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

[hbr@mailman.qth.net]

In a message dated 4/12/03 1:43:55 PM Eastern Daylight Time, 
waltah@earthlink.net writes:

> 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.  

I've thought of doing that sort of thing to match an xtal filter but being 
the lazy sort I've used LC matching instead. Some sets have used a slug-tuned 
coil and capacitive voltage divider arrangement (see ARRL handbook), others a 
tuned-toroid-with-link setup. The turns on the toroid link were determined 
empirically with a Q meter. 
>  
>  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. 

This just doesn't sound right. A 6ES8 Pullen making all that 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. 

Could the sours be part of the noise problem?
>  
>  (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.   

Cathode follower, just outside the VFO, maybe? 

>  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.   

The R-4C uses a frame-grid RF stage and mixer, followed by xtal filters. Only 
two stages between the antenna and the knothole, same as in my Southgate Type 
7. To avoid VFO switching, a premixer arrangement is used, which means mixer 
noise and spurs in the premixer have to be controlled. 

Another Idea:

Through the kindness of various amateurs over the years I have managed to 
acquire many of the parts to build the "Miser's Dream" receiver. 898 dial, 
390016 Oldham coupling, those 2215 kc. filters (thanks, Kees!), coil forms, 
7360s, lots of other goodies. No Polar tuning cap - yet. But I'm not too 
crazy about the front end - I think a double-tuned high Q circuit with nice 
big coils is preferable to the RF Q multiplier arrangement used in that rx. 
And the mechanical design could use a little work - build at least the front 
end on a removable plate so the thing could be used as a test bed for 7360 
vs. 6ES8 Pullen vs. 6JH8 or what have you. 

And I don't have the test gear to put numbers on how it actually performs.

Trouble is, back when I had the time and space I didn't have the parts, and 
now that I have the parts I don't have the time or space. Anybody interested 
in undertaking such a project?

73 de Jim, N2EY