K2 RX audio output - again

[n8ewy]

Isn't the position of the AF gain (volume?) dependent on the position of the
RF gain knob?  As I turn the "RF gain" knob clockwise, the AF gain (volume)
increases.  So it seems that the position of the AF gain knob is variable
with respect to the RF gain.

 

Just to get another thread going about this, I would ask where you best
position the RF gain knob when operating.

 

Dohn   N8EWY

 

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[Ron D'Eau Claire-2]

Dohn, N8EWY, asked:

Isn't the position of the AF gain (volume?) dependent on the position of the
RF gain knob?  As I turn the "RF gain" knob clockwise, the AF gain (volume)
increases.  So it seems that the position of the AF gain knob is variable
with respect to the RF gain.


Just to get another thread going about this, I would ask where you best
position the RF gain knob when operating.

----------------------------------------------------------

That's right Dohn.

Most communications receivers have two gain controls: an audio gain that
varies the amount of amplification in the audio stages and an "RF" gain that
varies the amount of amplification before the RF signal is converted to
audio. If you turn the RF gain down, you reduce the amount of audio being
delivered to the audio amplifiers by the product detector, so you have to
turn the AF gain up to maintain the same audio level.

Going waaaaaay back to when I was first building receivers, that RF gain
actually controlled the gain of the RF amplifier (preamplifier in modern
jargon) at the input to the receiver as well as the gain of the intermediate
frequency (i.f.) amplifiers ahead of the detector. Nowadays in
state-of-the-art receivers like the K2 we control the gain at the input by
switching the rf amplifier (preamp)into our out of the signal path or we can
add an attenuator (ATT) at the input to reduce the signal gain reaching the
receiver. The RF gain now controls only the gain of the i.f. amplifier, but
the term "RF Gain" is still used to describe that control (after all, the
intermediate frequency is still an RF signal).

When AGC is enabled, it controls the "RF" gain (gain of the intermediate
frequency amplifier) automatically. For the AGC to have full range of
control, you want the RF gain fully CW (maximum gain). Turning it down
over-rides the AGC, limiting the maximum gain available. If a signal is
strong enough to cause the AGC to try to reduce the gain below the level
you've set with the RF gain, it will do so, but the AGC cannot increase the
gain above what you have set with the RF gain. Watch the S-meter behavior as
you adjust the RF gain and you'll see how strong a signal is needed to
activate the AGC for any setting. It has to be a signal stronger than that
indicated on the "S-meter" bargraph.

So the normal position for the RF gain is fully CW if you have AGC on.

If you want to use the RF gain to control the loudness of signals in your
speaker or phones, first turn the RF gain all the way CCW (minimum gain) and
turn off the Preamp, then adjust your AF gain up until you hear background
noise in the speaker, then back it off a bit so that noise won't be
objectionable. In the K2, there's a fair amount of signal leak through the
I.F. amplifier even with the RF gain at minimum, so you might need to
disconnect the antenna to note the ideal position for the AF gain. Once
you've established that, you can reset it to approximately the same spot in
the future. It's not critical. Now use the RF gain control to control the
level of signals. I do that a lot and turn the AGC off because it makes
listening much, much nicer to my ear. With the AGC off, the background noise
stays down at the same level even though a signal may not be present. With
AGC, the gain is increased when a signal is not present, bringing up the
noise level to an unnaturally high level. That might help one hear a weak
signal calling, but to me it's just a nuisance in normal operating. I prefer
to adjust the RF gain manually when listening for a 'weak one'.

A related question that comes up here from time to time is "When is it
important to use the Preamplifier?"

If the K2 received only the lower frequency bands such as 160 meters, 80
meters and perhaps 40 meters, the preamplifier would be completely
unnecessary. Your K2 would receive the weakest signal possible without it.
That's because the noise picked up by your antenna will override any noise
produced by the amplifiers inside the K2. Atmospheric noise levels drop
dramatically on the higher frequencies. Depending upon your antenna, at some
point between 7 and 30 MHz the internal noise in the K2 may start to compete
with weak signals. That's when you use your preamp. The preamp is a low
noise amplifier that boosts the weak signal so it will be stronger than any
noise produced in the K2.

