Hans,
Thanks for the reference, I think that was what I was half-remembering.
However, wouldn't that mean that if I left the sampling rate at 48ksps
and changed the DSP buffer size from 512 to 4096 the baseline would drop
by 12db? This doesn't seem to happen.
Chris - AE6VK
-----Original Message-----
From: Hans-Juergen Hartfuss [mailto:[EMAIL PROTECTED]
Sent: Monday, March 05, 2007 12:07 AM
To: Christopher T. Day
Subject: Re: [Flexradio] Panoramic vertical scale
Christopher, plase see my e-mail to the reflector of December 19, 06
includede below.
73, Hans DL2MDQ
Dear FRS-Team,
an additional remark in the context of the SDR S-meter discussion.
This instrument measures the power within the passband selected.
In discussions with other users on the band, my impression is that the
exact relation S-meter power reading vs. the panadapter baseline
reading is not completely clear to everybody. Unfortunately the Manual
does not clarify the question.
Without any signal (50 Ohm at the receiver input) the panadapter
baseline gives the noise power as any other spectrum analyzer system
gives for a given bandwidth (noise power spectral density). However,
the question remains, what is the bandwidth the scale is refering to?
It would be nice the reading would correspond to dBm/Hz which is not
the case by comparison with the S-meter reading:
As an example under the conditions described before and at 3.7 MHz with
preamp HIGH, the panadapter baseline is in my system with the FA-66 at
-144 dBm.
Simultaneously the S-meter shows at
5 kHz bandwidth: -119.5 dBm
at 1kHz: -127.2 dBm
at 100 Hz: -137 dBm.
Refering all readings to 1 Hz bandwidth, we get in good agreement of
these 3 measurements: -157 dBm/Hz (an excellent S-meter!).
This means that the panadapter measures the noise power within a
passband which is 13 dB wider than 1 Hz that is 20 Hz.
If I understand the SDR-1000 correctly, the whole spectrum is
represented by a number of FFT bins which equals the buffer size
chosen. The 20 Hz bandwidth then results from the fact that I am using
48 kHz sampling rate (SR) and a DSP buffer size (BS) of 2048. For the
bandwidth of one bin we then get
48 kHz/2048 = 23.44 Hz
in sufficient agreement with my estimation of 20 Hz from above, which
means the panadapter gives the noise power in dBm measured within the
bandwidth of one FFT bin. One such bin as the smallest bandwidth
segment available seems to be chosen as the panadapter spectral density
reference.
It would first be nice to have this information, if correct and
complete, in the Manual and second why don't you use the information of
Sampling Rate and Buffer Size to correct the baseline in such a way
that the scale refers to 1 Hz (dBm/Hz) and not to dBm/(SR/BS) as
described?
Thanks in advance for your reply together with congrats once more for
this excellent product. Possibly all this has been discussed long
before, however, I am still relatively new in the field, since May 06,
and still discovering details.
Hans, DL2MDQ
Am Sonntag, 04.03.07 um 20:29 Uhr schrieb Christopher T. Day:
> I'm sure this has been answered many times, but the it seems to have
> fallen out of my brain.
>
>
>
> According to the Manual, the Received Signal Meter reads the actual
RMS
> power within the filter passband. This means it should scale with the
> width of the filter and it sure seems to do so.
>
>
>
> I can't find any similar definition for the vertical scale of the
> Panoramic Display. What does that reading mean? If it were the power
in
> the minimal FFT bin, I would expect it to scale with the DSP Buffer
> Size, i.e.., fewer FFT point => wider bin => more signal. Maybe I'm
> missing it, but it doesn't appear to scale that way.
>
>
>
> Is the vertical scale already divided out to be RMS power/Hz? Thanks
>
>
>
>
>
> Chris - AE6VK
>
>
>
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