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

Sat, 31 Jan 2004 22:50:45 -0500

Kees,

Keep in mind that the -55 db point on the filter may never come into 
play in your measurements.
I don't know exactly what the signal looks like that you are trying to 
measure but the reason that they may have specked a 3 khz bandwidth may 
have to do with the separation of the side band products in the signal.

Using a 3 khz filter will distinguish products as separate signals that 
are farther apart than that. Once you have separated a side band product 
from the main carrier then the filter band width will not overlap it. It 
will look like two signals rather than one signal with a wider filter.

With separate signal products visible the products at -55 db may be only 
a few db out of the noise floor. You will then be working close to the 
nose on the filter (-3 db point). So what you see then will be the true 
representation. Less the 3 db band width of the filter of course. But it 
will not be at the -55 db band width of the filter.

73
Gary  K4FMX

[email protected] wrote:
> Excellent !! My take on it............ 
> 
> Your input signal didn't change, however it shows me that a 10x RBW
> change affected the "measured" BW as follows:
> 
> The -3dB comparisons should be right at 10x, the -50dB numbers should be
> between 5x 
> and 10x depending on the filter characteristics.
> 
> Based on your data, Dave. 
> ---------------------------
> 300Hz to 3Khz       11x at -3dB, 7.2x (11.6/1.6) at -50dB, 6.4x
> (12.8/2.0) at -60dB
> 3Khz to 30khz        9.4x at -3dB, 5.8x at -50dB, 6.3x at -60dB
> 30Khz to 300Khz   13x at -3dB, 10.4x at -50dB, 10.3x at -60dB  
> 
> My input
> ---------
> That compares to my previous measurements in an earlier note (putting it
> in your format):
> 500Hz    500Hz @ -3dB     1.6Khz @ -25dB   2.4Khz @ -45dB     3.2Khz @
> -55dB
> 5Khz      4.7Khz @ -3dB    12.6Khz @ -25dB  21.6Khz @ -45dB  30Khz @
> -55dB  
> 
> My numbers (above) are going to be a little higher because the RBWs are
> wider, the measured signals are not the same, and the filters are
> different.
> 
> Based on my data.
> ------------------
> 500Hz to 5Khz      9.4x at -3dB, 7.9x at -25dB, 9x at -45dB, 9.4x at
> -55dB 
> 
> What that confirms/allows me to make, is a rough approximation (that will
> suffice) of the effect of a spec based on a 3Khz RBW and -55dB vs my
> measurement equipment which allows 500Hz RBW or 5Khz RBW in this range.
> However the 500Hz (few % of BW) RBW in "peak detect" mode will still
> provide a far more accurate representation of the ACTUAL signal BW per
> the FCC requirements. I think it also tells me that the filters in this
> Marconi are pretty good (steep sides). Looks like the AMA 3Khz RBW spec
> was put in place as "quick test" due to readily available test equipment
> in the field, ease of use (fast scan), not requiring peak detect, and
> adequately approximates the FCC spec for older R/C transmitters to
> prevent interference. 
> 
> And yes, the noise floor should drop 10dB for a 10x reduction in RBW
> ...and it does.
> 
> Thanks, guys.
> 
> 73 Kees K5BCQ
>  
> --------- Forwarded message ----------
> From: Dave Emery <[email protected]>
> To: [email protected]
> Cc: [email protected]
> Date: Sat, 31 Jan 2004 20:32:02 -0500
> Subject: Re: Fw: Re: Fw: Re: [Test-Equipment] Spectrum Analyzer BW
> measurement
> Message-ID: <[email protected]>
> References: <[email protected]>
> 
> On Sat, Jan 31, 2004 at 08:53:45AM -0600, [email protected] wrote:
> 
>>That, I understand. Thanks Dave. 
>>
>>Just for curiosity what do you read with your SA when you look at a
>>signal at the -55dB points using the "few %" RBW vs 10x that RBW. I
>>realize actual filter passbands are going to be different, today's
>>spectrum analyzers probably compensate, etc. but I'm interested if you
>>see something around 5x-10x BW at -55dB.
> 
> 
> 
>         I measured my antique IFR A-7550 (it being the one that
> was handiest and not needing work)...
> 
>         300 hz                 300 hz  @ 3db        1600 hz  @ 50 db  
> 2000 hz  @ 60 db
>         3   khz                3.2 khz @ 3db        11.6 khz @ 50 db  
> 12.8 khz @ 60 db
>         30  khz                30  khz @ 3db        68   khz @ 50 db   80
>   khz @ 60 db
>         300 khz                380 khz @ 3db   710  khz @ 50 db   820 
> khz @ 60 db
> 
>         I think its 300 hz and maybe 3 khz values were possibly somewhat
> contaminated by its inherent LO phase noise (incidental FM).   It
> ain't all that wonderful compared to better instruments, and its noise
> floor was about -78 dbC in this case.
> 
>         I tried a couple of different signals including a precision
> GPS derived 10 mhz from a standard and they gave similar results.
> 
> 