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

[w3jn]

Brian, good points.  Tight RF selectivity is ALWAYS good.  The reason the NF and dynamic range isn't that great on the HRO is due to its prehistoric 6K7 RF tubes, and the abysmal 6A8 (6B8?) mixer.  A popular modification of the day was to replace the 6K7 or 6K7 with a 6AK5, thereby improving the noise figure by some 15-20 dB.

One of the best mixers ever is the 7360 dual sheet-beam tube.  The Squires-Sanders SSR-1 used this tube as a mixer - and it didn't even need a RF tube to achieve relatively good .2uV for 10 db S+n/n ratio, but you could blast a couple volt signal 10 kC from a 10 uV signal and the weaker sig wasn't even affected.  I built a homebrew RX using this tube as a first mixer, directly driving 1.4 MC xtal filters pirated out of a Racal 6217, and its performance really isn't matched by anything else I've used.  Any of the DSP receivers I've used really don't hold a candle to some of the late tube designs.

73 John
  ----- Original Message ----- 
  From: Brian D. Comer 
  To: premium-rx at ml.skirrow.org 
  Sent: Thursday, February 10, 2005 11:12 AM
  Subject: [Premium-Rx] Recievers for MW and 160 meters.


  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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