[HBR] G2DAF receiver

[hbr@mailman.qth.net]

It appears to me that the early G2DAF receivers are entirely 
conventional double-conversion-with-tunable-1st-IF designs.   They 
should beat the W6TC designs on stability and since they use a 
mechanical filter, perhaps on skirt selectivity; otherwise they're 
unremarkable.   

What gives the Mk II version of these receivers such remarkable 
specs is the push-pull design of the front end.   By cancelling out 
second harmonic distortion, this makes the strongest third order 
products (which are a second order product combining with a 
fundamental) very much weaker.   

(For example, consider a weak desired signal at 3840 and very 
strong unwanted signals at 3800 and 3820.   Distortion in the RF 
stage produces the second harmonic of 3820 = 7640 and also gives 
the difference between this and 3800:  7640-3800=3840 which 
interferes with your desired signal.   "Third order intermodulation 
distortion dynamic range" (a mouthful if ever ...) is "How strong do 
the two unwanted signals have to be to produce a detectable 
interfering signal?"   

(But push-pull amplifiers tend to cancel out second harmonic 
distortion products -- that's why hi fi amps are push-pull designs.   
Less second harmonic means less to combine with the other signal's 
fundamental and a correspondingly higher 3rd order IMD dynamic 
range.)

One would not have to build the entire G2DAF receiver or even 
include the bandswitching feature in order to get that performance.   
Plug in coils for a push-pull front end would be a little more trouble 
than for the conventional design, because they need to be center 
tapped and tapping down for bandspread (as in the HBR-series) 
requires two tapes/coil.   However the narrow bands tuned by any 
ham-band-only receiver mean that balance will be determined by the 
shunt caps.   You would need a five section tuning cap of the regular 
sort, using four sections in two pairs to tune the push-pull RF stage 
and the remaining section to tune the 1st oscillator.   

Circuitwise there's nothing much to either the RF stage or the mixer.  

In other words, the critical idea can be grafted on to the regular HBR 
design.   It's just a different way to do the RF stage and mixer, not a 
wholly different receiver design.   

I speculate that you could make it even easier:  Omit the RF stage 
entirely -- in the G2DAF design it's just a controllable attenuator 
anyhow.   Drive the grids of the mixer in push-pull from the antenna.   
Inject the VFO on the cathode and take IF output push-pull from the 
plates.   The mixer's where most all the distortion comes from 
anyhow.   Three gang tuning cap that way; use a toroid for the 
antenna coil and keep the antenna coupling very loose.   

So many projects, so little time ...  Back to the 1MHBR!   

Walt
KJ4KV