On Mon, 2006-05-29 at 10:31 -0400, G wrote:
> I still can't understand why Yaesu and Icom don't use close in filters like
> the Orion2???
> I'll have to wait until the testing is done on this unit in close in 2KHZ
> dynamic range and where it falls in Rob's table.
> I sold my ProII for the Orion-2 and the difference in operation is small but
> there for me.
> George
>
Besides the difficulties of making a sharp filter at 45 or 75 MHz, there
are some other factors. For sure the Yeacomwood roofing filter has to be
15 KHz wide as a minimum to pass FM for the top end of ten meters and on
VHF when the radio has more bands. Then I suspect the first LO only goes
in 10 KHz steps with the fine tuning done in the second conversion
oscillator. Both are synthesized. So their roofing filter has to be 20
KHz wide or so to pass the signals for the fine tuning.
What I don't understand is why they don't put in a second mixer that can
handle the strong close in signals that got amplified in the high IF.
Just a cost issue probably. I know in the Yaesu FT-736 on 6m, 2m, and
220 they didn't terminate the first mixer properly and cost extra mixer
loss and take off 20 or 30 dB of 3rd order intermod capability. They run
the mixer right into a narrow crystal filter where there ought to be a
broad band termination to absorb the unwanted outputs from the mixer,
exactly as done in many Tentecs. It isn't as if Yaesu didn't know
better, they did better in the VHF modules for the FT-767 which predates
the 736.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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