[BC] NRSC and 80 dB floor...

[Burt I. Weiner]

Mike,

With regards to the 80 dB floor the problem is two-fold.  First, many 
spectrum analyzers do not have the low noise in the front end 
necessary for a low noise floor without an external pre-amp.  With my 
494AP I used a HP-461A pre-amp that I pick up from eBay.  I also had 
to play with attenuators at the antenna input and output of the 
amplifier in order to get a proper gain structure for the particular 
situation.  A time consuming adventure and a pain in the tukas.

Overload of the front end is another problem that can cause the 
appearance of spurious signals to be generated in the front end of 
the analyzer.  Chris Scott's filter can help to solve the overload 
problem and would probably help a great deal in my pre-amp 
lash-up.  I also highly recommend his loop antenna.

Since I've been using the Anritsu MS2721A I have been able to deal 
with low power stations and still maintain a low noise floor and not 
have overload problems from other nearby stations.

If you are going to get a Tektronix I would recommend the 2712 and 
then a 494AP in that order of choices.  If you can, try and find a 
2712 with the tracking generator but either way, avoid one that has 
the CISPR or EMI/EMC measurement capability.  That option trades off 
against the 300 HZ RBW capability.  Let me know how it goes and if 
you like you can contact me direct and I'll be glad to help.

Burt


At 02:57 PM 10/11/2007, you wrote:
>From: Mike Holderfield <mikeholderfield at gmail.com>
>Subject: Re: [BC] 492x -494x and 300 Hz RBW...
>To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
>Message-ID: <470D7732.1040009 at gmail.com>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>Good info Burt. I wondered about using 100 Hz for close in.
>Chris Scott seems to have a way around the 80 db problem:
>
>www.scott-inc.com/html/80db.htm
>
>Burt I. Weiner wrote:
> > Keep in mind that 73.44 only requires 300 Hz RBW for measurements with
> > 11.5 kHz of the carrier.  Beyond that you can use 1 kHz RBW.  When I was
> > using my 494AP for NRSC measurements I used 100 Hz RBW for the close in
> > measurements, which will accomplish the intent of 73.44.  I made a
> > statement in my reports in the methodology section about using these RBWs.
> >
> > One of the things you want to be careful of is that many of the older
> > analyzers did not have the low noise floor necessary to easily allow you
> > to go out to 1 Kilometer and be able to see 80 dB below the carrier
> > (assuming the man made noise would allow it with any instrument!),
> > especially on lower power stations.
> >
> > Burt

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California U.S.A.
biwa at earthlink.net
K6OQK