Dick wrote:
Craig wrote:
>Certainly the vast majority of
>>ham transceivers the imd performance is certainly very poor. Anybody looked
>>at the FT847? its a SHOCKER, maybe i just have bad examp
>
>How bad is the 847? The FCC has regs that specify how much IM a
>transceiver can generate. My guess is the rig is probably about 25 dBc
>or so. While this isn't good, let's remember that for every 3 dB drop,
>we cut our power in half. So if an IM product is 24 dB down, it's power
>is 1/256 of the carrier. At 100 watts carrier, that puts the IM at about
>0.4 watts. Significant? Sure, but not deadly.
Maybe not deadly at HF, but much worse on VHF where the background noise
levels are so much lower. That's a real issue for HF/VHF rigs because
the same standards do not apply... I mean OUR standards, not the FCC's.
The other question is how quickly the IM products go down into the
noise. It's possible to live with the 3rd and 5th order if the higher-
order IPs disappear. The real problem is when they hang up there,
because even IMD at the -70 to -80dB level can cause a lot of trouble to
other VHF weak-signal operators if it's spreading right across the band.
>
>Don't get me wrong. I am all for signals as clean as possible. But
>there are just tradeoffs. The FT-847 is a great rig, but it isn't
>Yaesu's top of the line by any means. You get what you pay for in rigs.
As far as IMD is concerned, it's "we" - all the rest of us - who get
what "you" pay for!
>
>In order to be able to put other features in and keep the cost down, the
>manufacturers gotta cut something somewhere. I would bet the PA is one
>place where the cut costs considerably. A smaller PA means less
>dissipation, less power requiremets, etc. So if you compared the FT-847
>to an FT-1000D, you'd probably see a huge difference in IM performance.
>
That's certainly true, in that only top-of-the-line rigs use 28V devices
and power supplies because of the added expense.
Also, several recent QST reviews of HF transceivers seem to suggest that
the IMD performance of solid-state finals is quite poorly controlled in
production.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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