Topband: Receivers

[Tree]

On Fri, Nov 12, 2004 at 12:11:22PM -0500, Bill Tippett wrote:
> W8JI wrote:
> 
> >IMO, even a 2kHz test is pretty wide for normal CW
> operation. When spacing moves closer in testing, rigs like
> the Orion and Omni get much better than DSP-only based
> radios.
> 
>         No kidding...when was the last time you heard stations
> only 2 kHz apart in the CQ 160 CW?  The only data I am aware
> of for 1 kHz spacings is in ARRL's Expanded Test Reports plus
> a few measurements on Orion by Sherwood.  Most other rigs' IMD
> is phase noise limited at 2 kHz and less (e.g. IC-7800 and K2),
> but Orion's excellent close-in phase noise plus 1000 Hz roofing
> filter yields decent results even at 1 kHz spacings.
> 
> IMD @ 1 kHz  84 dB (ARRL)
> IMD @ 1 kHz  85 dB (Sherwood)
> 
> Orion can also be improved another ~10 dB at 1 kHz spacings by
> using Inrad's #762 600 Hz roofing filter.
> 
> http://users.vnet.net/btippett/inrad_.htm

IMHO (getting really brave jumping in here - so emphasis on the H),
the "in band" IMD is a very important feature when using a receiver
that will be dealing with lots of noise (like Topband).

Think of noise as a bunch of little signals in your passband.  Think 
about how the signal you are trying to receive interacts with that
noise (which is about the same strength - right?).

I built up two crystal oscillators so I could produce two carriers at
about the same strength within my passband - and looked at the resulting
IMD products with an audio analyzer (using software with a soundcard).
Remember, the RX audio is what you listen to.

The performance of most receivers shows the first ordered products to
be down about 35 db.  This is when the RF gain is set by the AGC.  If
you back off the RF gain, you can generally imporve the IMD perforamnce
significantly, often over to over 50 db down for the first ordered 
products.

I have seen the 35 db point on radios like the FT1000MP and TS850.  I 
think the TS930 is actually better (based upon listening by ear), but
I have not actually made the measurement.  

I think this is why lots of us like to back off our RF gains.  

Something to think about the next time you are trying to pick a signal
out of the mud.

73 Tree N6TR
tree at kkn.net