Re: [time-nuts] RF Isolation Amplifier ...
Hi Corby, see: http://www.timeok.it/wp-content/uploads/2015/08/Frequency-Distribution-Design-Basic-Module-v-4.1.pdf Luciano Da "time-nuts" time-nuts-boun...@lists.febo.com A time-nuts@lists.febo.com Cc Data Mon, 15 Oct 2018 10:38:21 -0700 Oggetto [time-nuts] RF Isolation Amplifier ... Thanks for the inputs. Attached is the blurb from the manual on the 5Mhz isolation amps. I don't want to redesign as the 3 amps are matched for delay and temp and run at 45 degrees C. I'll check the output of the amp with a spec-A and select the capacitor as appropriate. Cheers, Corby ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] RF Isolation Amplifier ...
Am 14.10.2018 um 22:14 schrieb Richard (Rick) Karlquist: On 10/14/2018 11:20 AM, Dr. Ulrich L. Rohde via time-nuts wrote: Actually the BFT is out of production since quite a while there are more stable and higher Ft devices on the market. 73 de N1UL How is a higher Ft device more stable? Those attributes would seem to be mutually exclusive. Every mm has a nH all for itself. With 1.6mm for the board thickness, 2 mm for the transistor leads, 0.5 mm for the bond wires, we have > 3 nH in base, emitter and collector, and they are coupled. That can annoy the most well behaved RF transistor. [1] For time nuts purposes, I would submit this is a bad trend. What we want is higher DC beta, not higher Ft. The higher Ft just makes the device want to oscillate. For any designs I do, I put a 100 ohm resistor in series with the collector as an oscillation killer. There is a similar problem with gain block amplifiers having bandwidths into the double digit GHz. I routinely put a 10 pF capacitor directly from input to ground to kill high frequencies. A related problem is that newer devices have lower base spreading resistance. This does help with noise figure but again risks HF oscillations. I do love those Zetex BJTs with extra low base spreading resistance. We just discussed this in the usenet sci.electronics.design newsgroup. In "Art of Electronics edition 3 " by Horowitz? and Winfield Hill, there is a base band amplifier featuring 70 pV/rtHz voltage noise. I have verified that. One of the Zetexes was not enough, it took 16 in par to get Rbb low enough, just like demonstrated by Horowitz < https://www.flickr.com/photos/137684711@N07/31348447748/in/album-72157662535945536/ > anf the pictures to the right. ( the pics to the left are off-topic, but some of them nevertheless nice.) regards, Gerhard [1] analog to < http://gunkies.org/wiki/Vonada%27s_Engineering_Maxims > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] RF Isolation Amplifier ...
Well said Sent from my iPhone > On Oct 14, 2018, at 4:14 PM, Gerhard Hoffmann wrote: > > > >> Am 14.10.2018 um 18:47 schrieb Dana Whitlow: >> Corby, >> >> Now that I learn that the BFT66 is a "wild one", I'd remark that stability >> is >> indeed the likely issue. I was puzzled by the presence of that capacitor in >> the first place, as this circuit is clearly neither tuned nor of especially >> high frequency capability. > If the circuit really has a stability problem, one could try first to > harvest the low-hanging fruit and change the base resistors from > 2R5 to 22R, maybe even more. > > That won't damage the noise behavior, that has already been done by > the 180R in the input. The only possible reason I see for the 12 pF is > to kill the gain of the first stage at VHF+. It may be a disservice to > the 2nd stage, depending on the isolation performance of the R > between the upper emitter and the lower Collector. > > Also, the 100nF+10nF caps in the base divider will parallel resonate > at +/-10 MHz, depending on layout; and that means "no real decoupling" there. > > All in all, the circuit is somewhat weird; the BFT66 was a boutique part > even in it's heyday: low noise, high gain, high ft, high price and a package > that was not really RF-ish. The second stage is abt. as low cost as you could > get. > With all-3904/3906 that would be a text book AF amplifier. > > Have a nice Sunday evening, > > Gerhard, DK4XP > > > > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] RF Isolation Amplifier ...
