[HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 4

[Mike Feher]

Walt -

Took my wife out to lunch so only now have a chance to respond to your post
on your re-invention of the wheel, HI. I may be way off base on this, and
admittedly did not play any number games, but, it sure seems to me that a 13
dBm IP3 is pretty darn good. I have yet to think about the adequacy, one way
or another about the bottom end. Regardless, 13 dBm as you know is 20
milliwatts, measured in a wideband sense utilizing your techniques, and is
essentially equivalent to the same power integrated over the bandwidth
outside of your IF, and therefore is a heck of a lot of power. I believe
that is a lot more than most receivers will typically encounter. I plan to
look at these numbers more closely, but if 13 is what you are getting, in my
opinion right now, that is darn good. If it takes the sum of 2 X signals of
a total power equal to Y dBm (where this is equal to 13 dBm in your case )
that are out of band signals, for you to notice an inband signal that equals
13 dBm, then, again, I think you are doing real well. An often made
oversight is to look at the 1 dB compression point and equate it to the IP3
number. I do not believe you are doing that, however there is typically a 10
dB difference between the two. Maybe I have had too many 807s today, HI.
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: Saturday, April 12, 2003 1:43 PM
Subject: [HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 4


> (In previous updates I struggled to figure out how to do proper
> measurements of intermod free dynamic range {IFDR} and began
> sorting out the problems.)
>
> After perhaps a month of receiver design kindergarten revisited (note
> to self -- do not skip this grade next time) I homed in on the last
> stage before the crystal filter -- a cathode follower used as a driver
> mainly for the purpose of impedence matching.   I found that by
> changing my initial design to operate the c.f. at higher current and
> higher cathode voltage I could get the same results at the driver input
> as at the filter itself:  a third order intercept (IP3) of about 11.5 dbm
> (db above 1 milliwatt) and an IFDR of about 82 db.   By increasing
> the gain of the 1st IF stage (and decreasing that of the second to
> maintain the same overall IF gain) I pushed the IP3 up to 15.5 dbm
> without hurting the IFDR.
>
> An IP3 of 15.5 dbm and IFDR of 82 db is fairly respectable but of
> course there are three stages between these measurements and the
> antenna so I'm still a long, long way from a good receiver.
>
> Next stop, one stage earlier -- the 2nd mixer.   This stage combines
> a tunable IF signal 5520-6020 kcs with the VFO signal 8700-9200 to
> yield the fixed IF of 3180 kcs.   Measuring at the signal grid of the
> mixer (Pullen, 6ES8), I obtained an IP3 of +13 dbm (tolerable) and an
> IFDR of 65 db.   In other words, I lost nearly 20 db of dynamic range.
> Looking at what goes into the calculation of IFDR, the noise floor was
> up from -107 dbm to -84 dbm -- a 23 db increase in the receiver
> noise.
>
> Gee ... I think I found the next problem to work on.   Noise in this
> type of mixer can arrive by three routes:
>
> 1.  The tube itself -- some is unavoidable.
>
> 2.  The filament circuit -- since the cathode is 'hot', if the filament
line
> is noisy, noise will be picked up.   This seems very unlikely because
> there's a line filter and a pi filter in the filament line to the front
end
> tubes.   And since the cathode resistor is 470 ohms, the cathode is
> not extremely sensitive.
>
> 3.   The VFO.   No VFO generates a perfectly pure sine wave signal.
> For conventional oscillator circuits (PLL's have a whole new set of
> problems) there's a spectrum peaking at the nominal frequency and
> sloping away rapidly on both sides.   However, when you're looking at
> 3 volts of peak (desired oscillator injection) you have to slope down
> pretty far to get below the few-microvolt level that's the tolerable
> maximum at the 3180 kcs IF.   (One volt to one microvolt = 120 db!)
>
> It helps that the VFO frequency is so far away from the IF -- about 9
> Mcs to about 3 Mcs -- but 120 db is still a lot.
>
> The VFO is likely a problem.   If you disable the signal input to a
> mixer then when you also disable the oscillator input there should be
> only a slight change in the mixer output noise.   But as things stand
> now, the change is significant.
>
> (Even a pure oscillator signal would cause tube noise at frequencies
> 3180 kcs above and below the oscillator frequency to be mixed to the
> IF so it will produce some increase.)
>
> You can think of a VFO as a regenerative detector operating in
> oscillating mode.   We all know how a regen amplifies the tube noise
> -- that's the famous regenerative hiss.   But that hiss also modulates
> the pure sine wave you'd like to get from a VFO.   The problem can
> be minimized at the source by using a high Q tank and indeed a
> crystal oscillator causes much less trouble.
>
> Here we see the bite of a design compromise:  In order to get the
> oscillator to track the dial, I was forced to use a higher L/lower C
> combination than I wanted.   This is by design a relatively noisy
> oscillator.
>
> There's yet another potential oscillator issue.   By the nature of the
> design, the VFO is about 4" from the 2nd mixer.   Each circuit is
> grounded in its own location -- the usual good practice.   But that
> means that 4" of chassis is in series with the oscillator injection.
> Bypassing the injection grid to the cathode of the stage does much
> more to reduce the output noise than does bypassing it to the
> chassis nearby, suggesting that noise voltages on the chassis are
> an issue.
>
> Since the signal input to and output from the 2nd mixer are
> transformer coupled I can return the mixer signal grounds to the VFO
> -- the cleanest way would likely be via the shield of the RG-174
> injection cable.   At the same time I'll move the buffer plate tank
> circuit to the oscillator assembly (temporarily mounted at the mixer
> but radiation from it causes some humongous spurious responses)
> and provide a place to add a 3180 kcs trap on the oscillator output if
> needed to reduce oscillator spectrum noise.
>
> (Can't use an untuned buffer at 9 mcs because the stray
> capacitances are too large to allow a high enough plate impedance
> for the necessary gain unless circuit is tuned.   Another possibility is
> a filter with a peak centered on the VFO range and a notch at 3180
> kcs.   This is complicated by the need for a substantial bandwidth on
> the peak.   Note to self: study some filter design next time, too.)
>
> This means, of course, a substantial rewiring of the 2nd mixer plus
> removing the VFO assembly: I sure could have provided better
> access to the screws that hold the darn thing in place.
>
> I'm beginning to understand why there were few good receivers
> produced in the 1970 time frame.   Of course some of those
> designers knew a lot more about this stuff than I do so they'd have
> wasted a lot less time falling through holes in the learning curve.
> But most of the best of them were working for Collins ...
>
> One exception -- the Drake R4C is at the top of at least one list of
> high performance receivers.   With a noise floor of -138 dbm and an
> IP3 (@20kcs spacing) of -10.5 dbm it has an IFDR of 85 db -- truly
> excellent performance for its day.
>
> Anyhow, it's a hobby.   I don't have a deadline, although I'd like to
> have the project sufficiently wrapped up to set it aside over the
> summer.
>
> Walt Hutchens
> KJ4KV
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