[Premium-Rx] Recievers for MW and 160 meters.

[Brian D. Comer]

I realize that most of us always want to have a receiver that does
everything but it seams to me that if you want the best performance at
these frequencies the high IF HF receivers are far from optimum.
 
The problems in building narrow band filters at frequencies greater than
30 MHz are much greater than at lower frequencies. Phase noise increases
by the square of the frequency so using an LO that is about 40 times
what is needed makes no sense. The diplexers that have had all the
discussion are not realizable at 40 MHz with narrow band filters if one
looks at close in stuff. The only passive ways around this that I know
of are complex and expensive. Following the mixer with a broad band
amplifier is the most common solution to this problem. 
 
The point made by Micheal G8MOB that some of the older receivers using a
similar architecture to the HRO AR88 etc. may be a better choice is a
good one. However the HRO that I have has an IIP3 of 9 dBm and a noise
figure 15 dB worse than that of my Orion with the RF amplifier turn off.
This results in about a 30db lower dynamic range than that of the Orion.
I am certain that a receiver made with this architecture using today's
components would make the best High IF HF receiver look very bad.
 
Allowing for the fact that a lot of the noise at these frequencies is
aggravated by highly selective filters I believe that the best approach
today for these bands would be the use of constant delay filters for
some pre-selection and direct DSP. 
 
I think we have a bit of a dilemma that our interest in radios is
somewhat based doing things the hard way. We like to use these bands
more for their challenges than their advantages.  We find it hard to
part with the feeling of the tuning knob that had to turn a four gang
tuning capacitor and hence needed some very nice anti backlash gears and
a flywheel. Now we have to put up with a shaft encoder that runs so free
that a friction device has to be added to make it useable.  In the case
of the Orion this has been added to the knob and is probably partially
responsible for the wobble.  Gone is the analog feel as we now have
steps to deal with as a result of poor shaft encoder resolution. 
 
At this time I have an eddystone EA12  and an Orion on my desk. Most of
my listing is on 160, still trying to learn the code after 40 years! If
I want to just play listening the EA12 feels nice to tune and is great
fun. When it comes to being serious the Orion stays on frequency, is
easier to tune, has better adjustable selectivity, is orders of
magnitude better in handling QRM and is about 50% computer.
 
 The Orion's main receiver has no roofing filters in the normal sense,
it simply has crystal filter IF selectivity at 9 MHz and DSP       
 
73 Brian KF6C G3ZVC
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