Topband: Omni VII performance on 160m

[Michael Tope]

----- Original Message ----- 
From: "Petr Ourednik" <indians at xsmail.com>
 
> I was very surprised with the ARRL Lab tests which showing the close-in
> Ip3 (5kHz) +10/+6.5dBm (preamp off/on) only. Well somebody (like TT
> sales) saying "its great results far ahead from most of radios in the
> same price category"...but let me see my current Elecraft K2 (simple
> design) has Ip3 (5kHz) +21/+8.5dBm (preamp off/on) and the radio cost
> $600 instead of $3.5k for Omni VII.
> 
> Also if I found that there are NO any BPF's in the front-end just LPF
> (general coverage receiver architecture), 1st IF on 70MHz has fixed
> 20kHz wide roofing and after that is "distributed filtering" down to
> 300Hz on 2nd IF 455kHz ...I do not believe that it can be efficiently
> spent money :(
> 
> BTW: KEY (int. keyer) jack is not reachable on rear panel > available
> from front panel only.
> 
> Also I checked all articles about Omni VII performances from SEI, but
> nothing about Ip3 over there.
> 
> Can anybody (like Tom, W8JI or other guru) make some comments to the Ip3
> / filtering ect. please? I was dissapointed by the results from ARRL Lab
> for first time as my old FT-990 has an Ip3 (5kHz) around +6dBm with
> preamp off if I am right > but its dozen years old radio.
> 
> Thank You or help,
> 
> Petr, OK1RP


IP3 doesn't neccesarily tell the whole story, Peter. You can make 
any receiver have arbitrarily high IP3 by putting attenuation in the 
front-end of the receiver. The more important metric is the closed 
spaced (2KHz ) two-tone 3rd order IMD dynamic range and the 
blocking dynamic range. The Omni-7 looks to have good numbers 
in these two categories (good enough to put it into the top-ten box 
in Sherwood's list which is sorted by 2KHz dynamic range), but 
admittedly the 3IMD DR numbers are no better than the Elecraft 
K2 at 2KHz and slightly worse at 5 and 20 KHz.   

Also, if you look at the way ARRL measures 3IP, they can, 
depending on results use two different methods to calculate 3IP 
(see page 51 - 54 of the ARRL Test Procedures Manual 
http://www.arrl.org/members-only/prodrev/testproc.pdf). As 
indicated by the procedure, the two methods don't always agree. 
In fact, if you use the MDS method, the numbers you get for both 
the K2 and the Omni 7 differ significantly from the published 
numbers indicating that they use the "S5" method (the fact that they 
use the "S5" method to determine IP3 is stated explicitly in the 
Omni 7 review in July QST). The fact that they are using an 
s-meter reading as a benchmark level for 3IP is troublesome. This 
almost guarantees that the level at which 3IP is measured will be 
different for every rig. 

Based on the numbers (I haven't used either rig), I would say that 
the compromise in performance in going from the K2 to an Omni 
7 would be very small making it a worthwhile trade if you really 
wanted the remote control capability of the Omni 7. Dynamic range 
numbers aren't everything, however, so if it were me, I would 
probably wait until the Omni 7 was out in the field for a while 
before I bought one. 

As far as the "Distributed Roofing Filter Architecture" goes, I think
this is just a catch phrase dreamed up by Ten-Tec to put a positive
spin on the fact that the Omni 7 uses the standard KenYaesuCom 
triple conversion architecture with a very wide 1st IF filter roofing
filter (they are probably trying to distance themselves from the 
historical negative connotations of this standard architecture). That's 
not to say that this standard architecture is bad. If the intercept point 
of 2nd mixer is high enough, you can still get good performance with 
a wide 70 MHz 1st IF. I think what Ten-Tec has done with the 
Omni 7 is to try to do a very good job implementing an old 
architecture. Based on the numbers, it looks like they may have 
pulled it off. 

73, Mike W4EF................................................