Thanks.
To the best of my understanding the AGC is not audio derived. I recall an
firmware option with the Orion series that allowed the operator to choose
which system from which the AGC was derived. My recall is that the audio
derived system had several shortcomings.
The AGC is exactly that, an automatic gain control. Don't confuse this with
AVC {automatic volume control}systems known to be used in earlier radios.
These indeed were audio derived and usually with very long decay times to
minimize gain pumping. In most radios today, usually there's at least 2 AGC
loops, one controlled by the RF Gain and other circuits to prevent overload
and another inside the DSP loop to prevent overload of the DSP system. The
operator usually has control of the loop outside of the loop for the DSP
system. Since digital systems have a fixed number of bits for processing,
exceeding that bit range dynamically causes a big "splat" where the system
doesn't know how to process the excess number. Thus the need for the DSP
loop to control the dynamic range within the DSP system.
In reading through the posts, I see some dispersion in this discussion
between the Orion II and other Tentec radios such as the Omni VII and Eagle
that I have on the desk. Perhaps this where some of the confusion exists.
Someone made reference to the Omni VI Plus. This is a100% analog radio all
the way, with exception to the audio DSP system, thus with regard to
Attenuation and such pertaining to S meter display, all bets are off. I've
owned 2 of these and, over the years, repaired numerous ones of the Omni and
Paragon series.
The Collins 75S-3B RF Gain does affect the S meter reading in that reducing
the RF Gain causes the S meter to move up scale accordingly. Again this is
basically applying a DC control voltage to the RF and IF stages in effect to
reduce the gain. Hence the S meter then shows the amount of gain reduction
in S units. Any signal over that value is correctly displayed on the S
meter.
Sorry for any confusion between the radios.
73
Bob, K4TAX
----- Original Message -----
From: <k3miy@csonline.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Tuesday, April 24, 2012 9:51 PM
Subject: Re: [TenTec] 566 Orion II Firmware Version
V3.029---SpectrumDisplay&Tuning...
> Bob
>
> Best description of the system I've seen today. In the modern rig,
> is the S meter in the agc loop, and is the agc audio derived?
> Thanks
>
> Ron
> K3MIY
>
>
> Quoting Bob McGraw - K4TAX <RMcGraw@Blomand.net>:
>
>> 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
>> >> _______________________________________________
>> >> TenTec mailing list
>> >> TenTec@contesting.com
>> >> http://lists.contesting.com/mailman/listinfo/tentec
>> >>
>> >>
>> >
>> >
>> >
>> >
>> > -------------------------------------------------------------------
>> > This mail sent through CSOnline Webmail http://webmail.csonline.net
>> >
>> > _______________________________________________
>> > TenTec mailing list
>> > TenTec@contesting.com
>> > http://lists.contesting.com/mailman/listinfo/tentec
>> >
>>
>>
>> _______________________________________________
>> TenTec mailing list
>> TenTec@contesting.com
>> http://lists.contesting.com/mailman/listinfo/tentec
>>
>>
>
>
>
>
> -------------------------------------------------------------------
> This mail sent through CSOnline Webmail http://webmail.csonline.net
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|