And that leads to the next question, "Why would I turn the preamp off?
Signals are louder with it on."

Every receiver will overload in the presence of a strong enough signal, even
the K2. In an ideal receiver, all of the selectivity would be right where
the antenna connects to isolate the signal you want to hear from all the
other signals on the bands before it is amplified. If you need only listen
to one frequency, that's practical to do. But, in a receiver like the K2
that we expect to tune all over the band and even switch bands from one end
of the HF spectrum to the other, the incoming signals are first converted to
a fixed, intermediate frequency where the filter suppresses all the signals
except the one we want to hear. That works well, allowing us to use one
filter system tuned to the intermediate frequency in a receiver that covers
a wide range of frequencies, but it also causes a problem. Strong signals
can overload the stages in the receiver ahead of the filter!

The results of overload can take many forms. Commonly, you might hear
"phantom" signals QRMing other signals or the band might simply sound noisy,
as if there's something creating lots of QRN in the area (there is, your
receiver!). Often you'll hear both, depending upon the frequency.

The designers built the K2 to handle the biggest signals possible, but
there's still limits to what they can do. If you use your preamp on bands
where it's not needed, you are amplifying a broad spectrum of signals,
increasing the chances a strong signal that you don't want to copy will
overload the receiver. That's why the attenuator is provided as well. On
some bands, the noise picked up by the antenna is high enough the dynamic
range (range of signal strengths your receiver can handle without overload)
can be improved by reducing the strength of all the signals coming into the
antenna jack. That's when you use the attenuator.

Here's a simple way to see if you need the preamplifier. Turn the
preamplifier off. Tune to a quiet frequency (no signals) on the band you're
using, set your filter to the *narrowest* setting you use, then turn up the
RF/AF gains until you hear the background QRN. Now disconnect your antenna.
If the noise level drops, you do not need the preamplifier. The band QRN
(which dropped out when you disconnected the antenna)is setting the limit on
the weakest signal you can hear. The only way you can improve the receiver
sensitivity is to use a narrower filter(which removes more of the noise from
around the signal frequency)or reduce the noise coming in from the antenna
in some way. The noise blanker helps a lot with specific types of QRN, but
it, too can *cause* QRN with its switching action. That's why you should
only turn it on *if* it improves the reception.

Unless you have a very poor antenna, the preamp will do you no good from 1.8
through at least 7 MHz. Often it's not needed on 14 MHz either. The preamp
is usually a help up near 30 MHz and possibly as low as 14 MHz, depending
upon your antenna and whether you live in an area with very low QRN levels.

Ron AC7AC


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[Geoffrey Mackenzie-Kennedy-2]

Ron AC7AC wrote:

Going waaaaaay back to when I was first building receivers, that RF gain
actually controlled the gain of the RF amplifier (preamplifier in modern
jargon) at the input to the receiver as well as the gain of the intermediate
frequency (i.f.) amplifiers ahead of the detector. Nowadays in
state-of-the-art receivers like the K2 we control the gain at the input by
switching the rf amplifier (preamp)into our out of the signal path or we can
add an attenuator (ATT) at the input to reduce the signal gain reaching the
receiver. The RF gain now controls only the gain of the i.f. amplifier, but
the term "RF Gain" is still used to describe that control (after all, the
intermediate frequency is still an RF signal).

 -------------------------------------------------------------------------

Digressing from the subject of audio output for a moment, having the "RF
Gain" control the gain of the IF amplifier only can lead to some
undesireable effects unless the IF amplifier design is done very carefully
using suitable devices. The problem is that the linearity of some of the
more popular low cost amplifier chips actually gets worse as their gain
starts to be reduced, although some types of these chips will recover as the
gain is further reduced. Assuming a situation where the Preamp is "Off" and
the Attenuator is "In" leaving two or more signals are in the passband, some
strong and the one that you want is weak, any negative effect on linearity
caused by changing gain is of course unwelcome. The overall effect is
determined by the receiver's Gain Distribution and the characteristics of
the stages both before and after the controlled IF amplifier.

