What does a Q Multiplier do?
Rick
I never could quite figure out exactly what a "Q Multiplier" does.
There is a Heathkit model QF-1 Q Multiplier on ebay and that got me to
wondering.
I know what "Q" is regarding inductors and capacitors ("Quality factor",
reactance divided by resistance) but I don't know exactly what a Q
Multiplier does or how it connects (I assume it connects to a receiver but
am not even sure about that). Obviously it "multiplies the Q" but of what
component(s) or stage(s)?
Any of you boat anchor enthusiasts know about this?
Michael Black
confused by the recent equipment that gets posted about over there.
A Q-multiplier is a regenerative stage.
You might be familiar with the regenerative receiver, where positive feed
back is introduced to a stage so the same signal is amplified, fed back to
the input, and amplified again. This re-amplification allows for far
more gain than if the stage only amplified the signal once, and it
increases the selectivity of the tuned circuit involved. There is a means of
controlling regneration, so you can limit the amplification, or kick it up
so the receiver goes into oscillation, providing a beat note for reception
of CW signals.
A Q-multiplier is the same thing, though it doesn't bother to demodulate
the signal. Generally, the point is to increase selectivity in the IF
section of the receiver. It connects to the plate of the mixer with
a single connection, in parallel with that stage. INcrease the regeneration
of the Q-Multiplier, and it increases the selectivity. It gives a nice
peak, though does little to improve the skirt selectivity.
It's prime was in the days of the 455KHz IF, where the improved selectivity
would actually make a difference. At a higher frequency, it would still
increase the selectivity of the tuned circuit, but not enough to narrow
the passband for voice or CW.
It was often an add-on for low end receivers, because for the cost of
a single stage, you'd boost selectivity. Sometimes it was included
in the receiver (and sometimes the effect was done by adding some positive
feedback to the IF stage, so it could go into regeneration).
It was sometimes seen in more expensive receivers, where a variant
was often the reason. A rearrangement of the circuit, and it could
be used as a notch filter, to knock out a nearby signal, which was often
more useful in receivers with good IF selectivity where the peaking was
less of an improvement.
One early use for the Q-Multiplier, which never saw that much use,
was for boosting the received carrier in reference to the sidebands of
an AM signal. This peaking of the carrier made sure it was strong
relative to the sidebands, and made for better AM demodulation. The
Q-multiplier was good for this, since it peaked up the carrier but
the bad skirt selectivity didn't attenuate the sidebands too much.
The same basic notion that is the point of synchronous detectors
for better AM reception today.
For a brief while in the sixties, there'd be construction articles
with Q-Multipliers at the front-end of receivers. Someone described
an early single conversion receiver with a 9MHz IF, and put a front
end Q-Multiplier in to improve the selectivity of the single tuned
circuit at the front end, to improve image rejection. (Though, it
never fully made sense, since image rejection was much easier with
the high frequency IF.) The scheme was continued in a few other
receivers, basically because it had been done. And that was the
end of that.
Q-Multipliers faded as IF frequencies increased, and good filters
such as mechanical and crystal came along to replace a string of
IF transformers.
MIchael VE2BVW
Rick
Good afternoon, Michael.
Thank you very much for the very interesting and informative article on Q
multipliers. I learned a lot today. :-)
pltrgyst
>A Q-multiplier is a regenerative stage.
>
>It's prime was in the days of the 455KHz IF, where the improved selectivity
>would actually make a difference. At a higher frequency, it would still
>increase the selectivity of the tuned circuit, but not enough to narrow
>the passband for voice or CW.
So what about audio Q-multipliers? Do they function on the same principle?
-- Larry
The Shadow
"pltrgyst" <pltr...@spamlessxhost.org> wrote in message
news:pv8l13dtv4rs33eof...@4ax.com...
AUDIO Q-MULTIPLIER
http://cs.okanagan.bc.ca/ve7ouc/eng/kc6wdk-mirror/qmultiplier.html
Lamont
Richard Knoppow
Yes. The idea of a Q-Multiplier is to increase
selectivity. They use regeneration (positive feedback) to
decrease the bandwidth of a tuned circuit. The frequency can
be whatever is desired. Q-Multipliers were popular at a time
when most receivers had inadequate selectivity. They are
relatively cheap and, with some limitations, work pretty
well. As receivers with more sophisticated filters (like
mechanical filters or multi-pole crystal filters) became
available the Q-multiplier fell out of use.
BTW, the old regenerative receiver works on the same
principle. In fact, one can see a Q-Multiplier as a sort of
regenerative receiver working at the IF or audio
frequencies.
--
---
Richard Knoppow
Los Angeles, CA, USA
dick...@ix.netcom.com
--
Posted via a free Usenet account from http://www.teranews.com
Michael Black
Originally yes, though I've seen "audio Q-multipliers" described in
construction articles that are more strictly average audio filters.
They may even come out of the same place. The selectoject was described
in QST in the late forties, by O. G. Villard, and it uses the principle
of regneration to get better selectivity at audio. And O. G. Villard is
the name I'd associate with the use of the Q-Multiplier for better
reception of AM (there are references to a QST article by him on that topic
in an early ARRL SSB manual). But I've never seen a specific link between
the selectoject and the Q-Multiplier at the IF of a receiver, so I don't know
if he came up with the IF Q-multiplier as we know it, or someone else did.
Certainly O. G. Villard, who died only three years ago, came up with
quite a few things important to amateur radio, including a lot of work
in SSB in the early days.
Michael VE2BVW
Dick
wrote:
Interestingly, this month's QST in the 50 Years Ago column lists a
variable band width Q-Multiplier by Ronald Ives (April 1957.) The
article references the original Villard article in the April 1952
issue of "Electronics" magazine.
Dick - W6CCD
Jon Teske
multiplier was a hot item. Most of us could not afford the latest
and greatest receivers so the Heathkit Q-Multiplier was a desired
accessory . I believe is could also do a notching function. I never
had the Heath version myself, but many friends did. I had a
Hammarlund HQ-100 immediately upon its introduction and as far as I
can recall that was the first commercial radio to have a Q-Multiplier
built in as opposed to an add-on accessory. More expensive radios had
crystal filters and the Cadillac of the era, the Collins 75A4 had
mechanical filters. The $500 price tag of the 75A4 put it in the
"dream-on" category to those of us who only had part-time jobs or in
my case (I was 13 when licensed) too young to even work at anything
legally. It actually was pretty effective once you got to learn how to
use it. As I remember it was far more effective on CW than phone
(which was still heavily standard full carrier AM phone.) The
Hammarlund version only had a "peak"
function. I believe the Heathkit version's notch function could
be used for reducing heterodyne squeals although I have very
liitle experience with that. I believe the Heath also had a
third function, but I don't remember what it was.
Jon W3JT