[HBR] The long, SLOW HBR project

[Bill Cromwell]

On Fri, 2011-09-09 at 23:15 -0400, Walt Hutchens wrote:

> Sounds like a plan.  But ... why not just go with the 1.681 (or whatever it
> is) IF of the HR 10?   The crystal filter immediately follows the mixer
> circuit which is where you  want most of the selectivity and with two good
> IF stages at that frequency, you're in business.
> 
> You could do a lot worse than following the plan of the HR 20 from there on
> -- basically HR 10 + product detector.  Or go with the HBR setup but don't
> bother doing a second conversion.   The sets are actually a lot alike except
> for the bandswitching and cheaper construction of the Heath sets.
> 
> I would definitely use the circuitry of the HBR sets (but without the 2nd
> conversion); the Heath design is tuned for maximum cheapness.   Read the
> reviews of that set ... it was dead on 15-10M and there was no AGC for SSB
> and CW.   I'd bet on poor AGC, BFO pulling, an unstable LO ... the HBR
> designs will run rings around it.
> 
> The purpose of the second conversion in the HBR sets is to make selectivity
> easy and (with the parts of 1960) relatively cheap.   But you've got a
> perfectly good crystal filter; you don't NEED to convert to 85 kcs.
> 
> The far-out selectivity with a half lattice filter (as in the HR 10 and my
> current project receiver) isn't terrific, but it seems to be good enough.
> Adding an 85 kcs IF behind that would make it great but that's pretty much
> work for a problem I haven't noticed  yet.
> 
> There's an added bit of fussiness if you have high selectivity at two
> different frequencies: The conversion between them has to be perfect.
> Meaning that the 2nd LO has to be spot on frequency and no drift.   It's
> doable but it's a nuisance.
> 
> The HR 10 has just about all the electrical parts you need.   That's an
> interesting way to tackle an HBR project ...
> 
> Walt Hutchens
> KJ4KV

Hi Walt,

I wasn't thinking of tighter selectivity with the 85 kc second I.F.
except as it applies to something I have looked at that was labeled as
"tail end" filtering. That filtering is applied just ahead of the
detector and its purpose is to reduce broadband noise introduced in the
I.F. chain after that crystal filter up front. I think it was in one of
the W1FB/W7ZOI books. Their tail end filter was at the same frequency as
the star of the show up front and was "somewhat wider" bandwidth, for
example a "wide" SSB filter with narrower SSB and CW filters up front.

If I use an I.F. amp at the first I.F. and follow up with the 85 kc I.F.
I'll have to be careful about gain distribution, too. I understand about
the fussy alignment. I could make the second conversion oscillator
variable (VXO) to provide narrower/variable CW selectivity.

With two I.F. "filters" at two different I.F. frequencies it will be
like looking through the front window of a house and out the back
window. If you take a few steps in either direction the view through the
two windows narrows or is blocked completely. The "blocked completely"
analog is not useful but over a limited range the narrower or wider
analog is.

73,

Bill  KU8H