EMRFD Message Archive 5352

Message Date From Subject
5352 2010-10-05 10:31:50 Fernando Krouwel Re: [FtTuthill80] Fwd: [emrfd] A question to Dan Tayloe about audio
Hi Dan, good afternoon:

Thank you very much for your attention.

You answered exactly the way I needed, things are going to be even better here.

73´s
Fernando - PY2ETT


----- Original Message -----
From: Tayloe Dan-P26412
To: FtTuthill80@yahoogroups.com
Sent: Tuesday, October 05, 2010 1:31 PM
Subject: RE: [FtTuthill80] Fwd: [emrfd] A question to Dan Tayloe about audio filters




I cheated a bit going into the first stage of the low pass filter strip. This filter stage needed 1K of input resistance. I broke this 1 K filter input resistor into a pair of 2K resistors, one to the I phased output, the other to the Q phased output. On the undesired sideband, these two resistors acted like summing resistors. Thus on the undesired sideband the signal coming out of the two phasing section will be 180 degrees out of phase and thus cancel as they are summed using the input resistor of the filter. That allowed me to leave out a buffer stage that would normally be used to do this summing, I just needed two 2x filter input resistors. For the desired signal, the output of the I and Q are the same signal at the same phase, so my reasoning was that since the two output signals looked identical, both outputs could be viewed as the same output, so the two 2K resistors would "appear" to be in parallel, producing the desired 1K of filter input resistance.

When I was designing this filter using the TI software, I was trying to use the Chebychev configuration with a very, very small ripple (0.05 dB or less as I recall). The latest TI filter software is great, but I find the older TI software a bit more flexible in allowing me to pick specific capacitor values and then having the software compute the updated resistor values. The old software used to allow me to tweak the individual capacitor values until I exceeded a specified C1-C2 limit. It seems like the latest software forces an apparent fixed ratio of C1 and C2 in a given stage, which makes it more difficult to optimize for designs that try to stress minimal unique component values. I have often spent a lot of time tweaking filter cut off frequency, ripple, and capacitor values to be able to maximally reuse existing R and C values elsewhere in the circuit.

To answer your specific question, the two 2.2K summing resistors look like 1.1K of resistance for the desired sideband. Thus if you want to keep the 2.2K resistors, you should subtract 1.1K from your current 6.8K filter input resistance. Using a 5.6K resistor will likely be close enough. Alternatively, you could use two summing resistors close to 13.6K (a non standard value) and eliminate the 6.8K resistor altogether.

I use the TI filter output as a starting point. Depending on the Q of the stage, you can change values to something else close (like using two 15K resistors in the example above) and it will not impact the circuit much. That is where I simulate the circuit using one of the freeware simulators (like TI TINA or Linear Technology LTspice) to check the frequency response of the proposed component changes to see if they look ok or not.

Since my division is being purchased by another company (Nokia Systems Networks), my email will change one of these days and a forum such as this (or emrfd) may be the best place to find me.

- Dan, N7VE



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From: FtTuthill80@yahoogroups.com [mailto:FtTuthill80@yahoogroups.com] On Behalf Of Fernando Krouwel
Sent: Tuesday, October 05, 2010 8:41 AM
To: FtTuthill80@yahoogroups.com
Subject: Re: [FtTuthill80] Fwd: [emrfd] A question to Dan Tayloe about audio filters


Hi John, good morning:

Thanks for forwarding my EMRFD message to the Ft Tuthhill group.

In fact, at first I thought about sending it to EMRFD because the main question is about to get an advice about the best yet simplest way to coupling a phasing network into an audio filter to compose a phasing receiver (which has an entire chapter in the book), but I think you are right, it will be easier to find Mr Tayloe here, who is more familiar with the particular circuits in the discussion.

Thank you,
Fernando - PY2ETT
----- Original Message -----
From: John Stevens
To: FtTuthill80@yahoogroups.com
Sent: Tuesday, October 05, 2010 11:47 AM
Subject: [FtTuthill80] Fwd: [emrfd] A question to Dan Tayloe about audio filters



Cross posting.... Incidentally, the EMRFD list has quite a few interesting technical discussions relevant to the Tut80 and SDR radios.



john k5js




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