[HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 6

[Mike Feher]

Walt -

I have no idea of engines, so will not comment on them. I also feel you need
to do this your way, and while you say you appreciate my comments, you are
obviously determined to get to the solution on your own, and, more than
likely I am interfering more than contributing to that end. So, please keep
us informed of your progress, and I promise I will refrain from further
comments. 73 - Mike


Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-901-9193



----- Original Message -----
From: <waltah@earthlink.net>
To: <hbr@mailman.qth.net>
Sent: Tuesday, May 13, 2003 2:36 PM
Subject: Re: [HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 6


> Mike Feher wrote:
> > This implies that your receiver's NF (noise figure) is 31 dB. Does
> > that make sense? I would be surprised if it was over 15 dB. So, to
> > me, right now things just do not add up.
>
> In other words, on fundamental theory it ought to be a heck of a lot
> quieter (more sensitive) than it is.   I agree with that, and that's why
> I'm still working on it!
>
> Let's say a kid gets some bicycle wheels, a small block Chevy
> engine, the transmission from a riding lawnmower, a couple of V-
> belts, um..., a 4'x8' sheet of plywood and a few 2x4's.   He has
> ridden in some cars and once made a soapbox racer so he decides
> to build a real car.   An engineer friend calculates that with the gear
> ratio he has, an engine of that displacement (etc.) the top speed
> ought to be 72 mph but the kid can only get 'er up to 30.
>
> The engineer's theory fails to take into account Designer Error.
> There's simply a lot more to stitching together a complete car (or
> receiver) than the fundamentals tell you.   In this case the
> fundamentals apply only to a receiver in which a reasonable input
> stage dominates the noise.   Well, of course that's how it *ought* to
> be.   But I am not there yet, and going by the direct route -- simply
> cranking up the gain of the RF stage -- would (at the current state of
> things) compromise the dynamic range.
>
> One disadvantage I have is that when a design like this is done
> professionally the component parts are fairly well characterized by
> the manufacturers.   Custom parts are designed to specific targets.
> For example, the impedence of an IFT and the gain of a stage using
> a certain tube with that IFT would have been known before the stage
> was wired in a prototype.   Ditto the drive level for a certain crystal
> and the output a particular oscillator might produce with that crystal.
> Of course the optimum drive for a mixer would be known be before
> the prototype set was built.   And so on.
>
> I'm working with ideas and bits of circuits from other designs that
> were incompletely tested (or at least, incompletely reported) and
> parts of somewhat unknown properties.   (1415 kcs IFTs retuned to
> 3180 kcs, for example, for reasons of availablity and ease of
> modification.)   Some of the preliminary testing that I perhaps should
> do, I don't have the right instruments for.  Some I'm just too impatient
> to do -- I don't know what I *really* need until I see the problems and I
> fear spending a week studying something that turns out to be
> unimportant in the larger picture.   Probably this reluctance to do
> side-studies has hurt me, overall, but of course it's harder to see that
> one step at a time.
>
> And then there are the 'just plain problems.'    When I began this
> note yesterday I was optimizing the 2nd mixer.  I had tried a couple
> of different circuits, minor variations on one of them, and was down to
> tweaking component values.   The trend was to better performance
> (IFDR) at higher mixer plate currents so (since I've got more overall
> gain than I need) I decided to raise the cathode resistor of the 2nd IF
> stage and save some ma there.    Huh?   All of a sudden the noise
> floor and IFDR took about a 2 db hit.   There's no logical reason they
> should be the least affected by the 2nd IF -- it shouldn't be
> contributing any noise, and indeed when everything up to there is
> disabled, the receiver is completely dead, even at full volume.   AGC
> is disabled for these tests so that couldn't be it.   It had to be a
> subtle feedback effect, but bedtime intervened.
>
> In the early AM, sudden wakefulness.   There were a couple of
> obvious feedback paths from that last IF to early stages and I started
> by listening to the noise audio output with stages ahead of the 2nd
> mixer disabled.    Sure enough, there was a nearly inaudible signal
> that must have been coming in at around 3080.5 kcs -- so faint you'd
> never hear it with any other signal, let alone with an antenna
> connected.   I started poking around with bypasses and it was on the
> filament line, probably coming in through the power line despite a pi
> filter on the line input connector, the usual .01/2kv bypasses after the
> fuses and 0.1 bypasses at the main filament distribution point.
>
> If the filaments can propagate an external stray signal, they can do
> the same with an internal one -- there's a feedback path!
>
> I had wired all the filaments with 0.01 bypasses.  But the reactance
> of those at 3080 kcs is about 5 ohms -- not exactly a dead short,
> when filament levels of resistance (about 20 ohms per tube at
> operating temp) and tens or more of mmf filament to cathode
> capacitance are considered.   I got rid of the stray signal with 0.47
> bypasses where the filaments enter the front end compartment under
> the chassis.   And sure enough the noise level dropped a couple of
> db and the NF and IFDR are now the same with 150 ohms for a 2nd
> IF cathode resistor as with 68 ohms.   Most likely the path was from
> the cathode of the 2nd IF to the (unbypassed) cathode of the 2nd
> mixer, via the filament circuit.  The difference in feedback voltage
> when changing the cathode resistor was small, but there's a heck of
> a lot of gain (three very hot tubes) between those two cathodes and a
> 2 db change in the noise floor caused by regeneration isn't much.
>
> In the future I'll be rewiring the filaments again and when I do, the
> bypasses will go from 0.01 to 0.1 from the 1st mixer on.
>
> I think you learn this stuff only by practical experience of which I
> didn't have enough for this job. Theory certainly does inform, but it
> doesn't let me avoid all such several-hour pitfalls.  I have assorted
> handbooks, but the approach is either -- "Build it like this" with a
> circuit diagram and where to get the parts (in 1965), or "Here's the
> theory of a receiver" with the parts being square blocks of specified
> gain and noise figure.   I really need a lab manual -- from about 1960
> or so.   As I write the 'notes' for the receiver -- 40 pages and counting
> -- I am including all the pitfalls I hit, so hopefully they only have to
be
> hit once.
>
> The grand plan has been to build the receiver from the power supply
> and audio, thence one stage at a time to the antenna, get it
> 'basically working', then to repeat the journey from audio back to
> antenna making things as near 'really right' as I can.  I'm well along
> with the second pass; satisfied with everything from the filter driver on
> and probably close on the second mixer.  I need to finish up the
> optimization there, before moving back to the 1st mixer.
>
> A long story which I hope makes clear why a noise figure around 30
> db does not seem impossible right now, considering problems that
> may be lurking in the still only 'basically working' RF and 1st mixer
> stages.   I know for a fact that compared to really good receivers, this
> one is too noisy.  I strongly suspect the that 1st mixer has lower
> gain than it should because of insufficient injection.  The RF stage ...
> well, I'm pretty sure it's bad, although I can't tell you the details yet.
>
> I began with the belief that the set ought to be able to outperform an
> R-390 (IFDR around 80 db) because I've got better tube types and
> coils than were available in the 50's. I've seen nothing to change that
> view.  But it's still pretty far from there.
>
> As always, thanks for the comments, Mike.
>
> Walt Hutchens
> KJ4KV
>
>
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