Many modern (not ham) HF receivers designed to handle *very* strong signals
while allowing copy of an adjacent signal close to the noise still do use
"RF Gain" to control some type of front end variable attenuator, and a
strong
RF amp with fixed but very low gain embedded in reasonably high loaded Q
variable frequency tuned circuits. But these receivers consume a lot of
power.

73,
Geoff
GM4ESD







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[Ron D'Eau Claire-2]

Geoff, GM4ESD wrote:

Digressing from the subject of audio output for a moment, having the "RF
Gain" control the gain of the IF amplifier only can lead to some
undesireable effects unless the IF amplifier design is done very carefully
using suitable devices. The problem is that the linearity of some of the
more popular low cost amplifier chips actually gets worse as their gain
starts to be reduced, although some types of these chips will recover as the
gain is further reduced. Assuming a situation where the Preamp is "Off" and
the Attenuator is "In" leaving two or more signals are in the passband, some
strong and the one that you want is weak, any negative effect on linearity
caused by changing gain is of course unwelcome. The overall effect is
determined by the receiver's Gain Distribution and the characteristics of
the stages both before and after the controlled IF amplifier.

Many modern (not ham) HF receivers designed to handle *very* strong signals
while allowing copy of an adjacent signal close to the noise still do use
"RF Gain" to control some type of front end variable attenuator, and a
strong
RF amp with fixed but very low gain embedded in reasonably high loaded Q
variable frequency tuned circuits. But these receivers consume a lot of
power.

==================

Quite right, in my experience. That's true of tube-type amplifiers as well,
which is why the really low-noise state-of-the-art tube RF amplifiers were
fixed gain as well. They were most common in VHF/UHF applications.  

I believe that's why the K2 uses a fixed-gain wide-dynamic range I.F.
preamplifier (Q22) ahead of the I.F. filter, and puts the variable gain
MC1350 (U12) after the filter where it's reasonably well protected from
off-frequency signals. Of course, the K2 has a switchable RF amplifier and a
switchable attenuator which, while needing manual control by someone who
understands when to use them, go a long way toward making the K2 stand up
well in extreme conditions.

It helps, too, that the K2 is a straightforward single-conversion design.
More complex multiple-conversion receivers need more protection to do as
well because of all the extra mixers and amplifiers they have. Of course,
that complexity allows for some nice gadgets like front-panel "passband
tuning" but it puts a huge load on the design to stay competitive with a
more simple, straightforward single-conversion superhet like the K2.

One of my homebrew designs from the 70's, which also used the MC1350 (that
chip has been around a l-o-n-g time!), used pin diodes to control a variable
attenuator at the antenna input to the receiver in addition to varying the
gain of the MC1350. After fiddling with it for a long time, I reverted to a
simple step attenuator at the antenna input much like the K2 has. That
choice speaks to a basic question in the design of anything: which is the
more eloquent and desirable, extreme complexity that does anything and
everything as well enough and fully automatically, or a simple design that
does specific things very well but which may require a more knowledgeable
operator and greater operator intervention?

I don't think there's any one answer to that question. If there were we'd
not have both automatic and manual transmissions in cars.

Ron AC7AC


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[Jack Smith-6]

Geoff:

At least my Racal RA6790/GM labels the front panel control "IF Gain."

As a side note, I find the RA6790/GM to have the best ergometrics of any
receiver I've owned, although not the best AGC action.

