Fw: Re: Fw: Re: [Test-Equipment] Spectrum Analyzer BW measurement

Fri, 30 Jan 2004 16:53:11 -0600

Say Jim, since you also have a Marconi TF-2370......

What do you get on your's when measuring the BW of the internal 10Mhz
signal at -3dB, -25dB, -45dB and -55dB when using the "2" (normal) and
"3" (wide 10x) position ? 

I get:
-3dB 500Hz BW and 4.7Khz BW (+4.2Khz)
-25dB 1.6Khz BW and 12.6Khz BW (+11Khz)
-45dB 2.4Khz BW and 21.6Khz BW (+19.2Khz) 
-55dB 3.2Khz BW and 30Khz BW (+26.8Khz)

Some of the above are "a little difficult to measure" but it basically
holds to the "wide" position being 10x the "normal" position, which it
should be. But what this indicates to me, is that (me) measuring a real
20Khz BW signal with the 500Hz RBW may measure 23Khz BW at -55dB, but
measuring the same signal with a 5Khz RBW may measure 45Khz BW at -55dB.
That is what I see when measuring these transmitters ....around 17Khz in
the "normal" 500Hz RBW position and roughly 42Khz BW in the 5KHz RBW
"wide" position. I just want 1) to make sure my SA is OK and 2) how do I
correlate to a -55dB, 3Khz RBW specification. 

If I were a rough estimating man, I'd say 3Khz is roughly 6x "normal", so
about 18Khz plus the 20KHz signal, so at -55dB a real 20Khz BW signal
would measure approx 23Khz with a 500Hz RBW, 38Khz with a 3Khz RBW, and
45Khz with a 5Khz RBW. How does anyone get any good correlation data with
that scheme ??? I sure can see why you really should just go back to the
Old Rule of reducing the RBW/BW number down to a few % for good BW data
and let 'er rip.

I was asked if this was a paying job. No, I'm early retired and really
like this stuff. The transmitters being worked on are Radio Control units

for the club which I just decided to look at and maybe stir up some
interest with the appliance operators. But I need some more education
first.

73 Kees K5BCQ   

--------- Forwarded message ----------
From: [email protected]
To: [email protected]
Cc: [email protected],[email protected]
Date: Fri, 30 Jan 2004 15:53:43 -0600
Subject: Re: Fw: Re: [Test-Equipment] Spectrum Analyzer BW measurement
Message-ID: <[email protected]>

Kees and group,
I would guess it depends on the shape factor of the filter.  The Marconi
uses crystal filters, should be very steep skirts!  This correlation
issue is  an ongoing problem when you look at specifications closely. 
Typically, these issues are ignored or at best overlooked.
Comments?
Regards from Arkansas,
Jim
An interesting side note, I joined this group when I started
troubleshooting my Marconi SA.  I just plugged through and found the 10
ICs that had given up the ghost in the logic unit and the uncountable
open via connections.  Took months of off and on effort.  Now I am
collecting test equipment to check power levels.  

On Fri, 30 Jan 2004 14:57:27 -0600 [email protected] writes:
> Wait a minute, that "actual BW is roughly the measured BW minus the 
> RBW
> applies only to the -3dB point". The -55dB point of the internal
> oscillator, when measured with a 500Hz RBW is 3.2Khz and when 
> measured
> with a 5Khz RBW it's 30Khz. Approx a 10x relationship which is the
> specified RBW ratio. So now how do I correlate to a 3Khz RBW 
> measured on
> a different SA ? 
> 
> 73 Kees K5BCQ  