Fw: [Premium-Rx] bandscope frequency conversion question

[Michael O'Beirne]

----- Original Message ----- 
From: "Michael O'Beirne" <michaelob666 at ntlworld.com>
To: "H. L." <hl at sfsu.edu>
Sent: Sunday, June 24, 2007 12:28 PM
Subject: Re: [Premium-Rx] bandscope frequency conversion question


> Dear Hal
>
> Using a bandscope designed for 21.4MHz on an IF output at 455kHz via a 
> frequency converter is a complete waste of time.  The reason is that the 
> WJ bandscope (or any other designed for 21.4MHz by, say, Racal or 
> Eddystone) is designed for a very wide sweep width of up to 3MHz or more.
>
> By way of introduction, a bandscope (or "panoramic adaptor" as we call 
> them on this side of the Atlantic) is basically a superhet receiver but 
> the output is applied to the Y input of a CRT display rather than to a 
> loudspeaker.
>
> The IF filter bandwidth of the bandscope depends on the width of the 
> sweep. Basically, the wider the sweep the wider the IF bandwidth must be, 
> otherwise you get very bad distortion on the screen.  It is exactly the 
> same issue as with a spectrum analyser.
>
> Taking the old WJ SM-9404A signal monitor as an example, it has an IF 
> input of 21.4MHz and a variable sweep of up to 4MHz.  For a sweep of 
> 100kHz, the resolution (ie the ability to separate two signals by 6dB on 
> the screen) is 20kHz.  This means that you can distinguish one signal from 
> another on the screen only if they are separated by 20kHz or more.
>
> Now compare this with the total bandwidth of your 455kHz output - say 
> 10kHz for an AM signal at the -40dB points.  Such a bandscope is not going 
> to display anything useful.
>
> What you want is a bandscope designed specifically for an input of 455kHz, 
> eg the WJ-9188A.  This has a vastly narrower sweep width of either 5kHz or 
> 30kHz.  With a 5kHz sweep, the screen resolution spec of this bandscope 
> for a 6dB signal separation on the screen is now only 250Hz.  This means 
> that you can now see two stations separated by only 250Hz.  This is a 
> whole new ball game, and you will find that tuning the receiver with the 
> correct bandscope is very much easier.
>
> I hope this helps.
>
> 73s
> Michael O'Beirne
> G8MOB
>
>
>
>
> If the screen image is to be viewed in real time, then the selective 
> filter in the bandscope must be wide otherwise the image will be badly 
> distorted. Basically the wider the sweep the wider the filter must be.
> ----- Original Message ----- 
> From: "H. L." <hl at sfsu.edu>
> To: <premium-rx at mailman.qth.net>
> Sent: Sunday, June 24, 2007 4:48 AM
> Subject: [Premium-Rx] bandscope frequency conversion question
>
>
>> Just discovered the premium-RX group and am enjoying the archive very, 
>> very much.  Quick question. Has a circuit been published for an outboard 
>> mixer/oscillator that will convert a premium receiver's 455 kHz IF output 
>> to 21.4 MHz for connecting to a WJ (or other) panoramic bandscope 
>> spectrum analyzer. I understand that the display frequency range will be 
>> highly limited (or compressed) and that a lower quality Heathkit SB620 
>> bandscope will give a better, higher resolution display than an expensive 
>> WJ scope because it is designed for a 455 kHz input.  I don't understand 
>> the mathematics of this issue and would appreciate an article reference 
>> if available.  Regards, Hal, KK6HY (main premium receiver: Stoddart 
>> NM-26T --  with a 455 kHz IF output jack). My Stoddart radio history 
>> website: http://online.sfsu.edu/~hl/src.html
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>