[Icom] RE: Filtering at 9 MHz vs. 455 kHz

Sat, 14 Sep 2002 19:27:58 -0500

It might be interesting to note that one advantage of multiple-conversion
receiver architectures is that they allow effectively cascaded crystal
filters to be operated at different i-f frequencies. In this way, the
advantages of cascaded filters can be obtained without excessive steps
being required to minimize blow-by and related effects.
For each filter, however, at each i-f, the blow-by problem exists.
Sometimes extraordinary design and construction measures are required to
reduce blow-by such that 60-90 dB ultimate rejection can be realized.

On balance, there are both disadvantages and advantages to multiple
conversions. It  happens that reducing the blowby problem is one of the
advantages.

73/72, George    
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!

Adam Farson wrote:
> 
> Hi Johnny,
> 
> The RF blow-by issue does not arise at all in DSP IF filtering systems, as
> there is no case where the analogue IF signal ahead of the ADC
> (analogue/digital converter) can re-enter the signal path on the other side
> of the DSP. The analogue output of the DAC (digital/analogue converter)
> after the DSP is no longer at IF; it is at baseband (audio) frequency.
> 
> Please refer to <http://www.qsl.net/ab4oj/icom/pro_dsp.html>
> 
> In an analogue IF system with classical crystal filters, proper shielding
> will minimise blow-by. The filter should be mounted above a shielded
> compartment, in such a way that the filter case straddles a shield with the
> filter input and output terminals on opposite sides of the shield.
> 
> 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 16:53
> To: [email protected]
> Subject: Re: [Icom] RE: Filtering at 9 MHz vs. 455 kHz
> 
> Hi Stephen,
> 
> Based on your argument, would this mean that a DSP IF filter will not face
> the RF radiation problems?
> 
> Looking forward to your advice because I am not a radio profession.
> 
> TNX & 73
> 
> Johnny Siu VR2XMC
> 
> ----
> Your Moderator: Dick Flanagan W6OLD, [email protected]
> Icom FAQ: http://www.qsl.net/icom/

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