An AGC story

[Geoffrey Mackenzie-Kennedy]

Chen,

It is not really my place to comment on this subject not having a K3 to
measure, but I would like to offer the following thoughts related to the
"naughty things" you mention.

Whereas measuring the receiver's audio output level vs.the level of a single
signal injected into the antenna input port will obviously provide some good
information about the AGC's control characteristics, unfortunately this
measurement does not provide any hard data as to whether or not
intermodulation generated in the IF stages will become or is an issue when
two or more strong signals get through the roofer in use.

Given that the 1db compression point of a typical SA612 mixer is reached
when the *composite* power of all signals reaching its input is
approximately minus 25dbm, and given that positive gain exists between the
antenna port and the SA612's input when both the Preamp and Attenuator are
OFF, in the region of 6db I believe when a 400 Hz roofer is in place, then
in the *absence* of any HAGC action the SA612 would enter compression when
the *composite* power of all signals at the antenna port that eventually get
through the roofer is only minus 31dbm. At this level *without* HAGC applied
there would be, of course, a large number of of strong odd order intermod
products appearing at the SA612's output should multiple signals get through
the roofer.

Now bring in the HAGC.

Because part of the method used to control IF gain by the HAGC involves
current starvation of the J309 IF amplifier, it is *possible* that with HAGC
applied the Output Intercept (OIP3) of this amplifier might decrease,
thereby increasing the level of IMD products generated by this amplifier.

So, even though the HAGC might prevent the SA612 from approaching
compression, much of the HAGC's protective work in terms of reducing
unwanted IMD products *might* be undone by the J309 when it is starved of
current. Please understand that this is pure speculation on my part.

If I had a K3, before I changed the HAGC threshold my *first* measurement
would be to look at the output of the SA612 mixer (with a spectrum analyser)
while introducing two or more very close spaced signals into the antenna
port, so that these signals do get through the roofer - as in a pileup. The
objective, of course, would be to obtain an acceptably clean output from the
mixer with non intrusive products over a wide range of antenna input levels.
IMHO the higher the HAGC's threshold can be the better, provided that its
control is tight enough when activated to protect all what follows.

73,
Geoff
GM4ESD


Kok Chen wrote on Thursday, December 09, 2010, at 7:05 PM:

> We know that the S9+ signals have to be kept from saturating the A/D
> converter (or even kept in check so the large signal doesn't do
> naughty things to the IF amplifier and the SA612 second mixer -- I
> don't know which of these three is the wimpiest in the chain).  So
> some AGC is neccessary.

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[juergen piezo]

Hi Geoff

The noise power ratio test(NPR) might show more clearly any mixer IMD problems. The NPR test might also better characterize any roofing filter IMD as well.

The noise power ratio test  better characterizes dynamic receiver performance. The NPR test  also does not require expensive low phase noise signal generators.

73
John

--- On Sat, 12/11/10, Geoffrey Mackenzie-Kennedy <[hidden email]> wrote:

> From: Geoffrey Mackenzie-Kennedy <[hidden email]>
> Subject: Re: [Elecraft] An AGC story
> To: "Kok Chen" <[hidden email]>
> Cc: "Elecraft Discussion List" <[hidden email]>
> Date: Saturday, December 11, 2010, 5:26 AM
> Chen,
>
> It is not really my place to comment on this subject not
> having a K3 to
> measure, but I would like to offer the following thoughts
> related to the
> "naughty things" you mention.
>
> Whereas measuring the receiver's audio output level vs.the
> level of a single
> signal injected into the antenna input port will obviously
> provide some good
> information about the AGC's control characteristics,
> unfortunately this
> measurement does not provide any hard data as to whether or
> not
> intermodulation generated in the IF stages will become or
> is an issue when
> two or more strong signals get through the roofer in use.
>
> Given that the 1db compression point of a typical SA612
> mixer is reached
> when the *composite* power of all signals reaching its
> input is
> approximately minus 25dbm, and given that positive gain
> exists between the
> antenna port and the SA612's input when both the Preamp and
> Attenuator are
> OFF, in the region of 6db I believe when a 400 Hz roofer is
> in place, then
> in the *absence* of any HAGC action the SA612 would enter
> compression when
> the *composite* power of all signals at the antenna port
> that eventually get
> through the roofer is only minus 31dbm. At this level
> *without* HAGC applied
> there would be, of course, a large number of of strong odd
> order intermod
> products appearing at the SA612's output should multiple
> signals get through
> the roofer.
>
> Now bring in the HAGC.
>
> Because part of the method used to control IF gain by the
> HAGC involves
> current starvation of the J309 IF amplifier, it is
> *possible* that with HAGC
> applied the Output Intercept (OIP3) of this amplifier might
> decrease,
> thereby increasing the level of IMD products generated by
> this amplifier.
>
> So, even though the HAGC might prevent the SA612 from
> approaching
> compression, much of the HAGC's protective work in terms of
> reducing
> unwanted IMD products *might* be undone by the J309 when it
> is starved of
> current. Please understand that this is pure speculation on
> my part.
>
> If I had a K3, before I changed the HAGC threshold my
> *first* measurement
> would be to look at the output of the SA612 mixer (with a
> spectrum analyser)
> while introducing two or more very close spaced signals
> into the antenna
> port, so that these signals do get through the roofer - as
> in a pileup. The
> objective, of course, would be to obtain an acceptably
> clean output from the
> mixer with non intrusive products over a wide range of
> antenna input levels.
> IMHO the higher the HAGC's threshold can be the better,
> provided that its
> control is tight enough when activated to protect all what
> follows.
>
> 73,
> Geoff
> GM4ESD
>
>
> Kok Chen wrote on Thursday, December 09, 2010, at 7:05 PM:
>
> > We know that the S9+ signals have to be kept from
> saturating the A/D
> > converter (or even kept in check so the large signal
> doesn't do
> > naughty things to the IF amplifier and the SA612
> second mixer -- I
> > don't know which of these three is the wimpiest in the
> chain).  So
> > some AGC is neccessary.
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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>
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>

     
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[Geoffrey Mackenzie-Kennedy]

Hi John,

The NPR test is, I agree, certainly a very useful method to expose IMD
problems, but I think that when used as a *diagnostic* tool the requirements
placed on the notch filter's bandwidth limits this method's versatility -
unless one is prepared to build a set of band reject filters (probably
crystal) that may be required, whose IMD contribution is much less that that
of the receiver, device or sub-system being tested.

To obtain low phase noise from my IMD test arrangement I have been using low
noise crystal oscillators followed by +40dbm output amplifiers, which is I
admit a cumbersome approach, but it did provide useful results until I
damaged the equipment while measuring the IMD generated by some types of RF
transformer - a sad story!

If Santa gave me a K3 I could stop speculating about its multi signal
performance :-)

73,
Geoff
GM4ESD


"juergen" <[hidden email]> wrote on Saturday, December 11, 2010, at
6:18 PM:

> Hi Geoff

> The noise power ratio test(NPR) might show more clearly any mixer IMD
> problems. The NPR test might also better characterize any roofing filter
> IMD
> as well.

> The noise power ratio test  better characterizes dynamic receiver
> performance. The NPR test  also does not require expensive low phase noise
> signal generators.

> 73
> John




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[alorona]

Months ago I had heard a rumor that the P3 could eventually function as a
read-only display that could inform the operator of these various settings.

Al  W6LX

(Subject: line changed to reflect the current topic.)

>
There's a great need for some simple utilities for the K3
that will run with LP-Bridge or other CAT port sharing schemes:  AGC/EQ
management, expanded rig display, subrx control (avoid BSET), etc

Barry N1EU
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