Stan Barr ) writes:
On 7 Jan 2007 10:15:35 -0800, Nomad wrote:
The relatively inexpensive Yaesu FRG-7 also uses the Wadley loop. It's
an excellent performer for the price (although solid state).

Somebody once wrote about them that Yaesu were so pleased at getting the
Barlow-Wadley loop working in solid-state that they forgot to give the
receiver any rf performance :-)

There's a whole raft of mods to bring it up to scratch though...

The Wadley loop is often seen in mystical terms, presumably because
people aren't bothering to understand it. Obviously, it was a way
in the fifties to get rid of that need for crystals every 500KHz, yet
in retrospect I'm not sure that the effort couldn't have been put
into a synthesizer. After all, generating a first mixer signal every
500KHz requires about the same circuitry if done with a PLL. A few
years later there were receivers that used PLLs for such signals,
though over the years I've seen posts where people mistake those
PLLs with Wadley loops.

The problem with the Wadley loop is that it puts at least an extra
mixer in the signal path, and by definition you can't put
ultimate selectivity until three mixers down. Done right, as I'm sure it
was done in the Racal receivers, it works. But done carelessly, and you
have a receiver that uses a more esoteric design but automatically
comes out worse than something done a different way. After the Racal,
it mostly seems to have been used to cut costs, but once you reduce
costs the extra mixers in the signal path are a liability.

Moving the "synthesizer" out of the signal path means you don't
have that extra mixer in the signal path. And I'm not convinced
that making a decent PLL that only has to generate signals every 500KHz
is harder than all the filtering and isolation that the Wadley loop
requires.

People have confused the PLL in the HRO-500 and the mix-sixties Galaxy
receiver with the Wadley loop because on some level they are similar.
I've explained the Wadley loop in the past so I'm not going to explain
it again, but the HRO-500 PLL used a crystal oscillator at 500KHz,
generated lots of harmonics, and then the variable oscillator would be
locked to the harmonic of the reference. You'd be tuning the oscillator
with a manual variable capacitor, with a small varicap to actually tune
it to look by the voltage out of the phase detector. There is
similarity to the Wadley loop, but they aren't the same thing, and
you get the "synthesizer" out of the signal path in the receiver. The
circuitry would be about the same, if not a tad simpler, for the PLL.

SO in the end, I'm not convinced of the Wadley loop being anything
more than a neat trick, which at first seemed like a great solution
in the fifties but in retrospect may not have been.

Come the seventies with the FRG-7, there ultimately was no good
reason to use the Wadley loop in there, and using a PLL might have
given it better specs.

Michael VE2BVW