[HBR] Another Receiver Project -- HBR-4, Part 18

[YIR]

I am following your work on this project using the 6JH8 / 7360 series
mixers with great interest. You are actually doing what I am always
trying to find time for.  So for the time being I am vicariously
enjoying the work of your hands and your willingness to also put it out
here for the rest of us.  I am curious if you have actually measured
90 db with your same test setup on the R-390.
I have the schematic of the Squires Sanders SS-1R but don't yet have
it scanned or I would send it along also in case you don't have it.
By the way its 7360 mixers use 140 volts on the plates and 25 volts
on the deflection plates.  If you need other data from the manual
maybe I can help.

73,  Pat  K7YIR



----- Original Message ----- 
From: <waltah at earthlink.net>
To: <hbr at mailman.qth.net>
Sent: Sunday, November 14, 2004 6:20 PM
Subject: [HBR] Another Receiver Project -- HBR-4, Part 18


> 'Some progress' edition.
>
> With the antenna signal applied to one of the 6JH8 deflection
> plates (and assorted tweeking) the sensitivity is about back to
> where it was and the receiver sounds really great -- better than
> anything else I've got.    Testing time ... I hooked up my two
> homebrew signal generators through the homebrew hybrid
> combiner, dialed up 3800 and 3820 and the strongest signals
> they're able to deliver:  IFDR 77 db.   Not great, but another notch
> better.
>
> Unfortunately there's not enough gain to use these generators to
> test at higher levels so it would be possible to determine if the
> problem was in the receiver or was due to bad signals from the test
> setup.
>
> One way the test signals can be bad is if they contain harmonics.
> I built a 7-element lowpass filter (think: two pi nets hooked end to
> end) designed to pass the 75 meter band and kill 40 (where the
> second harmonic of my test sigs would be.   Gee, when I hook an
> antenna through it to the receiver, 80 is unaffected and 40 is gone.
> These are already *very good* signals; running through that,
> there'll be no significant harmonic content at all.
>
> That got the measured IFDR up to 81 db -- slightly the best figure
> for any receiver here, but I don't have anything really good.
>
> The way to pin down the bad guy is to do the test again at higher
> power levels.   Three or four tests, say, one at the noise floor, one
> at S-1, and one each at two S-unit intervals to S-9 and in general
> agreement, confirm the limit as being in the receiver.   If these
> measurements do *not* agree, the trouble is somewhere in the test
> setup or (for complex receivers) possibly in a receiver oddity.
>
> A low pass filter gets rid only of harmonic issues in the test
> signals.  (This could be caused by a non-linear amplifier in the test
> set, in fact, a little non-linearity is basically certain to be present,
> even in a class 'A' amp with a tuned output circuit.)   If, however,
> the basic problem is non-linearity of the combiner or
> crossmodulation of the signal generators, then we're looking at the
> two intended signals at 3800, 3820, being accompanied by
> spurious signals at 3780 and 3840 as they come into the set.
> There's no way to get rid of those with a low-pass filter!
>
> Higher receiver gain will allow testing at higher levels of mixer
> overload and since the receiver does need more gain, that's the
> next step.   However, the needed gain is in the range 10-20 db and
> the max increase in IFDR I can measure is only as great as the
> increase of the gain.   To get beyond that, I'll need to modify the
> generators for higher output -- say 1 volt p-p signals instead of
> 100,000 uV.   That's tougher than it sounds because the
> generators must also have very high isolation between the non-
> linear part of the generator (the tubes) and the output jack.
>
> Because tubes are non-linear, if you feed a signal in at the output
> jack of a sig gen, it will modulate the signal produced by the
> generator and in IFDR testing, that modulated signal looks exactly
> like the bad signal produced by a non-linear mixer in the receiver.
> So the tubes in two signal generators used for this kind of testing
> must be unable to 'see' each other.
>
> Part of the job is done by the hybrid combiner -- a weird little circuit
> using a small transformer and a couple of resistors that (ideally)
> takes 1/4 the power from each generator and adds those two sigs
> together at the output jack (a hybrid combiner has two input jacks
> and one output) and throws all the rest of the power away, so
> neither input jack (signal generator) sees anything from the other
> one.
>
> However, practical combiners don't provide much more than 40 db
> isolation between input ports.   So the signal generators
> themselves need to provide isolation.   It's conventionally done with
> a voltage divider.    My homebrew generators produce a 30-volt p-p
> signal and a chain of resistors takes that down to 100,000 uV
> across 50 ohms.   That's most of 50 db additional, giving (with the
> combiner) maybe 90 db isolation beween each generator and the
> output signal of the other.
>
> You can see the practical problem of increasing generator power.
> If I don't change anything else, then stepping up to 1V p-p output
> requires a 300-volt p-p signal on the voltage divider.   Among other
> nasty problems, what kind of sheilding will it take to keep that
> bottled up inside the sig gen case, so it won't go right past the
> attenuators?   I'm using well-shielded military signal gen cases, but
> they're not *that* well shielded!
>
> Even deciding to use less isolation won't work very well because a
> lower series resistance in the voltage divider chain will lower the Q
> of the amplifier tuned circuit, increasing any waveform distortion.
>
> I'm not eager to start on signal generator power increases.
>
> Another problem is that the receiver crystal filter is right behind the
> first mixer.   With a highly linear mixer, what's to stop a powerful
> input signal from destroying the CF?   Adding an AGC-controlled
> RF stage will help for in-channel signals, however nearby out-of-
> channel signals don't produce AGC voltage.   Very high
> performance receivers deal with this by putting a local AGC loop in
> the front end so that *any* extremely strong signal getting through
> the tuned circuits reduces the RF stage gain.   Typically these
> circuits start to take effect around S-9 to S-9+40 and they
> introduce their own set of new issues.
>
> Fortunately the filter I'm using is stolen from a Tempo-One which is
> a single conversion set.   So it should be a fairly robust filter.
>
> Next step: adding an RF stage.
>
> Walt
> KJ4KV
>
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