[HBR] HB-65 or HB-67 experience?

[Walt Hutchens]

Brian said:

> Anybody have any experience with either the HB-65 or HB-67 from the 1966 and
> 1967 ARRL handbooks? Inquiring minds, and all that (;->).

No experience, but -- this isn't going to surprise anyone -- an
opinion. As a way to use a mechanical filter that you have and want to
use this is good; otherwise it's a mixed bag.

The problem in HF receivers is NOT low noise or hearing weak signals.
At least up to 20M the problem is rejecting the vast amount of garbage
found in and near the ham bands. The mechanical filter solves part of
this problem -- near channel interference -- but unless used with a
great deal of selectivity ahead of the filter, it does not solve the
problem of images -- signals coming in on the other side of the LO
frequency. That's because the frequency is too low for a 1st IF.

A mechanical filter could work well at the start of a second IF,
following a 1st IF of 1.6 Mcs or more. However the design up to the
mechanical filter would have to keep signals small because these
filters don't handle large signals well.

One could adapt the W6TC designs to use a 455kcs 2nd IF instead of the
85 or 100 kcs frequencies that are standard. Then use a 455kcs
mechanical filter after the 2nd mixer. That way you've got those high
Q front end coils lopping off the image on the lower bands where it'll
be most troublesome.  

The HB-67 design (and relatives -- this design appeared at least
1965-69 in various flavors) could have been improved by the use of
very high Q front end coils a la W6TC, but since it's bandswitching
and predates the common use of toroids, not so.

I would probably also go with an RF stage run at close to unity gain
in order to simplify the coupling between the coils and allow AGC
ahead of the filter. The 6EH7 would be ideal for this. Of course that
does complicate the design.

For use right behind the 1st mixer CRYSTAL filters at higher
frequencies are an ideal choice. The Heath HR-10B filter -- two
1680kcs crystals in a half-lattice arrangement -- is simple and works
well.

These filters don't handle super-large signals either -- the crystals
can fracture, as found in some of the HR-10s out there, probably due
to the owner transmitting with the antenna still hooked to the
receiver. But as far as normal signals are concerned they're good and
being located at the 1st mixer makes it easier to keep signal
amplitudes down. Up to the point of fracture I don't think non-linear
effects are a problem.

In fact, crystal filters and 'mechanical' filters are both mechanical
filters. The ones called 'mechanical' couple electrical signals to
mechanical resonators by magnetostriction while the crystal filters do
it via the piezoelectric effect, but in both cases you're using
mechanical vibration of a solid as a tuned circuit analog.

An additional minor issue with the HB-67 design is that it 'uses' a
beam deflection mixer basically as a high gain pentode mixer. To get
the advantage of balanced mixing (so that the input signal is
canceled at the output) it would need a balanced output circuit,
meaning some sort of coupling transformer ahead of the mechanical
filter -- or a filter specifically designed for balanced input.

Walt
KJ4KV