[Elecraft] Fwd: An AGC Story

[Bob Henderson]

Sorry Geoff.

My setup is fairly basic but I believe quite adequate.  I use two +10dBm low
noise xtal oscillators operated within the 40m band.  The output of each is
attenuated 20dB before combining via a Minicircuits SCP-2-1.  Thereafter the
signals pass through an 8MHz LPF to a step attenuator.  The SCP-2-1 provides
around 33dB port to port isolation at 7MHz.  Isolation between the
oscillator output stages is therefore around 73dB.

73 Bob, 5B4AGN


On 12 December 2010 10:04, Guy Olinger K2AV <olinger at bellsouth.net> wrote:

> Thank you, Bob.
>
> But you have robbed Geoff of his favorite whipping boy.  We will have
> to take Geoff out to a pub and help him drown his sorrows.
>
> What was your setup?
>
> 73, Guy
>
> On Sat, Dec 11, 2010 at 11:11 PM, Bob Henderson <bob at 5b4agn.net> wrote:
> > Bottom posted for context.
> >
> > Geoff GM4ESD wrote
> >
> > "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 input levels.  IMHO the higher the
> > HAGC's threshold can be the better, provided its control is tight
> > enough when activated to protect all what follows."
> >
> >
> > I made within DSP b/w IMD measurements on my K3 and comparable
> > measurements on my K2. These were included in a post dated 5 December
> > but you may have missed it.
> >
> > The following measurements relate to K3 S/N 4904 on 7MHz with 2.7kHz
> > roofing filter and 2.7kHz DSP b/w.  Default slow AGC was selected.
> > The table shows input level at the K3 antenna connector for
> > each of two carriers spaced 500Hz.  At each input level, the level of
> > the strongest IMD product observed at the AF output using an HP8568B is
> recorded
> > alongside.
> >
> > -73dBm  Better than -55dBc  (Input S9)
> > -63dBm  -55dBc
> > -53dBm  -50dBc
> > -42dBm  -45dBc
> > -33dBm  -42dBc
> > -23dBm  -37dBc  (Input S9+50)
> >
> > These figures are good and IMHO entirely acceptable.  Some older
> > transceivers would struggle to provide -35dBc at S9.
> >
> > I made a similar set of measurements with my old K2 S/N 997
> >
> > -73dBm  -50dBc
> > -63dBm  -48dBc
> > -53dBm  -45dBc
> > -43dBm  -48dBc
> > -33dBm  -45dBc
> > -23dBm  -15dBc
> >
> > Broadly speaking the K3 is as good or better than the K2, which is
> > itself good. It is possible later mods to the K2 may have made it even
> > better.  The most recent mods to my K2 were carried out over 5 years ago.
> >
> > 73 Bob, 5B4AGN
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