[HBR] Pullen mixer

[Walt Hutchens]

Donald posted:

> As to strong signal handling ability, the 6gm8 (Pullen) is able to hear a
> signal on 630-with only minor noise from WSM on 650.  Same is true on the
> other side of 650.  670 in Chicago comes in like a local with no shot noise
> from WSM.  This with a long wire end fed at about 40 feet high, input on
> directly on the grid.

You are able to hear strong signals that you are tuned to in the presence of
off-channel strong signals.  This isn't the exact meaning of strong signal
handling as the term is used for mixers, receivers, etc.   It's a good
example of adequate selectivity -- that would be in your IF, since you're
feeding straight from the antenna.

'Strong signal handling' refers to the following issues:

1. You are listening to a weak one and suddenly the signal disappears and
the atmospheric noise drops too.   Your first thought is "what's wrong with
my receiver" but tuning around you discover an extremely strong station has
just come on, 20 kcs away.   When he's transmitting your receiver is
semi-dead over much of the band.    This is 'receiver blocking.'  The usual
cause is that some early stage -- RF amp or more likely the mixer -- is
being driven into grid current which drops the gain dramatically.

2. You are listening to a weak one on 3798.   There are strong stations on
3818 and 3838 and you can hear a distorted mixture of those transmissions
over the top of the one you are trying to listen to.   Of course this exact
combination is very unlikely but 3806 and 3814, 3791 and 3784, 3838 and 3878
-- and all the rest of such pairs -- work the same way, so on any busy band
there are bad combinations all the time.
 
This is 'intermodulation distortion.'   In a linear stage, 3820 and 3840 IN
will yield 3820 and 3840 OUT -- AND NOTHING ELSE.   But for every stage
there is a signal level that leads to NON-linear operation,  In that case,
3820 and 3840  IN yields the same two out PLUS the same two frequencies
modulated by the difference of 20 kcs.   So you get 3800 and 3860 as well.
Punch that stage with enough signal and you'll get 3780 and 3880 too ...
'second order intermodulation distortion.'   However the first order problem
comes first so that's the one we worry about.

Short story:  Good strong signal performance means being able to listen
without disturbance to a weak signal when there are strong signals elsewhere
on the band.   One of the best tests is 40M in the evening in the section
that is shared with broadcast.   Second to that, 80M in the evening.   A
really good receiver may get chatter from the guy only 5 kcs away (that's a
channel spacing problem -- your receiver can't reject what is actually
within its passband), but whatever you can hear will sound as clean as at
any other time -- not mushy and hard to understand.

One can say "But I don't do weak signal stuff under those conditions."   But
every round table has some stations that are weaker than others, often some
that are MUCH weaker than the others.   A good receiver hears those stations
well if the channel is reasonably clear even if the rest of the band is full
of megastations.   A crummy receiver (one that doesn't handle strong signals
well) makes the weaker stations sound mushy, muffled.

It's also true that intermod distortion makes broadband noise --
thunderstorms are a common example -- worse.   It's 'everywhere' and it is
strong, so in a set with poor intermod performance more of it will be heard
on the channel you're tuned to than would be the case for a better receiver.

'Interference free dynamic range' is a way to give a number to
intermodulation distortion performance.   Take 'a weak signal' as your
standard -- that might be 1 uV or it might be something less; the exact
number doesn't matter.  (A common standard is the signal that gives a 10 dB
S/N ratio.)   Now take two strong signals located 20 kcs and 40 kcs away
from the weak signal:  How much stronger do those two signals (of equal
strength) have to be to produce a spurious signal of the same strength as
the weak signal, on the frequency of that weak signal?

For poor quality receivers -- the early transistorized ham sets were
examples -- the number could be 40-50 dB or so.  Cheap tube ham receivers
like the S-38 and home 'shortwave' sets might be near the lower end of the
range.  I'd guess the HR-10 at 60-70 dB: vacuum tubes are better than
transistors and that crystal filter eliminates problems after the mixer.
Plus using the RF gain as a volume control (as recommended for CW/SSB) will
reduce the RF stage gain and help both it and the mixer.   The R-390 should
hit 80 dB and the SS-1R gets around 90 dB or even higher with a bit more
oscillator injection.

With a receiver having an IFDR over 80 dB you will rarely have a problem
hearing a weak station that's in the clear, regardless of what's on the rest
of the band.   Receivers like the HBRs that have separate gain controls for
the RF stage will also do very well if you back down the RF gain some when
the band is full of strong signals, even if the one you're listening to is
weaker.   

There is no magic about the choice of 20 kcs spacing for the test signals.
It's just a convenient number that's big enough to be clearly
out-of-channel.  Tests are sometimes done with 80 kcs or 100 kcs spacings
and these tell you other things about the set:  While a 20 kcs test result
will only be slightly improved by a high-Q tracking front end (as on the
HBRs), the difference between such a front end and a broadband one (many
modern ham sets) on a 100 kcs test will be dramatic.   (That's why modern
sets all have switchable attenuators ...)

All of that ignores gain and internal noise level.   And that's what one
should (mostly) do when thinking about MF-lower HF receiver designs.   Up to
20M or so front end noise simply isn't an issue but strong signal handling
is very much so.   When using vacuum tubes most designs will be quiet enough
to do well even on 15M and often on 10M as well (IF they have enough LO
injection!) -- but they'll only handle strong signals well enough to be top
performers on 160-80-40 if you design for that.

On the lower HF front end noise only matters in the very worst cases -- like
a 6SA7 mixer with no RF amp -- and gain can be made up anywhere, even in the
audio section.   But once the signal you want to hear is trashed by a
non-linear front end, no later stage of a receiver can fix it.

Walt 
KJ4KV