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Date: Sun, 07 Nov 2010 20:15:22 -0500
From: John Andrews <w1tag@charter.net>
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Subject: Re: LF: exciter for 137 and 500 kHz
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Andy,

I used that program when I did my 500 kHz exciter a few years ago. At 
the time, I went back to some old source material at work, and compared 
the hand-calculated results with the program. The two produced the same 
results, so the program is certainly OK.

You do raise an excellent point with component tolerances, however. For 
that filter, I used a very accurate Phillips digital RLC meter at work 
to select capacitors from a batch of 2% units that I had in stock. Armed 
with the measured capacitance, I hand-selected resistors from ample 
stocks of "close" 1% values to equal the reactance of the capacitors at 
the break points. The 10k0 resistors were already 0.1%, so I didn't 
bother trying to grade them out.

The resulting plot of phase vs. frequency may be seen at:
http://www.w1tag.com/pix/600M_Phase.jpg
As you noted, it does have a problem at the high end, probably due to 
the op-amp input capacitance, which was not considered. A mental note 
was made at the time!

The only schematic I can find is hand drawn, but using the program, the 
parameters were F1=100Hz, F2=3500Hz, C=10.0nF and 5 filters (per side, 
for a total of 10 op-amp sections. I tweaked the cap values to only use 
100, 10 and 1 nF. The critical thing in tweaking is to maintain those 
breakpoint frequencies that represent Xc=R.

John, W1TAG

On 11/7/2010 3:46 PM, Andy Talbot wrote:
> That's a neat bit of software!
> But I've noticed in all the all-pass design procedures, there
> are choices of optimising, Monte Carloe-ing, adjusting and tuning the
> RC  part of the networks.  But none seem to take into account the effect
> of mismatch of the nominally matched pair of resistors around each
> opamp.   Or of residual amplitude mismatching.
> But it does seem that if you are prepared to spend the time getting all
> the Rs and Cs within a micro percent of the correct values, then apply
> the same effort to amplitude and phase matching the channels, a quite
> phenomenal performance wrt. sideband suppression is theoretically possible.
> But I guess in any practical scenario, most constructors will tweak for
> an acceptable performance and leave it alone.   For upconverting at LF,
> we're only interested in narrow band signals - the entire WSPR segment
> is only 200kHz wide - and a simple 2 stage (one opamp in each of I and
> Q) is adequate.  In that case a pair of preset or variable resistors can
> be tweaked quite quickly.   40 - 50dB supression seems almost guaranteed
> over a 100Hz using a predictable mixers like fast CMOS switches, with a
> comparable level of carrier suppression.  I've built two now, one with
> FST3125 and a balanced output transformer,  and one with a dual quad
> switch, FST3257.  The latter gives somewhat better performance, but is
> slightly more complex.
> Andy
> www.g4jnt.com <http://www.g4jnt.com>
>
>
> On 7 November 2010 19:33, Johan H. Bodin <jh.bodin@telia.com
> <mailto:jh.bodin@telia.com>> wrote:
>
>     Andy,
>
>     here's another nice program for all-pass I/Q filter design:
>
>     http://www.tonnesoftware.com/quad.html
>
>     73
>     Johan SM6LKM
>
>     ----
>
>     Andy Talbot wrote 2010-11-06 14:59:
>      > Very Interesting.
>      > Where did you find the design data for the allpass network?
>     I've only been
>      > able to find the values for up to 3+3 allpass stages.
>      >
>      > Mind you, at that number of chips a DSPic begins to look attractive.
>      >
>      >
>      > Andy
>      > www.g4jnt.com <http://www.g4jnt.com/> <http://www.g4jnt.com
>     <http://www.g4jnt.com/>>
>      >
>      >
>      > On 6 November 2010 11:52, <jrusgrove@comcast.net
>     <mailto:jrusgrove@comcast.net> <mailto:jrusgrove@comcast.net
>     <mailto:jrusgrove@comcast.net>>>
>      > wrote:
>      >
>      >     Nothing earth shattering ... another way to skin the cat. The
>     137 and 500
>      >     kHz units have been in use for a few years ... just now
>     getting around to
>      >     the write up.
>      >
>      > http://www.w1vd.com/137-500Phasingexciter.html
>      >
>      >     Jay W1VD  WD2XNS  WE2XGR/2
>      >
>      >
>
>
>
>
>
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