Topband: SDR-1000 IMD-DR, blocking dynamic range, IP3, and IP2 measurements

Tom Rauch w8ji at
Thu Feb 15 21:08:37 EST 2007

I live on a dirt road in Georgia and work at home.  Now that 
that's out of the way, let's look at what the ARRL says. :-)

> In EVERY single case where I have seen numbers similar to 
> those written
> up and distributed here,  the radio was set up incorrectly 
> or there was
> some other problem.  Working with one of the good 
> suggested sound
> cards,  the radio has consistently, in EVERY SINGLE CASE, 
> measured 100
> dB IMD dynamic range with an IP3 hovering around 30 dBm.

I measured:
 IM3 DR between 76.2 and 94.1dB on 160 meters depending on 
preamp and other settings and the particular IM product.
98.7dB blocking on 160 meters.

The ARRL measured   (later 
date Flexradio data):

between 81 and 96dB IM3 DR on 80 meters depending on preamp 
between 105 and 111dB blocking on 80 meters depending on 
preamp setting

The ARRL didn't do 160 and I didn't do 80, but the numbers 
aren't exactly miles apart. I quoted an average value in my 
post and I did a different band.

Maybe everyone would be a little happier if I quoted the 
absolute maximum, maybe the setup wasn't exactly optimum, 
and maybe there is a few dB difference from radio to 
radio.....but that wasn't the main point. I prefer to let 
the numbers speak for themselves in that regard. Use the 
ARRL's 80 meter numbers on 160 instead of 160 meter numbers 
if you like.

> The big difference between this receiver and any other 
> receiver I have
> EVER measured is that these same numbers are true at 20 
> kHz,  5 kHz,
> 1KHz, and lower.

...and therein lies the problem that stopped us from using 
the radio as we hoped.

Even 80-85dB close spaced IM3 DR is enough here if the 
receiver only has to handle a window a few kHz wide, but the 
problem is the entire 160 band is just packed full of strong 
stations during a contest. While a regular superheterodyne 
receiver only has to handle things in a couple stages that 
can easily be made very stout before hitting a filter, a 
direct conversion receiver has to handle the entire mess. 
There is a huge increase in overall signal power in that 
entire wide window.

While the FT1000MP MK V with preamp on (-135dBm sensitivity) 
had a blocking DR of around 140 dB at wide spacing and 
around 135 dB at 20kHz spacing, the SDR radio only had  ~ 
99dB blocking at any spacing (with preamp set to essentially 
the same sensitivity as the Yaesu). Wide-spaced dynamics 
that were already marginal decreased by 30 to 40dB, and 
meant the radio could not be used for what we were trying to 

This certainly is not a slam against the SDR, but just a 
factual observation in a particular real life use that 
differs from other situations. Someone claimed he used a SDR 
in an even worse environment to find weak new stations while 
transmitting, and we thought that would be a useful tool. 
The problem is it just couldn't come close to doing that.

I'm sure (as I already said) in a band that was 
significantly less crowded with very strong signals and we 
were not trying to receive while transmitting, or in a case 
where the local noise floor was 30-40dB stronger, the SDR 
would have been useful as we tried to use it. This doesn't 
mean it is a bad radio for everyone, it just means in a high 
power crowded contest situation in a low noise environment 
there are much better choices...the Orion and even the 
FT1000MP MKV being two.

73 Tom 

More information about the Topband mailing list