[HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 1

[hbr@mailman.qth.net]

Small signal performance (what is the weakest signal you can hear 
when the band isn't crowded) was simple -- around 0.5 uV sensitivity 
for 10 db s/n.   

Large signal performance is generally more important for HF 
receivers because few bands are uncrowded and even when the ham 
band is clear there are many high power SW broadcast stations and 
local sources of interference.   I can measure one SW BC station at 
0.1 volts on my antenna terminals; when I lived near Washington, 
DC, the total antenna voltage was around a volt.  

One measure of strong signal performance is the 'two-tone third order 
dynamic range.' Forgetting the third order part (nobody much 
measures anything else) this is "How strong do two equal signals 
have to be before non-linearity in your receiver produces detectable 
false signals?"   

For example if you put in signals at 7200 kcs and 7220 kcs, you will 
at first hear nothing else.   But as you keep increasing the two 
signals, eventually you'll hear two new signals at 7240 and 7180 kcs. 
For the best typical vintage receivers able to hear an 0.5 uV signal 
you might get spurious signals of detectable strength at around 
5,000 uV -- an 80 db dynamic range.   

How good is 80 db?   S9 is most often 50 uV which is 40 db above 
an 0.5 uV signal.   40 db over S-9 is 80 db above 0.5 uV.    On any 
good night on 80 meters, some of what this typical receiver hears, 
isn't there -- it is spurious signals generated by internal distortion.   
And not only is this receiver hearing signals that aren't there, but the 
signals that *are* there, are distorted -- muddy sounding.   If sigs are 
muddy on your transmitting antenna but clear on a clip lead, your 
receiver doesn't have enough dynamic range for the operating 
conditions and you're experiencing 'intermodulation distortion.'

My HBR2K tested at around 60-70 db -- disappointing and certainly 
not adequate.   Back to the workbench!

Fortunately the set covers 160 meters and I have a 5 Mcs scope so I 
could actually look at the distortion.  The problem can be studied just 
as you would distortion in an audio amp, by injecting a signal and 
seeing where it gets messed up.   You have to use much larger 
signals to see distortion than you do to hear it, but my URM-25 goes 
to 100,000 uV.   WOW -- when you stick that in, the voltage reaching 
the 1st RF grid is 2 volts p-p due to the high Q of the antenna tuned 
circuit!   That's a whole bunch for a low noise RF amplifier and all but 
impossible to handle in a conventional mixer, especially if the RF 
stage jacks it up some more.   

Sure enough, a beautiful 2 volt sine wave in at the RF stage grid gave 
a clipped sine wave between the two sections of the tube and a real 
mess at the cathode of the 1st mixer.   (1st oscillator disabled so the 
waveform would be easy to understand.)   

I switched to a cascode (triode plate drives the cathode of another 
triode) RF amp circuit; that helped some.   Then by increasing the 
cathode resistors on both sections I reduced the gain to just a few 
db.   The situation in the RF stage was much better but the dynamic 
range didn't improve much because the big problem is in the mixer.

I decided that the Pullen mixer isn't a good choice in this situation.   
Although I have an article claiming a two-tone dynamic range 
approaching 120 db with this circuit I can't see how that's possible if 
an RF stage is used and I don't have enough information to figure the 
situation out.   

I changed both mixers to a 'push-push' design favored for large signal 
handling at the expense of lower gain.   That gave a good over-all 
gain distribution -- receiver noise is now dominated by the RF stage, 
as it should be.  But still only a 70 db range.  

The reason is clear enough: the gain of the 1st mixer is low so the 
RF stage gain has to be relatively high.   But that means that the 
signal voltage reaching the mixer is too large for linearity on the input 
signal path -- both the RF stage itself and the mixer are pushed into 
non-linearity at signals of only 1000 uV or so.   

The 1st mixer gain is low partly because of the circuit but also 
because of the low injection voltage from the 1st oscillator.   And 
*that's* low because with the small crystals of the FT-101 my simple 
Pierce oscillator wouldn't deliver more without drift.   

Very interesting design issues here!   There are several ways to go.

D'oh!  Department.    I heard a whistle in the mornings.   It was there 
on more than one band and did not depend on tuning so obviously it 
was a signal coming directly in to the second IF at 3180 kcs.   But 
how?  There are three very good tuned circuits ahead of there.   Then 
I looked at where the antenna connector was mounted.   I had used 
the original FT-101 location but later added a cathode follower to drive 
the 3180 kcs crystal filters, locating the new stage *right by the 
antenna connector*.   Suddenly the problem wasn't that hard to 
understand -- the connector and wiring was radiating directly to the 
filter driver stage, less than an inch away.   I moved the antenna 
connector to the left rear corner of the chassis (shortening the leads 
considerably in the process) and the problem disappeared.

By the way, because the RF stage does really well when there's 
some AGC voltage the receiver works fine in the usual situations.   
Weak signals on a quiet band are okay, strong sigs on a noisy band 
are okay too.   It's the weak signals on a noisy band (that's what 
dynamic range measures) that's a problem.   I still think it ought to 
be possible to build an all-round outstanding receiver along these 
lines and I'm not yet ready to quit trying.

Walt Hutchens
KJ4KV