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

[hbr@mailman.qth.net]

In a message dated 2/23/03 2:16:35 PM Eastern Standard Time, 
waltah@earthlink.net writes:

(lotta good stuff snipped)

> 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.   

Ayup.   

>  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.   

Two words: Buffer stage.
>  
>  Very interesting design issues here!   There are several ways to go.

>  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.

Here's how I handled the same problem in the Southgate Type 7 and all its 
predecessors. Brute force solution learned from W6TC and W2LYH, master 
homebrewers and rx builders. (Anybody know what happened to W2LYH's homebrew 
rig?)

The Type 7 is single conversion, using a 6EH7 RF amp and 7360 mixer. After 
the mixer is an 8 pole 500 Hz xtal filter which I think was originally meant 
for a RACAL receiver. The RF amp has its own manual gain control, which is 
run just high enough to hear band noise. The IF also has its own gain 
control, which is used most of the time. Adequate injection is provided by 
the premixer/buffer setup used to generate the LO signal. 

I don't know the actual dynamic range, but under contest conditions on 40 
meters there is very little I cannot dig out. 

Not a perfect solution but worth considering.>

Also see:
  
Feb 1972 QST, "An Experimental Receiver For 75 Meter DX Work" or something 
like that. By 
W1KLK (SK)
  
- the front end has 4 tuned circuits (2 double tuned and ganged circuits) 
between the antenna and mixer. Very sharp. Slug tuned. Not ganged to the main 
tuning.

- the RF amp is a dual triode grounded grid (both sections parallelled). The 
basing of the 7044 is such that a single shield across the socket will ground 
both grids and shield input from output. Most dual triode pinouts won't allow 
that. The 2C51 and 5687 have similar bases. The RF amp has only enough gain 
to overcome the loss of the 4 tuned circuits - it's not there to boost the 
sensitivity much. 

- the 7360 mixer does not used fixed bias - it uses cathode bias. However, it 
is on the "RF" gain control line (also AGC) along with the IF amps. This 
mixer uses different resistor values than are usually seen, and the B+ is 250 
volts. But it is a single-ended design. 

- there's a choice of xtal or variable BFO

- the audio AGC is considered marginal by the author, but he also says that 
it is not used most of the time anyway.

 The achieved goals were very high sensitivity/selectivity and excellent 
adjacent strong signal elimination.  

6JH8 or 6ME8 are possible subs for the 7360.

73 de Jim, N2EY