanding it and
hams willing to live with the Guess Meter manufacturers wont bother.

There is a fax station in Germany running 20 kw or so, into a vertical
antenna on 13,882 or somewhere around their. Its quite useful comparing
it to the many ham stations running beams and even low power with beams.
Its also a useful propagation beacon. Its been on for years and i
wanted to measure the signal strength and compare it to what VOACAP
predicts.

In private emails some people have indicated that Winradio has a
calibration software option for their receivers. So that may be another
option. Some users have also indicated that the RFSPACE DSP receiver
likewise is quite accurate with amplitude measurements.

There are options...

Will