Transistor choices and Active filters for RF? - Page 1
Forum section: electronics / rf-microwave

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 08, 2016, 07:53:20 pm
content: These are two totally unrelated questions. The first is about transistor choises in two different schematics.So the first one is this pretty simple it called the "Amazing All Band Receiver" Its basically an RF detector with audio out.http://www.techlib.com/electronics/allband.htmSo they chose to use a 2n4401,DataSheet, now ill admit ive only been doing RF a little while most of my experince is lower speed digital, i.e micros, hombrew z80 etc. In most circuits I design I stay away from transistors when I can and try to use FETS or OpAmps if at all possible. Basically I understand you want to pick a transistor based on gain(hfe) and speed(ft) so the hfe for the 2N4401 is 50 max with an Ft of 250mhz. Based on this im guessing this all band receiver doesn't make it past VHF. My real confusion is why this transistor is there something special about it that I am missing? The Ft for a 2n3904 and 2n222A are higher.. although there hfe is only 300, is the extra gain og the 2N4401 really needed? What if you just threw a much fast power transistor in the thing and used a pot to set bias/gain? is the 2n4401 something I should have on hand as a jelly bean?Second is the ever popular PARD0T Active Mini Whip,here is the schematic. I really really want to listen to VLF and ELF and especially WWVB. This antenna seems like a pretty decent solution to running a mile long wire. Its also good to 20mhz so I can use it with my SDR up converter. This thing is only good to 20mhz though.... it is using two hi speed amplifiers. I get the JM310 is there to bring the noise floor down most likely because s/n is everything. What I dont get is the2N5109 (DataSheet)it doesn't have a very high hfe which is understandable since this is all about s/n not overall db gain, but the ft is 1.2Ghz.... That seems like overkill for a VLF/HF amplifier, way beyond nyquist. These guys arent cheap either at $2.75 a pop. I acually plan on ordering a few for other things but why use it here and not just a 40mhz low hfe transistor? Is there something besides Hfe, Ft, that makes this a good choice for the 20mhz mini whip? Also you can get 10 of these for 8 bucks on E-Bay from China, should I trust that? I order from china all the time but I have had bad luck with higher end components being falsely advertised?Lastly and hopefully an easy answer.. when building filters in RF we all build multi pole passives. My understanding is that there is no place for active filter simply because opamps aren't fast enough. Isnt that kind of outdated? I have 2 TI opamps in front of me rated for 1.7ghz theTI LMH6702I also happen to know these were used in the front end of a an agilent signal generator so they must work well. Is there an issue with building active filters using chips like this? There only 4 dollars ill pay that for no insertion loss, and even gain! I also have some fairly pricey jfet based opamps that run up to 30mhz, im actually thinking of using these to make a low frequency mini whip...

author: edavid
author_rank: Super Contributor
timestamp: July 08, 2016, 08:37:54 pm
content: Quote from: rwgast_lowlevellogicdesin on July 08, 2016, 07:53:20 pmThe 2N4401 is only amplifying audio, so the Ft is irrelevant. Another kind of transistor would be fine though.QuoteYour link is bad.Anyway, the 2N5109 is used as a high current class A amp, so it won't saturate at high signal levels. If you don't care about strong signals and/or you add a preselector, you could use a cheaper/weaker transistor.QuoteNo, active filters are still impractical for RF.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 08, 2016, 09:18:51 pm
content: Thank you short and to the point. The transistor options make a little more sense now. For anyone else PARD0T schematic the site i linked the pic from ishttp://radioaficion.com/cms/pa0rdt-mini-whip/the schematic is the first pick on the page.Can you maybe explain why a low noise 1.7ghz op amp isn't practical for RF filtering applications? I built this recentlyIt is a 3.3v 28.8 TCXO with a 5 pole tunable low pass filter, this is to feed an RTL SDR and cut down on the internal harmonic noise from the LO. It works well I have brought the spike down 20db or more, but I want to get rid of the harmonics totally. My problem is the SDR wont work if the VPeakToPeak of the oscillator is under about 90~95mv this varies between sticks. I would have gone much farther and kept adding poles until I was content but there is just enough loss with 5 poles to tune from 60mv to 112mv. Adding another 2 or 4 poles would definately attenuate the voltage far to low to drive the R80T2 tuner chip. If we could use active filters even if it requires expensive op amps we could get much closer to perfect and almost eliminate most or all birdies from the LO to the RF Front end and the USB clock. So im very interested to know why hi speed opamps are still a bad choice :/ im sad to get that answer? Also what if your hetrodyning down to 455Kc are active filters acceptable on the 455khz IF?

