hi Neil, TPS79333 is quite cheap and adm7150 is 60 times the price of it. These 2 ldo has different package, it not easy to replace it directly. I have tried power the system with 18650 battery, the performace is almost the same with 5v from laptop USB port. So I think psrr might not be a issue. If the noise generated by LDO is the problem, can I filter it out by puting more caps?
Neil Schroeder <gign...@gmail.com>于2015年1月3日星期六写道: > I would reconsider the LDOs while you have some time to play with them. > The TPS79333DBVR is not even remotely "ultralow noise" at most offsets, > despite what TI may say. > > For 5V5 and under up to about 600ma, I would suggest you take a look at the > ADM7155 (adjustable) or ADM7154 (fixed). If you need 800ma, the 7150 is > here for you. > > http://www.analog.com/en/power-management/linear-regulators/adm7155/products/product.html > http://www.analog.com/en/power-management/linear-regulators/adm7150/products/product.html > > The TI part has nearly 35 uVRMS of noise at just 2.8a with not very good > PSRR. The 7155 will produce less than 1.0 μVRMS Total Integrated Noise > from 100 Hz to 100 KHz and 1.6 from 10 to 100 KHz,and that's without an RC > noise compensation network. Its a fabulous part for a sensitive > application like yours. > > I'm hoping I am right in assuming that the noise performance of the LDOs is > a concern, and don't mean to even dare suggest that you've done anything > but a great job! :-) > > NS > > On Sat, Dec 27, 2014 at 5:34 AM, Li Ang <lll...@gmail.com> wrote: > >> Hi Charles, >> In my circuit, the VCC is 5v. I've noticed my bias and emitter resistor >> is something need to be changed. I will play with the resistors and see if >> it improves. Thanks. >> >> 2014-12-27 6:42 GMT+08:00 Charles Steinmetz <csteinm...@yandex.com>: >> >> > Li Ang wrote: >> > >> > RF pnp transistor is harder to get. I would like the front end works >> >> at 300MHz. >> >> >> >> My questions: >> >> 1) why the difference of DC bias of the 2 NPN matters? I thought only >> the >> >> frequency part is useful to a counter, amplitude information is useless >> >> right? >> >> >> > >> > You want the circuit to switch near the mid-point of the input sine wave, >> > and at exactly the same place every time. How you bias the transistors >> > determines how well this is accomplished. >> > >> > You also want the output to switch fast and cleanly between a low voltage >> > very near 0v ("ground") to a high voltage very near 3v (Vcc, logic high). >> > An NPN cannot do that, biased the way that you have them connected (the >> > emitter of the output transistor Q301 can only pull the output down to a >> > little less than 1v due to R315, which may sort of work but is not a >> proper >> > way to run 3v logic). This operation also saturates Q301, which is bad >> for >> > performance. See simulated results below. >> > >> > In order for an NPN to provide a useful output for 3v logic, (i) its >> > emitter must be grounded, and (ii) it must either be run into saturation >> or >> > use a Baker clamp. Running the transistor into saturation must be >> avoided, >> > particularly if you want to reach 300MHz, and a Baker clamp raises the >> > "logic low" output voltage to >0.5v (not a good thing with 3v logic). >> So, >> > it is very much better to use a PNP differential pair. For a 300MHz >> > circuit, I would use BFT93 (and even that barely gets you to 300MHz). >> > >> > 2) what's is the C4 in your circuit for? >> >> >> > >> > C4 makes Q1 and Q2 a differential (emitter-coupled) pair at RF >> > frequencies, but not at DC. So, the circuit has no gain at DC and >> > therefore the DC errors between Q1 and Q2 cause much less output error >> than >> > they would if the emitters were connected directly together. >> > >> > 3) If the noise is more important than the gain, what kind of transistor >> >> should I choose? The Ft near 300MHz ones(BFS17, 2SC9018) or Ft far >> beyond >> >> 300MHz ones(BFP420, BFP183,BFR93) ? >> >> >> > >> > Far beyond. The Ft is the frequency where a transistor completely runs >> > out of gain. You want to operate at a much lower frequency where the >> > transistor still has substantial gain, particularly with fast RF >> > transistors, which generally have much lower DC hfe than general-purpose >> > transistors like 3904 and 3906. Note that the simulation of the circuit >> > you published (simulated results below) barely works at even 20MHz. As I >> > noted above, even the BFT93 barely gets you to 300MHz with a 1Vrms input. >> > >> > Best regards, >> > >> > Charles >> > >> > >> > _______________________________________________ >> > time-nuts mailing list -- time-nuts@febo.com >> > To unsubscribe, go to >> > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> > and follow the instructions there. >> > >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.