[R-390] Tuning SSB

Larry Haney larry41gm2 at gmail.com
Mon Sep 30 14:40:05 EDT 2024 

Barry,  The 2nd diode (across R547) could be switched in and out
successfully and would be of benefit, but the 1st diode (across R546) can't
be due to high levels of IF at that point.  That's why I added the cap in
series and res in parallel.  That version of the fix improves the agc for
signals that are not real strong and using only the agc fast speed.  This
limits the quality of the ssb reception, hence the need for the 6au6 agc
amp to provide higher gain.  The higher gain in the agc amp improves the
quality of the audio for strong signals and in all agc speeds.  This is in
reference to the schematics on pages 5 and 8 in my doc: R390A Fix Lankford
2 diode SSB AGC
<https://www.r-390a.net/R390A%20Fix%20Lankford%202%20diode%20SSB%20AGC.pdf> on
our website.  This circuit has minimal effect on AM reception.

Regards, Larry

On Mon, Sep 30, 2024 at 7:36 AM Barry Scott <72volkswagon at gmail.com> wrote:

> Regarding the "original" two-diode mod, is that something that is supposed
> to be in-circuit all the time or should those be switched out for AM and CW?
>
> Thanks,
> Barry - N4BUQ
>
>
> On Mon, Sep 30, 2024 at 3:25 AM Larry Haney <larry41gm2 at gmail.com> wrote:
>
>> As already noted, easy clear SSB reception on a 390 needs changes in 2
>> areas: 1. AGC operation and 2. BFO injection level into the 'envelope'
>> detector.  I've done both and have had very good success, but for good
>> weak
>> low signal level SSB reception, a 'product' detector is required (mainly
>> due to the much lower noise level in it compared to an 'envelope'
>> detector).  And then with 'product' detectors there is quite a range of
>> noise levels in different designs and implementations.  The 6be6 method is
>> ok, but there are better (lower noise) choices.  See my doc on our
>> website: Improving
>> Lee Prod Det and SSB AGC.pdf
>> <
>> https://www.r-390a.net/Improving%20Lee%20Prod%20Det%20and%20SSB%20AGC.pdf
>> >.
>> It has a link to this doc: R390A Fix Lankford 2 diode SSB AGC.pdf, which
>> has an improved AGC circuit for much improved SSB reception.
>>
>> Another important 'feature' of using a product detector is its natural
>> reduction of interfering noise coming in on your antenna.  The amount of
>> noise reduction depends on the type of noise it is, but can be from 40% to
>> 75%.  I'm currently working on a circuit to allow correct use of the built
>> in AM noise limiter circuit (designed by Jacques Fortin) to reduce most of
>> the rest of it.  I'm in the final testing stage and it looks very
>> promising.
>>
>> Regards, Larry
>>
>>
>> On Sun, Sep 29, 2024 at 2:32 PM Jordan Arndt <Outposter30 at shaw.ca> wrote:
>>
>> > The type and specs of the AGC circuit plays a major role in SSB
>> > demodulation...
>> > I'm not too familar with the 51J4 AGC circuit but some of you are...
>> >
>> > I had an R-390 that already had a 6BE6 product detector with wiring and
>> > switching very similar to the Lee circuit. I had to add a small relay to
>> > switch diodes in and out when the BFO was selected on the front panel...
>> >
>> > It worked quite well and allowed excellent Exalted Carrier reception of
>> > low
>> > power tropical AM broadcast stations on the low bands and did well for
>> SSB
>> > with the diodes added to the AGC ckt...
>> >
>> > 73...Jordan VE6ZT
>> >
>> >
>> > ----- Original Message -----
>> > From: "Bob Camp" <kb8tq at n1k.org>
>> > To: "Ing. Giovanni Becattini" <giovanni.becattini at icloud.com>
>> > Cc: "R-390 Mailing List" <r-390 at mailman.qth.net>
>> > Sent: Sunday, September 29, 2024 3:18 PM
>> > Subject: Re: [R-390] Tuning SSB
>> >
>> >
>> > > Hi
>> > >
>> > > A BFO is not typically set up to provide great audio. A “product
>> > detector”
>> > > is optimized for lower audio distortion. Yes, there are other
>> > differences,
>> > > but they get into the “how did they do it” side of things.
