Portable Software Defined Transceiver

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Re: Roofing Filter

Postby rsdio » Tue Dec 27, 2011 8:53 pm

jason wrote:
rsdio wrote:I must be misunderstanding your question about feedback. The filter itself requires no feedback beyond what is shown in the schematic to work. Each of the four 2nd-order sections produces a 12dB/octave lowpass response and the 3dB cutoff points are chosen in order to create the Bessel response. I used filter design software (Nuhertz) to verify the response of this filter

Are you perhaps referring to some other type of feedback in the circuit?

Have you actually built the circuit to verify that your schematic is correct?

Look at the Wikipedia entry for Sallen-Key. Figure 1 shows feedback from the output to the inverting (-) input of the op-amp. All op-amps require feedback for stability, unless you're designing a comparator, which the Sallen-Key is not.

Your schematic shows no link from output to input of any op-amp, thus you have no feedback. As mentioned in the blog comments, and op-amp will quickly slam up against either the positive or negative supply rails if you run without feedback, thus making your circuit fail with only DC output. Check.
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Re: Portable Software Defined Transceiver

Postby Squonk » Wed Dec 28, 2011 10:36 am

jason wrote:As for the QSD mixer (Dan Tayloe did not invent it despite the fact that many people attribute his name to it)

Can you elaborate on this, as there is an US patent (http://www.freepatentsonline.com/6230000.pdf) on it?

Unless it is not doing its job, there is only a little chance that the USPTO would have granted it without a serious "previous art" search.

Alos found another original paper by Dan Tayloe: http://www.norcalqrp.org/files/Tayloe_mixer_x3a.pdf, and a very good list of resources on analog switches used as mixers: http://www.oe1ira.at/sl/mix.html.
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Re: Portable Software Defined Transceiver

Postby sqkybeaver » Wed Dec 28, 2011 6:08 pm

jason wrote:
Squonk wrote:
jason wrote:As for the QSD mixer (Dan Tayloe did not invent it despite the fact that many people attribute his name to it)

Can you elaborate on this, as there is an US patent (http://www.freepatentsonline.com/6230000.pdf) on it?

Unless it is not doing its job, there is only a little chance that the USPTO would have granted it without a serious "previous art" search.


The USPTO has become a joke, but that's a subject for another thread. Suffice it to say that there exists prior art for a rather large percentage of patents they are granting today. Tayloe's patent is no exception.

Follow this link and you can see that an article with prior art in it was published in Japan in 1993, effectively negating the US patent:

http://web.archive.org/web/200804200455 ... merigo.htm


all in all calling it a Tayloe mixer would not be incorrect, the majority of people familiar with SDR would be familiar with the circuit.
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Re: Portable Software Defined Transceiver

Postby sqkybeaver » Wed Dec 28, 2011 7:55 pm

most ic that use the technique are called quadrature something receivers, "QSD" is as good a name as it gets.

as far as Dan Tayloe's"Motorola" patent goes, usually first to patent is the first to the bank
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Re: Portable Software Defined Transceiver

Postby rsdio » Thu Dec 29, 2011 3:39 am

jason wrote:Yes, I have built and tested the filter. It worked as advertised, but not as drawn! Earlier when you mentioned I was missing feedback, I checked my notes and didn't know what you meant. However, I just looked at the schematic I drew in Altium Designer, and I see that when I drew the first 2nd order stage, I left out the connection between the inverting input and the output of the op amp! That mistake was duplicated on the remaining 7 stages in the schematic when I pasted it!

Anyway, thank you very much for catching that mistake and I'm sorry I didn't think to check the posted schematic rather than my notes earlier. Since this mistake is particularly grievous, I have edited my earlier posting to correct the error.

See if Ian can update the blog entry, too. Someone commented there before I had a chance. They caught the same error in the schematic.
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Re: Portable Software Defined Transceiver

Postby Squonk » Thu Dec 29, 2011 4:31 am

Thank you for these references!

Although I completely agree with you on the fact that the US patent system is broken, it is difficult in this particular case to incriminate it, as the patent #5,506,548 by Shigeki Kajimoto and Weimin Sun only cites Nozawa's "Merigo method" paper (in Japanese) in the references. And this paper is still extremely difficult to find, even using today's omnipotent search engines, so try to imagine the situation 10 years ago :-)

BTW, I found a better copy (esp. pictures) of the "Merigo Method" article here: http://ja2kai.freei.me/merigo.htm

EDIT: ... and according to this excellent paper from "A Software Defined Radio for the Masses" series by Gerald Youngblood (AC5OG), there are even older articles on the subject, at least back from 1990:
http://www.marcradio.org/re-09-1990/the ... gnals.html

More references on phase shifters can be found at http://home.earthlink.net/~christrask/pshift.html.

OK, let's call it "QSD mixer", then!
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Re: Portable Software Defined Transceiver

Postby ian » Fri Dec 30, 2011 3:54 am

I will add a note to the post.
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