Well some of the 7 GHz and higher Ft transistors have internal matching networks to prevent such oscillations, but yes it is an important issue. 73 de N1UL Sent from my iPhone > On Oct 14, 2018, at 4:14 PM, Richard (Rick) Karlquist > wrote: > > > >> On 10/14/2018 11:20 AM, Dr. Ulrich L. Rohde via time-nuts wrote: >> Actually the BFT is out of production since quite a while there are more >> stable and higher Ft devices on the market. >> 73 de N1UL > > How is a higher Ft device more stable? Those attributes > would seem to be mutually exclusive. > > For time nuts purposes, I would submit this is a bad trend. > What we want is higher DC beta, not higher Ft. The higher > Ft just makes the device want to oscillate. For any designs > I do, I put a 100 ohm resistor in series with the collector > as an oscillation killer. > > There is a similar problem with gain block amplifiers having > bandwidths into the double digit GHz. I routinely put a > 10 pF capacitor directly from input to ground to kill high > frequencies. > > A related problem is that newer devices have lower base > spreading resistance. This does help with noise figure > but again risks HF oscillations. > > Rick N6RK ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] RF isolation amplifier question
Hi What you have there is known as a cascade buffer amplifier. The common base stage combined with the common emitter has some well known issues at high frequencies ( like UHF / microwave). That’s the tradeoff for having really good isolation at HF and usually at VHF. Put a sniffer loop near the circuit and look with a good wide range spectrum analyzer. Drive the circuit over a range of input levels. Try terminating the output with odd loads. Watch for birdies popping up and then fiddle things to kill them. As others have noted, those may not be the best parts to use for this. I would not replace it with something simple, it should have a lot of isolation when it works correctly. Bob > On Oct 14, 2018, at 11:54 AM, wrote: > > Hi, > > I removed this amp from the EFOS2 Maser as it was intermittent. > > Of course now I can't get it to fail on the bench! > > I have fabricated a new PC board to replace it and have the components on > hand. > > Note the 12pfd cap on the collector . > > Some notes I have indicate is select at test and that is it a glass > capacitor. > > The as built unit has a johansen 50pf that looks to be an early version > of surface mount with leads soldered on. > > How do I determine what value to install on the new amplifier? > > Cheers, > > Corby___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] RF Isolation Amplifier ...
Am 14.10.2018 um 18:47 schrieb Dana Whitlow: Corby, Now that I learn that the BFT66 is a "wild one", I'd remark that stability is indeed the likely issue. I was puzzled by the presence of that capacitor in the first place, as this circuit is clearly neither tuned nor of especially high frequency capability. If the circuit really has a stability problem, one could try first to harvest the low-hanging fruit and change the base resistors from 2R5 to 22R, maybe even more. That won't damage the noise behavior, that has already been done by the 180R in the input. The only possible reason I see for the 12 pF is to kill the gain of the first stage at VHF+. It may be a disservice to the 2nd stage, depending on the isolation performance of the R between the upper emitter and the lower Collector. Also, the 100nF+10nF caps in the base divider will parallel resonate at +/-10 MHz, depending on layout; and that means "no real decoupling" there. All in all, the circuit is somewhat weird; the BFT66 was a boutique part even in it's heyday: low noise, high gain, high ft, high price and a package that was not really RF-ish. The second stage is abt. as low cost as you could get. With all-3904/3906 that would be a text book AF amplifier. Have a nice Sunday evening, Gerhard, DK4XP ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] RF Isolation Amplifier ...
On 10/14/2018 11:20 AM, Dr. Ulrich L. Rohde via time-nuts wrote: Actually the BFT is out of production since quite a while there are more stable and higher Ft devices on the market. 73 de N1UL How is a higher Ft device more stable? Those attributes would seem to be mutually exclusive. For time nuts purposes, I would submit this is a bad trend. What we want is higher DC beta, not higher Ft. The higher Ft just makes the device want to oscillate. For any designs I do, I put a 100 ohm resistor in series with the collector as an oscillation killer. There is a similar problem with gain block amplifiers having bandwidths into the double digit GHz. I routinely put a 10 pF capacitor directly from input to ground to kill high frequencies. A related problem is that newer devices have lower base spreading resistance. This does help with noise figure but again risks HF oscillations. Rick N6RK ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] RF isolation amplifier question
Corby - >From the schematic it does not look like anything special but only a 50 ohm in and out circuit with fairly high isolation due to the common base of the second BFT66. You may be able to simply use a newer MMIC from one of the many sources to replace the assembly if you know the gain. Get one with more gain than needed and then use a resistive attenuator for additional isolation. I did not see the variable Johanson on the schematic and there are no tuned circuits that I can see, at least not LC wise. I would think you could measure the Johanson by removing it from the board. Since I do not know the exact purpose of the amp I do not really understand why it has to be so critical. Regards - Mike Mike B. Feher, N4FS 89 Arnold Blvd. Howell NJ 07731 848-245-9115 -Original Message- From: time-nuts On Behalf Of cdel...@juno.com Sent: Sunday, October 14, 2018 11:55 AM To: time-nuts@lists.febo.com Subject: [time-nuts] RF isolation amplifier question Hi, I removed this amp from the EFOS2 Maser as it was intermittent. Of course now I can't get it to fail on the bench! I have fabricated a new PC board to replace it and have the components on hand. Note the 12pfd cap on the collector . Some notes I have indicate is select at test and that is it a glass capacitor. The as built unit has a johansen 50pf that looks to be an early version of surface mount with leads soldered on. How do I determine what value to install on the new amplifier? Cheers, Corby ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