>> > >
>> > > The R390 came out before SSB was “a thing to use”. Even the 390A was
>> > right
>> > > at the start of SSB being something the military was looking at. Move
>> a
>> > > few years down the road and the designs did have a “can do SSB” check
>> > box
>> > > on the design requirements.
>> > >
>> > > Bob
>> > >
>> > >> On Sep 29, 2024, at 4:50 PM, Ing. Giovanni Becattini via R-390
>> > >> <r-390 at mailman.qth.net> wrote:
>> > >>
>> > >> Hi,
>> > >> I don’t answer …because I “know", but just because I find the theme
>> > >> intriguing and have similar doubts. This is a picture from the 51J-4
>> > >> manual, which I think should be good also for the R-390A:
>> > >> <Screenshot 2024-09-29 alle 22.11.58.png>
>> > >> Because we must rebuild something similar to an AM signal but with
>> just
>> > >> one side band, I believe we must keep the BFO 1.5 kHz above the
>> center
>> > >> frequency of the filter for LSB and below for USB. And, obviously, we
>> > >> need to “move” the received signal (upper or lower band) to stay
>> > centered
>> > >> on the filter using the VFO.
>> > >>
>> > >> In other words: tune the VFO so that the band (upper or lower) is
>> > >> centered on the filter, and move the BFO +1.5 kHz above if the band
>> we
>> > >> want to read is the lower, and vice versa.
>> > >> <What is SSB: Single Sideband Mo dulation » Electronics Notes.png>
>> > >>
>> > >> This interpretation seems to be confirmed by the manual itself:
>> > >> <Screenshot 2024-09-29 alle 22.21.53.png>
>> > >>
>> > >> And this should be true also for SSB. In addition, it lets me think
>> > that
>> > >> with the 6 kHz filter, the dial reading does correspond to the
>> carrier
>> > >> frequency of station.
>> > >>
>> > >> All that assumes that the filter is centered on the IF channel, even
>> if
>> > >> not specified by the 51J-4 manual (left), but specified by the R-390A
>> > >> manual (right)) and however rather obvious
>> > >> <Immagineallegata-1.png><Immagineallegata-2.png>
>> > >>
>> > >> I am not sure that I am not saying something wrong, so I hope that
>> some
>> > >> true expert can help us to clarify the things….
>> > >>
>> > >> Gianni
>> > >>
>> > >>
>> > >>
>> > >>
>> > >>> Il giorno 29 set 2024, alle ore 19:29, Barry Scott
>> > >>> <72volkswagon at gmail.com> ha scritto:
>> > >>>
>> > >>> I hope I'm not opening a can of worms but I have some questions
>> about
>> > >>> tuning SSB signals with the R-390/URR.
>> > >>>
>> > >>> I know it works best if the LOCAL or LINE GAIN control is at maximum
>> > and
>> > >>> to
>> > >>> adjust the RF GAIN for a comfortable audio level.  What I'm
>> wondering
>> > is
>> > >>> what the proper way is to set the BFO.
>> > >>>
>> > >>> I've always set it +1 for LSB and -1 for USB but I can also just
>> leave
>> > >>> that
>> > >>> at zero and am still able to tune either sideband and now I'm
>> > wondering
>> > >>> if
>> > >>> setting the BFO + or - is mainly to get the dial to reflect the
>> > received
>> > >>> frequency.  Is that an over-simplification?
>> > >>>
>> > >>> I'm asking because I'm never really sure what the transmitted
>> frequency
>> > >>> is.  If I set the BFO + or -, it's only a matter of how I determine
>> > what
>> > >>> sounds good (e.g. no Donald Duck, etc.) as to what the dial reads.
>> > >>>
>> > >>> I apologize if that's something that should be more obvious but
>> > reading
>> > >>> up
>> > >>> on it on the web doesn't quite make full sense to me.  The
>> discussions
>> > >>> seem
>> > >>> to revolve around whether the signal is in the IF's passband, etc.,
>> but
>> > >>> like I said, it seems I'm centering the signal in the IF even if I
>> > keep
>> > >>> the
>> > >>> BFO at 0.
>> > >>>
>> > >>> Thanks for any insight on this,
>> > >>> Barry - N4BUQ
>> > >>> ______________________________________________________________
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