According to Art Collins, generally credited with the adoption of the S
meter system, it was intended to give meaningful signal strength reports.
The old "green magic eye" on radios was a tuning indicator. Yes, the
original value per my Collins 75A-3B manual shows S-9 = 100 uV.
Sitting here this evening in front of my Omni VII listening to WWV on 10
MHz, at 100% RF Gain the S meter indicates S-9 +6dB. Switching in the ATTN
in 6 dB steps to 18 dB the S meter stays at S-9 +6dB. The answer is simple.
Add 18 dB of attenuation to the input, which automatically adds 18 dB of
gain to the S meter circuit. The opposite is true when the preamp is
engaged. Add 12 dB of gain to the receiver circuit which automatically adds
12 dB of loss to the S meter circuit. I refer to this as a smart S meter
system. SMART = Signal Measurement, Accurate, Reliable, True
The Eagle, no matter what one does in terms of Attenuation or RF Gain or
Preamp activation, the S meter reading does not change. Again, same
concept.
Think about it, if you are receiving a station at S-9 + 20 dB and you switch
in 18 dB of attenuation, did you change his signal strength at the input of
your radio? The answer is NO. You changed the signal at the input to the
receiver first stage. Then shouldn't the S meter always reflect the signal
strength regardless of what one did to their receiver gain system? Hence
you can always be assured that the S meter system in the Tentec Omni VII and
Eagle will reflect the correct value at the input to the radio.
Even with a signal at the input of -23 dBm or 15,811 uV which is 50 dB over
S 9, reduction of the RF Gain to 0% the S meter does not change. It still
shows the signal at 50 dB over S 9, hence the system inversely adds gain to
the S meter circuit to make up for the reduction in RF Gain. Therefore, the
S meter is correct reading regardless of RF Gain setting. Of course once
you remove the 50 dB over S 9 signal with RF Gain at 0% you will see the S
meter display drop to 40 dB over S 9. The point being that the RF Gain does
not affect the accuracy of the S meter reading above AGC threshold.
Now what one does see with no signal applied, as one reduces the RF Gain the
S meter moves upscale in 6 dB increments or 1 S unit. This reflects the
threshold of AGC reference to S 9 being 50 uV. At 100% RF Gain, as one
engages the ATTN function in 6 dB increments you will observe the S meter
indication move up scale 1 S unit per 6 dB. Now once reaching 18 dB, engage
the Preamp and you will see the S meter move down scale by 2 S units
indication that 12 dB of gain has been added.
All of the math works and so does the radio respond accordingly.
73
Bob, K4TAX
----- Original Message -----
From: <k3miy@csonline.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Tuesday, April 24, 2012 11:10 AM
Subject: Re: [TenTec] 566 Orion II Firmware Version V3.029---SpectrumDisplay
&Tuning...
You're right, Ray. In the old days, the S meter was regarded as a
tuning aid, and nothing else.
Ron
K3MIY
Quoting Rsoifer@aol.com:
Not only that, but the old Collins spec was 3 S units = 10 dB, not the
modern standard of 6 dB per S unit.
More broadly, I'm afraid I just don't understand the continuing
preoccupation with S meter and spectrum display issues. Neither has much
to
do with
performance, which is where I'd like to see T-T focus their attention.
Flak
jacket on!
73 Ray W2RS
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