[Icom] Filtering at 9 MHz vs. 455 kHz

Sun, 15 Sep 2002 01:41:36 -0700

Hi Johnny,

The new page is done! The new page is on http://ww.qsl.net/icom.

Click on P"aired IF Filters in Icom Radios"

or go to http://www.qsl.net/icom/xfilter.html

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Note new e-mail address:
mailto:[email protected]

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Note new e-mail address:
mailto:[email protected]

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Siu Johnny
Sent: Saturday, September 14, 2002 06:48
To: [email protected]
Subject: Re: [Icom] Filtering at 9 MHz vs. 455 kHz

Hi Adam,

Very informative message.  How about intergrating it somewhere in your web
page if you have not done so?  I shall then advise local hams to make
reference to that information.

73

Johnny Siu VR2XMC
----- Original Message -----
From: "Adam Farson" <[email protected]>
To: <[email protected]>
Sent: Saturday, September 14, 2002 2:24 PM
Subject: RE: [Icom] Filtering at 9 MHz vs. 455 kHz

> Hi Guys,
>
> Here is an argument for installing 9 MHz and 455 kHz filters with equal or
> comparable bandwidths and shape factors:
>
> Assuming that the filter passbands overlap (PBT or Twin PBT in centre
> position), the composite adjacent-channel selectivity at the stopband (-60
> dB typ.). will be superior to that of one filter alone.
>
> In addition, for PBT or Twin PBT to operate correctly, the passbands of
the
> two filters should at least be similar. Thus, for example, the
FL-80/FL-44A
> pair ensures a much tighter and more aggressive PBT action than
FL-80/FL-96.
> Pairing 250 and 500 Hz CW filters (in any order) will make the PBT very
> sloppy.
>
> A pair of 500 kHz CW filters in a radio equipped with PBT or Twin PBT is a
> very acceptable compromise. Whilst the adjacent-channel selectivity with
the
> effective bandwidth reduced to 250 Hz via the PBT is not quite up to that
of
> a pair of 250 Hz filters, it is still very acceptable in many cases.
>
> In a receiver, the filter closest to the first mixer does most of the work
> in terms of determining the IF bandwidth and filtering out the unwanted
> sideband, out-of-band signals etc. If only one filter is installed, it
> should be the 9 MHz filter. In the original, unmodified IC-765, the 9 MHz
> filter is bypassed when enabling IF Shift. This allows strong out-of-band
> signals to ride down the IF strip and overload the third mixer (9 MHz/455
> kHz). This degrades close-in dynamic range.
>
> Conversely, in an analogue transmitter which shares the filters with the
> receiver, the first filter should be as close to the balanced modulator as
> possible, to ensure adequate suppression of spurious products. This means
> that the transmitter should always have a 455 kHz filter.
>
> Thus, we end up with a transceiver fitted with crystal filters at both
IF's.
>
> You can view the passband curves of popular Icom IF filters here:
> http://www.qsl.net/ab4oj/icom/filters/filters.html
>
> Best 73,
> Adam, VA7OJ/AB4OJ
> North Vancouver, BC, Canada
> http://www.qsl.net/ab4oj/
> Note new e-mail address:
> mailto:[email protected]

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