work and was obviously untested at the time of printing .... The first
error is trying to bias the LVDS output amps like they are the
internal comparator in the 9854 ... An LVDS amp is a differential amp
like a balance microphone amp (The article also uses one after the
mixer for an example of a diff amp) and wants to see a differential
(180 degrees out of phase) signal with NO BIASING .... the LVDS amps
are interally biased to act similar to a comparator as was as acting
like a differential amp and it's purpose is to receive a digital
signal from a cable, amplifiy it and clean up the losses due to cable
capacitance which 'squares up' the output ... the LVDS was a good
choice and Gerald kept it after correcting the errors, I measured
about a 5-6DB increase in spur reduction over the stock outputs of the
IQ-VFO kit ... part of this is due to using 200 ohm filters which
lowers the necessary current and partly due to the LVDS amps
The second mistake is you need a low pass filter on each differential
output, 4 total in the case of the 9854 .... here's a picture of the
final design of the SDR-1000 and on the left, just above the
heatsinked 9850 you can clearly see the 4 filters
http://www.tracey.org/wjt/sdr1k/sdr-board-pics/trx-top.jpg
If these things aren't corrected you will not get a 90 degree
quadrature output, or anything even close enough to be software
corrected, which sounds exactly your situation now
I have a schematic of a working design using a MAX9172 which is
Maxim's version of the chip Gerald uses and is 100% compatable ... My
low pass filter is different than others (After numerous simulations
I'm convinced an elliptic filter is a poor choice for low Q surface
mount inductors and a Chevyshev low pass is better using surface mount
inductors. If you use higher Q [and much larger} inductors then the
Elliptic makes sense) but you can just substitute 2 more of the low
pass filters you already have and you can see how to correctly feed
them to the output amps
If you are indeed using the LVDS amps like in the QEX article and
it's OK with Alberto I can upload the schematic and a picture of my
circuit laid out on a section of SMT protoboard
JR
--- In [email protected], "jeanrenier2004"
<[EMAIL PROTECTED]> wrote:
>
> Hi everyone,
>
> I am new to this discussion group, so I may comment about items that
> have been covered before, please bear with me.
>
> I am currently finishing an home build SDR receiver front end
> project (very much) based on Gerald Youngblood's design published
> back in 2003. It is based on an AD9854 IQ local oscillator and a QSD
> plus a pair of amplifiers, the whole being preceded by a set of HF
> filters.
> I have written a little application in VB6 in order to control the
> DDS frequency and the HF filter bank using the parallel port.
> Nothing fancy, just to run some tests.
> Currently I use the following set-up: the audio output of the SDR is
> connected to an SB Audigy SE sound card of my desktop PC (the laptop
> has no stereo audio inputs) which is running SDRadio (version 0.99),
> while the control part is connected to the parallel port of my
> laptop PC (the desktop having no free parallel port). It is bit
> awkward, but it OK for testing, I'll buy a LPT/PCI card for the
> desktop one of these days.
>
> SDradio works great, it is a really remarkable piece of programming,
> good demodulation in AM and SSB (I did not try NBFM yet). The low
> frequency noise, some 3kHz both sides of the 0Hz is very well
> visible (and audible), but can be avoided altogether by tuning
> SDRadio some 5...10kHz up or down.
>
> I have a problem though, for some reason I can not fully calibrate
> the skew of the SB Audigy board, both the phase and amplitude
> cursors should actually be set to some position outside their range.
> I did the same exercise with an old SB PCI128 board, this one can be
> calibrated all right within the range of the cursors.
> I prefer however to use the SB Audigy board, as it has the full 96dB
> dynamic range when used at 16 bit resolution, which is some 10dB
> more than the SB PCI128 board.
>
> Another thing that would be nice, is the possibility to set the
> centre frequency of the spectrum display of SDRadio by an external
> program, the little application in VB6 in my case. So that it would
> be possible to actually change the frequency including the spectrum
> display in one single GUI action. Maybe having some API on SDRadio
> or possbly a global variable that would be read by SDRadio.
>
> As I use to listen in on the digtal modes (RTTY, HF fax, ...) I use
> other programs such as Hamscope connected to the output of SDRadio.
> This is done by actually patching the analog signal from the line
> out of one sound board (the one with SDRadio) to the line in of the
> other sound board (the one with Hamscope). It would be nice if
> SDRadio would have some means to perform this on the internal
> digital signal, avoiding a DA and AD conversion.
>
> Thanks for any help or advise and all the best,
> Jean.
>
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