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

[Michael O'Beirne]

----- Original Message ----- 
From: Michael O'Beirne 
To: Brian D. Comer 
Sent: Thursday, February 10, 2005 11:31 PM
Subject: Re: [Premium-Rx] Recievers for MW and 160 meters.


Dear Brian,

Many thanks for your thoughts.  I agree entirely that it is madness running the LO at a far higher frequency (and with that, the unneeded phase noise) if one can run it at a lower frequency.  The main reasons why we place the first IF at VHF of course are:-
    (1) to simplify the design of the synthesiser and 
    (2) to avoid having to build a complicated and expensive tracked preselector to remove the image.  A simple LPF in the antenna input is vastly cheaper!

It is sometimes quite a revalation to go back to tracked preselector technology and to be surprised at how good some of the oldies were.  I would certainly not advocate the HRO.  That really is far too old and the dial is not pleasant to tune, quite apart from the hopeless scale.  On the other hand an AR88 or SP600 with appropriate new components is a different matter.  Their scales were not bad at MW.

It may interest readers to know that one of the conservators at the Royal Navy's communications museum at the training base, HMS Collingwood just outside Portsmouth, rebuilt a real boat anchor, the venerable Marconi CR100 (as used on every British warship in WW2).  We Brits call it a "door stop". I was about as surprised as he was to find that with modern components and new tubes it really was very good.  The frequency scale is not much cop but the logging scale is fine and it was very sensitive and selective.

My point is that if you want to hunt out weak AM stations on LW and MW why use a fancy synthesised radio that has no decent selectivity (or in many cases, no selectivity) where you need it most - ie in the front end.  With a fully tuned front end, there is rarely a requirement for an IP3 of +20dBm or more.

An interesting radio that takes this philosophy a stage further is the fairly modern Redifon R500.  This dates from the late 1980s and is still used on many RN warships. I don't think it has yet made it to Fred's big book.  I shall have to mail him some pics and details.   It has just one IF, at 1.4MHz.  This means that there is only one potential Achilles heel, and it means that the first sharp crystal filter  can be placed directly after the mixer - the best place. 

To remove the image it has an extremely sharp four gang preselector tuned by a stepper motor/gearbox controlled by the synthesiser.  There are two tuned circuits before the low gain RF amp and two before the mixer.  The range switch is actuated by another stepper motor.  The mixer is a ring of four FETs, quite similar to those in the RA1792 with an IP3 at 25kHz off tune of about +27dBm, so pretty bombproof, but further improved by the preselector.  The preselector is a big module about half the width of the chassis.  However, not all versions of the preselector cover MW.  The military ones omit it as there was no need to tune below 1.6MHz and just use a simple LPF.

The excellent IF filters are made by the Canadian company, Snelgrove, and interestingly are cascaded with low gain amps in between to make up the gain.  The 300Hz filter is thus in series with the 1kHz, 3 kHz and 6kHz filters.  For AM types it has a very good synchro detector built round a Plessey SL 623C demodulator.  The internal standard runs at a nominal drift of around 5 in 10^8, pretty OK.

The bad news is that there is no tuning knob - its a 4 x 4 button pushpad plus fine tune up and down in 10hz steps and of course full remote control.  For fixed frequencies - eg the SWL, it is excellent.  I sometimes use it on the end of the IF output of other receivers to improve their AM performance, for example the 30kHz IF output of the Siemens E311 and the 100kHz output of the RA17L.  It's a strange mix of technologies but it works well.

These sets are available in the UK for about £500 in good condition.  I have a copy of the manual if anyone comes unstuck.

73s to all our group and a very good night,
Michael
G8MOB
  ----- Original Message ----- 
  From: Brian D. Comer 
  To: premium-rx at ml.skirrow.org 
  Sent: Thursday, February 10, 2005 4:12 PM
  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



-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.qth.net/pipermail/premium-rx/attachments/20050211/bd3702a7/attachment.htm