[Premium-Rx] Preselectors

[John Reed]

----- Original Message ----- 
From: "Michael O'Beirne" <michaelob666 at ntlworld.com>
To: "Premium-Rx" <premium-rx at mailman.qth.net>
Sent: Saturday, December 01, 2007 10:51 AM
Subject: [Premium-Rx] Preselectors

>
> 2.    A pro told me a couple of years back that he worked at a large Air 
> Traffic Control centre near London where his building was stuffed full of 
> QRM-producing computers.  They had to monitor a particular HF channel for 
> which they used an old Eddystone 958 (which has 3 tuned circuits before 
> the 1st mixer).  After 20 years or so they needed a replacement.  None of 
> the new "PR" type receivers trialled were any good at that location until 
> he tested a JRC NRD-535, which (to his considerable surprise) worked very 
> well and was a lot cheaper.  He reckons the internal preselector was doing 
> its stuff in reducing the unusually high level of wideband QRM to a level 
> acceptable to the 1st mixer.
>
He may have found it acceptable, but I have a problem with the
NRD-525,535,545 preselectors.  I have a 525 and 545.  The design of the
preselector is very clever and is similar on both.  There are 5 switched
bands covering the medium and short wave bands, and in each band a voltage
is supplied by the tuning logic to voltage variable capacitor diodes to tune
the preselector through that band.  Very nice design except for one problem:
When a strong broadcast station's voltage hits those tuning capacitors it
sets up cross modulation of that station on all the other signals.  I have
that situation here, and the only way to stop it is to switch in the
attenuator.  In fact, the receiver does better in this situation with the
preselector switched out, but I still have to use the attenuator to get rid
of the problem.  My Racal RA-6793A handles this situation best.  I also have
a Harris RF-590, however the synthesizer is too noisy to do much good close
to the strong signal due to excessive phase noise.  With a quieter
synthesizer the Harris would probably do well.  Neither of these receivers
shows any sign of the cross modulation problem.

John Reed, KA5QEP