[two tone test 400 + 1900 Hz]
FT847 transmitter
intermodulation performance
by OK1VPZ
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[two tone test 400 + 1900 Hz] |
10 W PEP | 20 W PEP | 30 W PEP | 50 W PEP |
| FT847 1998 144 MHz 2kHz/div |
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| FT847 2002 144 MHz 2kHz/div |
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| FT847 1998 144 MHz 10kHz/div |
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| FT847 2002 144 MHz 10kHz/div |
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| FT847 1998 432 MHz 2kHz/div |
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| FT847 2002 432 MHz 2kHz/div |
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| FT847 1998 432 MHz 10kHz/div |
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| FT847 2002 432 MHz 10kHz/div |
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Note: two FT847 were measured - one from 1998 and the second from 2002 production. Into microphone input was applied AF two tone signal of 400 and 1.9kHz. This combined audio signal was attenuated on the front of FT847 to be on the same level as fully modulated speach peaks. RF speach processor was switched off, because in case of switch on, intermodulations in the nearest neighbourhood were much worse. On the output of tranmitter was connected precise directional coupler with aprox 40 dB tap attenuation and 150 W 50 Ohm load. Spectrum analyzer with 300Hz RBW was connected to the tap output of directional coupler.
Differencies between these FT847:
- older rig has in TX chain only ceramic Murata SSB filter, while new FT847 has
Collins one. This differency is slightly visible in the "stop band" of
intermodulation shape factor on the bottom of IMD curve on 10kHz/div pictures.
- older rig has slightly lower gain of both PA, which is resulted by slightly
worse IMDs shape on 50W PEP - but it is not created by PA, but by PA driver
- newer rig has slightly lower BIAS of both PAs, what is resulted by slightly
worse IMDs on the level of - 60 dB and more
- FT847 with ceramic filter in TX chain has more rippled modulation chain and it
requested individual adjustment of both AF carriers level (± 1.2dB), compare the
newer rig with Collins SSB filter, which has better flatness (± 0.5dB)
Conclusion: both FT847 do not meets SSB linearity specs, because two tone test IMD3 products are not for 50W PEP attenuated by at least 30dB below PEP. Newer rig IMDs are generally slightly better. Suprisingly no major differencies are between 20 and 30 W output level, and it seems, that for driving of additional high power PA, power output up to 30 W from FT847 would be highly recommended, however when input of additional PA is well matched. Using of RF speech processor (compressor) creates much worse IMDs near TX signal, but 30 kHz away (due to influence of FM cover filter) are IMD products more less the same.