[Lowfer] SDR

[Art]

Laurence,

If you mean the softrock type receivers, I can help although my LF 
experience is minimal.

The QSD has a problem with odd harmonic response, much more so than 
typical hetrodyne type superhets. For this reason, several that have 
tried softrocks on LF have been less than happy due to interference 
from the third and fifth harmonics of the frequency of interest.

The answer is a totally bullet proof front end. Don't try any 
wideband front ends, those that would be just fine on a superhet will 
disappoint you severely if tried on a softrock. I built a 7 element 
bandpass for 160 to 190 KHz and the performance is awesome!

There is a German on the softrock mailing list that discovered the 
same thing. I can look up his email address if you want to holler at 
him directly-although he built a very fancy tunable front end with 
active amps for his.

The other thing is don't even think about running a softrock with a 
random wire. This is an area where I do have much experience::> And, 
I learned the hard way. Even with a bullletproof front end, using a 
random wire will be disappointing. The random wire is just fine as 
long as it presents a relatively low impedance, anywhere between 10 
to 100 ohms. The reason for this is that the antenna impedance is an 
integral part of the radio-that impedance is in series with each of 
the 4 integrating caps (because the switching 'on' resistance is 
relatively low). Use a resonant tuned loop with a sampling loop to 
convert the impedance to a low Z and you won't be disappointed.

I should also say that the higher atmospheric noise on LF means that 
the gain of the post integrating caps is much to high-ideally these 
should be reduced to improve the dynamic range. In a pinch, such as a 
portable operation, an attenuator can be used-be sure it's a 50 ohm 
in/out attenuator. On HF, the gain is 10 db too high-on LF it might 
be 30 db to high. The LF gain is just a guess, so don't hold me to 
it. Overload in a softrock based on analog switches is exactly the 
same as overload in a superhet-if you see output voltage approaching 
1 or 2 volts p-p on the I and Q outputs, you are definitely overloading.

The softrock is an exceptional performer, but it has quirks-just as 
any type of receiver does. Applying superhet MO's to a softrock will 
result in less than spectacular performance.

Enjoy-feel free to write me off list if you like.

Art



At 08:59 AM 3/6/2007, you wrote:


>Anyone any practical experience on using the sdr iq or similar 
>"radio" on LF/MF and any interference/noize issues from PC noize? Im 
>trying to reduce the volume and shipping weights of the 
>radio/computing system a bit as I wander. I know the IQ model is 
>very new so maybe not a lot of experience yet..
>
>Anyone had any experience on the 897 or 857 for LF with a preamp up 
>front on LF image/imod etc issues? Not that impressed with 817 on LF 
>as you may remember even with preamps.
>
>Thanks -
>
>We leave Shanghai for the last time around May 20th and will turn up 
>in Singapore a few weeks later.
>
>Laurence VR2 /KL 1X
>wet and cool Hong Kong