[hpsdr] Janus frequency response ?

[David Woodhead]

Ahti,

I figured out the problem. The Janus board input has series resistors
(39K) to lower the signal level from the SDR1000. These are followed by
emi filters which = 1000pF to ground. The combination is a nice LPF!
Shorting the series resistors fixed the problem and resulted in a nice
flat noise spectrum with just a few dB roll off on the edges (same as
Delta 44). Ofcourse the gain is too high now so I must come up with a
better solution. I plan to drive the A/D with a differential signal
(presently I'm only driving it on one side) which should improve the
dynamic range a little. I'm not sure this is new information - I just
didn't pay attention to it first time around.

I wonder if anybody has experimented with different LPF values on the
QSD for 192KHz sampling?

73's

David.
KM5TZ

-----Original Message-----
From: Ahti Aintila [mailto:oh2rz.sdr at gmail.com] 
Sent: Monday, June 04, 2007 10:26 PM
To: David Woodhead
Cc: hpsdr at hpsdr.org
Subject: Re: [hpsdr] Janus frequency response ?

David,

In my case with Janus at 96 kHz I get about 4 dB signal (and noise
floor) drop at the display edges and about 10 dB with 192 kHz sampling.
I did not do the test with 48 kHz sampling because the signal drooping
was so small.
However, in these tests we have to remember that the QSD output
frequency response is not flat. It has the bandwidth given by the
formula Eq 8 ( in Gerald's article QEX Jul/Aug 2002, page 20) BWdet =
1/(pi*n*R*Cs)

I checked the input signal to Janus and did not find any significant
difference between the input signal and display frequency response.

I don't understand why do you get so much different readings between
Janus and D44. Unfortunately, I don't have the Delta 44 to compare with.

73, Ahti OH2RZ

On 04/06/07, David Woodhead <dwoodhead at bms-inc.com> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> I have my Janus/Ozy working with a homebrew SDR1000 clone and noted 
> the following in comparing with a Delta 44.
>
> With the panoramic display on maximum span and sweeping a signal 
> generator across this span I see the signal drop about 10dB on the 
> display edges with Janus but only drops 2 or 3 dB with a Delta44. Both

> tests with 48K sampling. I also see this in the noise floor which 
> slopes to about -10dB at the panoramic display edges vs. DC zero 
> (noiuse floor is much flatter with Delta 44). This suggests to me that

> my Janus board has a frequency response problem and I wonder if others

> are seeing the same thing?
>
> I also see a 2nd harmonic component in the panoramic display that is 
> about 10dB worse with Janus than my Delta 44. This is with a signal 
> input about 10dB below saturation (this is twice the input frequency 
> on IQ inputs to Janus). I have not checked to see if this is worse in 
> I or Q inputs yet.
>
> I'd be interested in others observations?
>
> Thanks!
>
> David
>
> KM5TZ
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