[hpsdr] ALEX - Call for Comments - III

Thu Mar 8 07:56:30 PST 2007

Larry,

Most NP0 and C0G capacitors have extremelly low distortions. I have 
performed tests on these type caps with as many as 132 carriers and 0dBm 
each and the performance is very very good. As you start going up in value, 
the materials required to increase the capacitance per unit area become more 
and more unstable and non linear. Some of these materials exhibit a 
capacitance over voltage characteristic. So as the voltage on the cap 
varies, so does the capacitance. This tends to created mixing products since 
a change in capacitance (modulated by one carrier's voltage) causes the 
whole filter/resonator to be modulated in phase and amplitude.
An even bigger problem are the ferrite beads that are used in the high value 
chip inductors. These can have very non linear characteristics specially if 
the core is conductive and the enamel coated wire touches the core.
The powder iron cores from Micrometals are also extremelly good as well as 
the wirewound surface mount inductors that have no ferrite.

Pieter, N4IP

>From: "Larry Gadallah" <lgadallah at gmail.com>
>To: grahamh at verizon.net
>CC: High Performance Software Defined Radio Discussion List 
><hpsdr at hpsdr.org>
>Subject: Re: [hpsdr] ALEX - Call for Comments - III
>Date: Wed, 7 Mar 2007 21:30:59 -0800
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>***** High Performance Software Defined Radio Discussion List *****
>
>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
>PGP Sig: 616D 4E52 CF1F 3FEC FFFB  F11B 7DB9 C79A EA7E B25B
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