I found Carl's original Message from last year that addresses the addition
of his RX-340 4 kHz roofing filter. Pay particular attention to the last
paragraph.
-Paul, W9AC
----- Original Message -----
From: "Carl Moreschi" <n4py@earthlink.net>
To: "tentec" <tentec@contesting.com>
Sent: Thursday, April 25, 2002 18:05 PM
Subject: [TenTec] Rx340 Close in dynamic range modification by N4PY
> In a email to the reflector several months ago I reported that
> the Rx340 close in dynamic range for signals closer than 10 khz
> to the desired signal was only 40 db. I have done some careful
> measurements on this and have found the figure is actually 65 db.
> This is a much better number.
>
> However, I decided to go ahead and see if a simple mod could
> improve on this parameter by adding a tighter filter at the beginning
> of the 455 khz IF chain.
>
> At this point some review of how the Rx340 works is in order.
> This is a gross simplification of how things work.
> All signals are upconverted to 45.455 mhz. Here, the signals
> are filtered with a 20 khz filter. Once filtered, the signal
> is then converted to 455 khz. The AGC in the Rx340 is divided
> up with 80 db of hardware AGC and 65 db of DSP AGC.
> Up to the point of converting to 455 khz, no AGC is used
> at all. All signals at this point are "full blast" inside the 20 khz
> roofing filter. At this point the 455 khz signal is first amplified
> with an amplifier under hardware AGC control. The added filter
> must go before any hardware AGC is used.
> It is then converted to 16.67 khz and digitized for DSP. The DSP
> then has the job of filtering the desired signal (amoung many tasks).
>
> Because the first local oscillator only moves in 1 khz steps to
> convert to 45.455 khz, the 455 khz IF center frequency is actually
> 455 to 456 khz, depending on the actually tuned frequency. The DSP
> performs the job of recentering over this 1 khz range to allow for
> 1 hertz tuning.
>
> What I did was order a 4 khz filter module from Kiwa Electronics.
> This filter has a 4 khz bandwidth at 6 db and 6 khz bandwidth at 60 db.
> It centers on 455 khz. I installed this early in the 455 khz IF,
> ahead of any hardware AGC control, with
> reed relays to switch it in and out. The filter module is equipped with
> its own amp to make it a zero db insertion loss filter.
>
> Anyway, with this filter installed, the close in dynamic range for signals
> between 3 and 10 khz away is now improved to over 100 db.
> Of course, the dynamic range for signals past 10 khz was always
> very very high. There is no change for signals less than 3 khz away.
> That figure is still 65 db.
> Because of the 1 khz center frequency tunig of the DSP, I cannot use
> a tighter filter than this. The mod was not too hard to install but did
> require
> cutting one land trace on the second mixer board ( a little scary!).
>
> I am now using this mod and looking for a situation where it might help.
>
> Carl Moreschi N4PY
> Franklinton, NC
|