Avis éclairé de F6DFZ concernant la comparaison du FT2000 avec d'autres postes (en anglais)

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Avis éclairé de F6DFZ concernant la comparaison du FT2000 avec d'autres postes (en anglais)

Message  F5PBG le Jeu 10 Mar - 8:41

Yes, but which charts ?

I have a lot of respect for the work done by Rob Sherwood. But its chart, and the way receivers are classed, describes only a very small part of receivers performances, even if it has the merit to provoque the interest of prospective buyers and by way of consequence the interest of marketing departments of ham equipment manufacturers.

2kHz spacing IMD3 describes the behavior of a receiver in presence of close spaced signals, mainly CW and data. A good figure can be had by a tight selectivity near the frontend obtained by a narrow bandwidth roofing filter (as narrow as 200 Hz for the K3), and/or by a wideband roofing filter and judicious gain distribution (as the RS XK2100 does).

As Rob pointed it, 2 kHz IMD3 has little to mean for SSB because IMD generated by nearby transmitters is more prevalent than IMD generated by the receiver itself ; there, selectivity obtained by the DSP is the main factor.

The chart of Rob Sherwood is very incomplete ; for exemple, he could add image rejection, and IMD2 and IMD3 generated by remote signals.

Image rejection is not a problem with a good up-conversion design, figures well over 100dB are mesured (FT-2000). But it's not the same case with down-conversion designs (K2, K3, high in the Rob chart, Eagle, FT-5000, TS-590). With a 9MHz hign IF, it's difficult and costly to reject images over 70dB on the higher ham bands, even more difficult with a lower IF (K2).
One must understand that these low figures are a real threat when cycle 24 will peak. Can you imagine your "chart topping" $5k+ receiver with the upper ham bands full of BC and utilities image signals ?

Remote signals IMD2 and IMD3 are even more prevalent. These measurements describe the behavior of a receiver in presence of signals for which the sum and/or difference (F1+F2 or F1-F2 for IMD2, 2xF1-F2 or 2xF2-F1 for IMD3) falls were you want to receive.
Take real life exemples, described in CQDL magazine in the 80's by the late DL1BU. Imagine for IMD2 large BC signals from the 31m band combining with large BC signals from the 25m band. If your receiver has poor IMD2 (see ARRL review of some "chart topping" receivers), your 15m ham band will be full of strong combs of BC signals each 5kHz.
The same problem exists for 31m and 19m BC bands falling inside the 12m ham band, and for 21m and 19m BC bands falling inside the 10m ham band, but there are a lot of other combinaisons doing the same.
For IMD3, the 41, 31 and 21 m BC bands can generate tremendous spurious signals on the 40, 30 and 20 m ham bands.

Speaking of real life, I bought in the mid 80's a brand new TS-830s, a very good reveiver, with down-conversion and a preselector. This receiver is quite high into Rob chart, and highly regarded (see eHam.net). When, for the first time I connected it to my Sommer XP507 antenna (a kind of log periodic wide band beam), the receiver was useless during certain parts of the day when the propagation was good on the BC bands. I could solve the problem from IMD2 with 2 switchable high-pass filters (18 MHz high-pass for 17 and 15 m ham bands, and 24 MHz for 12 and 10 m ham bands). For 40, 30 and 20 m ham bands, the only solution was to use the 20 dB attenuator to reduce IMD3. I could also have used an outboard preselector.

I can attest that with the peak of cycle 24, if you have a large and/or broadband antenna, these same problems will occur if your receiver has poor IMD2 and remote signals IMD3. I can also certify that 2kHz IMD3 will have very little interest because statisticaly you will have a few ham signals inside your roofing filter, and thousands large BC and utilities signals outside your roofing filters. RF selectivity (VRF or Digisel), relays switching of RF band-pass filters will be most important, not 2 kHz IMD3, even if it has its own relative importance.

It's not honnest to write that the FT-2000 is an "awfull" rig as I read it in Rob's comments. If you make your own chart, but with a different ponderation made on measurements, FT-2000 should top the chart in a few very important departments (RF selectivity, IMD2, remote signals IMD3, image and IF rejection), just to name a few.

Think about your trafic, about your antenna, make your own jugement and enjoy your rig, no one is perfect, but the FT-2000 is certainly one of the best money can buy. Think, it's just a hobby.

Best regards to the group.

Georges F6DFZ

A bientôt,
Ludovic - http://inforadio.free.fr

Date d'inscription : 08/06/2008


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