Jack K8ZOA

Geoffrey Mackenzie-Kennedy wrote:

> Ron AC7AC wrote:
>
> Going waaaaaay back to when I was first building receivers, that RF gain
> actually controlled the gain of the RF amplifier (preamplifier in modern
> jargon) at the input to the receiver as well as the gain of the
> intermediate
> frequency (i.f.) amplifiers ahead of the detector. Nowadays in
> state-of-the-art receivers like the K2 we control the gain at the
> input by
> switching the rf amplifier (preamp)into our out of the signal path or
> we can
> add an attenuator (ATT) at the input to reduce the signal gain
> reaching the
> receiver. The RF gain now controls only the gain of the i.f.
> amplifier, but
> the term "RF Gain" is still used to describe that control (after all, the
> intermediate frequency is still an RF signal).
>
> -------------------------------------------------------------------------
>
> Digressing from the subject of audio output for a moment, having the "RF
> Gain" control the gain of the IF amplifier only can lead to some
> undesireable effects unless the IF amplifier design is done very
> carefully
> using suitable devices. The problem is that the linearity of some of the
> more popular low cost amplifier chips actually gets worse as their gain
> starts to be reduced, although some types of these chips will recover
> as the
> gain is further reduced. Assuming a situation where the Preamp is
> "Off" and
> the Attenuator is "In" leaving two or more signals are in the
> passband, some
> strong and the one that you want is weak, any negative effect on
> linearity
> caused by changing gain is of course unwelcome. The overall effect is
> determined by the receiver's Gain Distribution and the characteristics of
> the stages both before and after the controlled IF amplifier.
>
> Many modern (not ham) HF receivers designed to handle *very* strong
> signals
> while allowing copy of an adjacent signal close to the noise still do use
> "RF Gain" to control some type of front end variable attenuator, and a
> strong
> RF amp with fixed but very low gain embedded in reasonably high loaded Q
> variable frequency tuned circuits. But these receivers consume a lot of
> power.
>
> 73,
> Geoff
> GM4ESD
>
>
>
>
>
>
>
> _______________________________________________
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>

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[Geoffrey Mackenzie-Kennedy-2]

Ron AC7AC wrote:

>I believe that's why the K2 uses a fixed-gain wide-dynamic range I.F.
>preamplifier (Q22) ahead of the I.F. filter, and puts the variable gain
>MC1350 (U12) after the filter where it's reasonably well protected from
>off-frequency signals.

--------------------------------------------------------------------

A post mixer amp such as Q22 is in some cases required to reduce the IF
system noise figure, and to protect the mixer from the large variations in
input impedance vs frequency exhibited by many narrow bandwidth filters, in
and close to the passband. Several types of mixer including diode rings
generate additional intermodulation products when their load is allowed to
vary, and the same is true of amplifiers unless the amplifier design
addresses the problem.

While the filter can offer protection to later stages, the $64 question is
what happens all along the receiver chain to the audio output when one or
more big signals *do* get through the filter with little attenuation,
alongside that weak signal which you are trying to copy?

----------------------------------------------------------------------

>It helps, too, that the K2 is a straightforward single-conversion design.
>More complex multiple-conversion receivers need more protection to do as
>well because of all the extra mixers and amplifiers they have. Of course,
>that complexity allows for some nice gadgets like front-panel "passband
>tuning" but it puts a huge load on the design to stay competitive with a
>more simple, straightforward single-conversion superhet like the K2.

------------------------------------------------------------------------

I won't be drawn into this discussion <g>

--------------------------------------------------------------------------

>One of my homebrew designs from the 70's, which also used the MC1350 (that
>chip has been around a l-o-n-g time!), used pin diodes to control a
>variable
>attenuator at the antenna input to the receiver in addition to varying the
>gain of the MC1350. After fiddling with it for a long time, I reverted to a
>simple step attenuator at the antenna input much like the K2 has.

-----------------------------------------------------------------------------

Understood. Sometimes variable attenuators using transformers wound on
ferrite toroidal cores with a third 'DC' winding used to vary the core's
permeability are useful, but draw too much current for use in a portable
rig.

-----------------------------------------------------------------------------

>That choice speaks to a basic question in the design of anything: which is
>the
>more eloquent and desirable, extreme complexity that does anything and
>everything as well enough and fully automatically, or a simple design that
>does specific things very well but which may require a more knowledgeable
>operator and greater operator intervention?

>I don't think there's any one answer to that question. If there were we'd
>not have both automatic and manual transmissions in cars.

------------------------------------------------------------------------------

There is some middle ground between the extremes which you mention!

73,
Geoff
GM4ESD









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