[1000mp] QST Product Reviews whitewash

Mon, 15 Apr 2002 13:10:00 -0400

Charles Harpole [mailto:[email protected]] writes:

> Major problem with this review:  clearly the xmitter is a dirty little box

> with multiple order signals that will further clutter our bands.  When
guys 
> get on with a big linear and TURN IT UP too far, also, they will be all
over 
> the bands with this one,liks so many others, sadly.

> But, the QST review that mentions this factor (and the dit shortening)
does 
> not POINT OUT THE MEANING of its notation in passing that the 
> xmitter is filthy.  Tell everyone what the numbers MEAN with the same 
> glee that you praise its other features!

The Review said:

"Unfortunately, one number we'd like to have seen change greately in the
IC-746PRO did not -- two-tone, third-order transmit IMD. This number
indicates whether the unit will generate undesireable intermodulation
products that can lead to splatter. On the worst-case band -- 10 meters in
both cases -- third-order products were down by about 23 dB in the original
'746 and about 25 dB in the 'PRO. In contrast, the IC-756PROIII's worst-case
third-order products were down 30 dB -- not terrific, but getting there. The
third order numbers were about the same on the worst-case VHF band, 2
meters.

"On the postive side of the ledger, the IC-746PRO did a better job than the
original model at surpressing fifth-order products on the worst-case bands
-- by about 7 dB on 10 and about 9 dB on 2 meters. (Fifth-order products
were down about 40 dB in the '756PROII on its worst-case band, 10 meters;
that unit does not have 2 meters.)

"Transmitted phase noise on the worst-case band -- 80 meters on the
IC-746PRO and 20 meters on the IC-746, was also slightly worse in the 'PRO
model, which also exhibited a few prominent spikes."

Did you read this part of the review?  

In practice, the higher-order transmit intermod products are generally more
important to hams than the close-in stuff. If we get some splatter from the
adjacent channel, it is just as apt to be our own receiver selectivity as it
is the other guy's transmit IMD.  A QSY of a few kHz solves the problem.
However, if a station has significant splatter up and down the band from the
higher-order products, that presents a much bigger problem.  As I look at
Figure 1 in the 'PRO review from the May issue, I do not agree that the rig
is a "dirty-little box," looking at the overall performance.  I think that
the Product Review hit the markk in saying what it did about transmit IMD
and phase noise.

> Now, I see why ICOM trumpets its rcve selectivity so much...  it is to try
to 
> cope with other ICOM rigs that are sending out barn sized signals.

Receive selectivity won't help with transmitted intermodulation signals.

73, 
Ed Hare, W1RFI
ARRL Lab
225 Main St
Newington, CT 06111
Tel: 860-594-0318
Internet: [email protected]
Web: http://www.arrl.org/tis

> -----Original Message-----
> From: Charles Harpole [mailto:[email protected]]
> Sent: Sunday, April 14, 2002 6:33 AM
> To: [email protected]
> Subject: [1000mp] QST Product Reviews whitewash
> 
> 
> Another glowing whitewash of a "Product Review" in the latest 
> QST on the 
> ICOM 746PRRO.  The text is more pumped up than ICOM's 
> advertisements..... 
> Who writes these reviews, a retired cheerleader?
> Major problem with this review:  clearly the xmitter is a 
> dirty little box 
> with multiple order signals that will further clutter our 
> bands.  When guys 
> get on with a big linear and TURN IT UP too far, also, they 
> will be all over 
> the bands with this one,liks so many others, sadly.
> 
> Now, I see why ICOM trumpets its rcve selectivity so much... 
> it is to try to 
> cope with other ICOM rigs that are sending out barn sized 
> signals.  (the 920 
> specs are little better, but bless the Mark 5 option to send 
> out a cleaner 
> signal).
> 
> But, the QST review that mentions this factor (and the dit 
> shortening) does 
> not POINT OUT THE MEANING of its notation in passing that the 
> xmitter is 
> filthy.  Tell everyone what the numbers MEAN with the same 
> glee that you 
> praise its other features!
> de K4VUD life member ARRL
> 
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