author: edavid
author_rank: Super Contributor
timestamp: July 08, 2016, 10:36:29 pm
content: I think the idea of sine wave drive to the RTL SDR is pretty questionable... doesn't it just square up the clock internally? Maybe you just need to control the slew rate.But if you do want a sine wave, I think you would be better off with a bandpass filter instead of a lowpass filter.Or forget the TCXO and use a sine wave XO in the first place.At 28MHz, an LC filter will give you higher Q than an active one.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 09, 2016, 05:39:16 am
content: Hmm my LO filter isnt really where i wanted to take this but ill definately take good advice. First off why bandpass, the dongle only tunes to 25mhz so any undertone harmonics dont effect anything. I know that the tuner chip probably wants a square wave but it doesnt seem to dislike a sign and it clears up a lot of harmonics! I got the idea from a guy using a 3 pole on a si5351his results look goodi jist had a tcxo on hand and decided to use elsie to help me design a 5 pole. I would definatly like a 28.8mhz ovenized oscillator if you know where i can get one ill snatch it up! Please tell me!What i really want to know is why a 1.7gig opamp cant be used in a filter up to 850mhz especially if a wide q is ok? In a broadband radio it would be nice to stack bp filters for instance on every tv broadcast band, theres no wat to do this without suffering lots of insertion loss. I can accept a fast opamp cant be used as an active filter for rf. I just want to understand why not? Obviously if it could be done it would have already.

author: tggzzz
author_rank: Super Contributor
timestamp: July 09, 2016, 08:34:25 am
content: Quote from: rwgast_lowlevellogicdesin on July 09, 2016, 05:39:16 amWork out what the noise floor, IP2 and IP3 of such a filter would be.

author: edavid
author_rank: Super Contributor
timestamp: July 09, 2016, 04:17:56 pm
content: Quote from: rwgast_lowlevellogicdesin on July 09, 2016, 05:39:16 amTo reduce the insertion loss you were complaining about.QuoteDesign your own oscillator using a crystal?Quote1. Closed loop gain of 2 isn't nearly enough to make a good filter.2. The passive components you would need don't exist.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 09, 2016, 06:39:22 pm
content: Well im not designing my own crystal oscillator because I need a stable reference, Im doing a lot of narrow digital and satellite stuff. I know you can make a pretty stable oscillator using a wein bridge and light bulbs, thats out of the question though. I have also seen people strap heating elements or power resistors to there crystals but not only does this take a lot of power but im not sure how well it works.... I would guess you would have to get the crystal over the highest expected ambient temp (in this case 130F) as fast as you can? I just found this,http://www.ebay.com/itm/Precision-Crystal-Heater-for-HAM-radio-RTL-SDR-OCXO-TCXO-transceiver-/282071583591?hash=item41acc6cb67:g:WLIAAOSwLN5Wi7E3looks like it turns any hc size xtal in to a tcxo, im not sure if its worth the 20 bucks considering it could be junk. I put a lot of work in to that filter the insertion loss is actually something I wanted in order to lower the LO power to the absolute minimal. If i did scrap it and went BP would that save the IL enough to get a 9 pole filter?I did have 5Mhz OCXO from ovenair that was so old and had the strangest pinout and socket that I could never get the thing to work and calls to at least 10 companies associated with the former ovenair, or who repair oven OCXOs had never heard of my model. I decided to take it apart I would like to figure out how to use the oven at some point and stick an oscillator in there.You seem pretty knowledgeable so maybe you could help me with this, according to my scope as long as the LO has at leas 90mv going to the SDR there is no drift. Shifting around to some frequency's that I would think would be dead on my TCXO is rock solid stable no drift, but I may be up to 20ppm off. Where I live I cant receive any NOAA NFM stations at 162.***, I dont even have a GPS antenna but I am going to build an L band patch today (i need this for Iridium anyways) so I can check my TCXO against GPS. If I am a few PPM off is there anyway to fix that in hardware? I know you can type an error factor in to SDR# but not in most other software. Does the screw on the TCXO board calibrate center frequency or the oven? If it calibrates the oven can I somehow use a trimming cap or something/anything to get the thing dead on 28.8?right now I am working on a patch antenna since there are no GPS antennas laying around my house

author: dmills
author_rank: Super Contributor
timestamp: July 09, 2016, 07:50:43 pm
content: The real reason to place a tight BPF at the output of an oscillator is usually as much about phase noise as spurs.You may wish to investigate the possibilities of crystal ladder filters, bucketloads of Q and reasonable loss.While you can use modern opamps in RF circuits, they usually do not have a particularly wonderful combination of IP3 and Noise figure, it is usually possible to do much better with a suitable transistor or two especially in a narrowband design.Now on HF (As Opposed to VHF and up) receiver noise figure is usually not a major issue, keep it less then 10dB or so and it is good enough that sky noise is usually the limiting factor, by the time you hit 70cms not so much.Regards, Dan.

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 01:53:39 am
content: HiA Wein Bridge oscillator will have phase noise that is roughly 100 to 150 db worse than a crystal oscillator depending on offset frequency .... not a good way to go at all. It's stability will be around 1,000X worse as well (also a bit of a problem). There *is* a reason why people use crystal oscillators.Bob