[Premium-Rx] Preselectors

[L D Ritta]

Tell me how do the RA6790/GM and RA6793A compare?

73 Lee VK5ABC
----- Original Message ----- 
From: "John Reed" <ka5qep at sbcglobal.net>
To: "premiumRX" <premium-rx at mailman.qth.net>
Sent: Sunday, December 02, 2007 10:59 AM
Subject: Re: [Premium-Rx] Preselectors


>
> ----- 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
>
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