[hpsdr] ALEX - Call for Comments - III

[Larry Gadallah]

Hi all:

One more comment regarding bandpass/lowpass/highpass filters: Using
very small SMT components for these may cause problems with IM,
dynamic range, and non-linear operation. I had an early model of the
AOR AR7030 receiver, and there were problems in the 1.6 - 2.5 Mhz
range because of SMT parts in the front-end bandpass filters and the
high signal levels found at the lower end of this range (i.e. MW
broadcast stations). AOR issued an ECO
(http://www.aoruk.com/7030bulletin.htm#ip3) whereby they replaced the
SMT components with normal types and this solved the problem. I have a
hard time visualizing how a SMT capacitor becomes non-linear, but
apparently they do.

Cheers,

On 3/7/07, Graham Haddock <grahamh at verizon.net> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> All:
>
> Thanks to all for good continuing input and discussion.
>
>
> General:
> Interesting reading suggested by other emails earlier today:
> "G3SBI H-Mode Mixer as tested by PA3AKE"
> Specifically - preselector comments/measurements on Page 9.
> Best I can tell, we are using the improved topology he refers to.
>
>
>
> 1.) Name...
> I'll take offline with Don.
>
>
>
> 2.) Control...
> Lyle's KK7P comments on control lines allowing coupling to the
> input and output of the filters and allowing "filter bypass"
> are noted.
>
> A question for Lyle or Phil C or someone familiar with the CPLD parts...
> If I don't care about output rise and fall time, what is the
> largest capacitor (I am thinking 0.01 uF or 0.1 uF, .01 ohm ESR)
> that I can throw on the output line of a CPLD?  Will it current
> limit and protect itself until the capacitor charges, or will
> it trigger life issues after repetitive cycling?
>
> Phil C. - I will research the EPM240 CPLD.
>
> Also, I am thinking about inverted signals from the decoder, versus
> distributed inverters for the RF switches, versus a distributed
> SPI decoder and respective impacts on PCB layout and
> potential RF coupling.
>
>
>
> 3.) Filter Design and selection...
> I posted the individual filter performance plots on the bottom of the ALEX
> page on the Wiki, for those of you that wanted to look at them.
>
> These look like a "contest" or "field day" grade set of preselectors
> appropriate for use in "Overload Alley".
>
> Read Phil H's comments on the robustness of the native
> Mercury receiver (yesterday).  It does not sound like a preselector
> is always necessary on a Mercury.  The story may be different
> on other receivers.
>
> I also played around a bit with opening this filter topology out
> to about 40 percent bandwidth, (you have to relax the ripple
> spec from 0.01 dB to 0.1 dB) and posted a sample of that to the
> Wiki, too.
>
> It looks reasonable, which means that a ten section overlapping
> filter continuous coverage preselector is also possible, with just
> L and C value substitution.  I assume that MW and down would
> be a single low pass filter, and 1.7 to 30 MHz would be 9 overlapping filter
> sections of about 40 percent bandwidth each.
>
>
>
> 4.) Physical design...
> Nothing to report
>
>
>
> 5.) Preamp...
> The preamp guys came alive last night.  I need a recommendation
> for a 6M preamp of known performance/layout to add to the board.
>
>
>
> 6.) Transient Protection
> I don't think that the neon light will work in this application.
> The weak link is the Hittite RF switches.  Their spec
> sheet does not say, but I can't imagine a GaAs FET
> in this part surviving anything over 30 volts.
> I think we need to consider biased diodes or something.
> Does someone have a circuit of known performance/layout to
> add to the board?
>
>
> --- Graham / KE9H
>
> ==
>
>
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-- 
Larry Gadallah, VE6VQ/W7                          lgadallah AT gmail DOT com
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