[hpsdr] Janus frequency response ?

[Phil Harman]

Hi David,

I did post this issue regarding the input attenuator some time ago.  You may 
like to consider connecting the balanced inputs of the Janus card directly 
across the QSD capacitors and not using a pre-amp at all. You should find 
that the noise figure of the Janus card is low enough for that to work and 
also give a useful increase in dynamic range.

73's Phil...VK6APH


----- Original Message ----- 
From: "David Woodhead" <dwoodhead at bms-inc.com>
To: "Ahti Aintila" <oh2rz.sdr at gmail.com>
Cc: <hpsdr at hpsdr.org>
Sent: Wednesday, June 06, 2007 12:10 AM
Subject: Re: [hpsdr] Janus frequency response ?


> ***** High Performance Software Defined Radio Discussion List *****
>
> 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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