30 MHz LPF on LFRX daughterboard?
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30 MHz LPF on LFRX daughterboard?
by John Orlando
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Hi all,
I was just reading the specs/schematic on the LFRX RF daughterboard, and noticed that the literature indicates that there is a 30 MHz low-pass filter after the differential amplifiers. Looking at the schematic, there is a 50 ohm resistor + 20 pF capacitor on each leg of the differential amplifier output. I believe this would mean that, for each of the differential amplifiers, the effective resistance is really 100 ohms (two 50 ohms in series) with 10 pF (two 20 pF in series). If it really is 100 ohms + 10 pF, the passband of the filter is much larger than 30 MHz. Even taking into account some input capacitance on the A/D converter on the USRP (10-20 pF?), it still would be larger than 30 MHz. Am I missing something here? Thanks much, John Orlando www.epiq-solutions.com _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@... http://lists.gnu.org/mailman/listinfo/discuss-gnuradio |
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Re: 30 MHz LPF on LFRX daughterboard?
by Matt Ettus
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John Orlando wrote:
> Hi all, > I was just reading the specs/schematic on the LFRX RF daughterboard, > and noticed that the literature indicates that there is a 30 MHz > low-pass filter after the differential amplifiers. Looking at the > schematic, there is a 50 ohm resistor + 20 pF capacitor on each leg of > the differential amplifier output. I believe this would mean that, > for each of the differential amplifiers, the effective resistance is > really 100 ohms (two 50 ohms in series) with 10 pF (two 20 pF in > series). If it really is 100 ohms + 10 pF, the passband of the filter > is much larger than 30 MHz. Even taking into account some input > capacitance on the A/D converter on the USRP (10-20 pF?), it still > would be larger than 30 MHz. Am I missing something here? The caps on each leg are 82pF, or 41pF in series. That gives a 3dB bandwidth of about 38 MHz. Matt _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@... http://lists.gnu.org/mailman/listinfo/discuss-gnuradio |
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Re: 30 MHz LPF on LFRX daughterboard?
by John Orlando
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On Fri, Oct 23, 2009 at 7:08 PM, Matt Ettus <matt@...> wrote:
> John Orlando wrote: >> >> Hi all, >> I was just reading the specs/schematic on the LFRX RF daughterboard, >> and noticed that the literature indicates that there is a 30 MHz >> low-pass filter after the differential amplifiers. Looking at the >> schematic, there is a 50 ohm resistor + 20 pF capacitor on each leg of >> the differential amplifier output. I believe this would mean that, >> for each of the differential amplifiers, the effective resistance is >> really 100 ohms (two 50 ohms in series) with 10 pF (two 20 pF in >> series). If it really is 100 ohms + 10 pF, the passband of the filter >> is much larger than 30 MHz. Even taking into account some input >> capacitance on the A/D converter on the USRP (10-20 pF?), it still >> would be larger than 30 MHz. Am I missing something here? > > > The caps on each leg are 82pF, or 41pF in series. That gives a 3dB > bandwidth of about 38 MHz. Thanks Matt...I'm looking at the LFRX schematic found here: http://gnuradio.org/trac/browser/usrp-hw/trunk/basic-lf/lfrx.pdf Is there a more up-to-date version I should be looking at? This one still shows the 20 pF caps instead of the 82 pF caps. Regards, John Orlando www.epiq-solutions.com _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@... http://lists.gnu.org/mailman/listinfo/discuss-gnuradio |
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Re: 30 MHz LPF on LFRX daughterboard?
by Matt Ettus
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The PDFs are out of date. I'm going to update all of them and post new ones. Matt John Orlando wrote: > On Fri, Oct 23, 2009 at 7:08 PM, Matt Ettus <matt@...> wrote: >> John Orlando wrote: >>> Hi all, >>> I was just reading the specs/schematic on the LFRX RF daughterboard, >>> and noticed that the literature indicates that there is a 30 MHz >>> low-pass filter after the differential amplifiers. Looking at the >>> schematic, there is a 50 ohm resistor + 20 pF capacitor on each leg of >>> the differential amplifier output. I believe this would mean that, >>> for each of the differential amplifiers, the effective resistance is >>> really 100 ohms (two 50 ohms in series) with 10 pF (two 20 pF in >>> series). If it really is 100 ohms + 10 pF, the passband of the filter >>> is much larger than 30 MHz. Even taking into account some input >>> capacitance on the A/D converter on the USRP (10-20 pF?), it still >>> would be larger than 30 MHz. Am I missing something here? >> >> The caps on each leg are 82pF, or 41pF in series. That gives a 3dB >> bandwidth of about 38 MHz. > > Thanks Matt...I'm looking at the LFRX schematic found here: > http://gnuradio.org/trac/browser/usrp-hw/trunk/basic-lf/lfrx.pdf > > Is there a more up-to-date version I should be looking at? This one > still shows the 20 pF caps instead of the 82 pF caps. > > Regards, > John Orlando > www.epiq-solutions.com > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@... > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@... http://lists.gnu.org/mailman/listinfo/discuss-gnuradio |
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