I gave the specs earlier in the thread. Knowing those in advance of
recommending a device improves SNR.
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
On 4/13/2022 18:36, Lothar baier wrote:
toggle quoted message
Show quoted text
Since I don¡¯t know the specs of the
original device I cant really judge which device would be
feasible , I chose the BFP740 because I used it in the past in
low phase-noise oscillator circuits , there is a slew of other
devices out there with lower FT
?
A BFP740 is, in a sense, much too fast. By
that I mean that other parameters have been traded off in
exchange for high ft. BVCBO is too low, and so is the power
dissipation spec, for example.
--Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
On 4/13/2022 18:16, Lothar baier wrote:
For best phase-noise I would recommend
finding a SiGe Device like the BFP740 or similar , Infineon
and NXP both make great devices
?
?
Re: Ft overhead: Keep in mind that you
need more gain than the minimum that sustains oscillation in
the absence of a load -- you need enough to support the
power delivered to a load. On top of that, you don't want
the frequency of oscillation to be affected too much by the
transistor's own phase shift -- you want the resonator to
control it. All those considerations argue for a healthy ft
margin. I'm sure you could sub a somewhat slower transistor
and still get output, but you will sacrifice some of the
famous (and hard-fought) stability for which the HP8640B is
prized.
-- Cheers
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
On 4/13/2022 16:44, Flannel Tuba wrote:
Thank you, Arie for finding a link to the actual scanned?
article! Having the pictures along with he text is quite a
nice addition.
Meanwhile, I have ordered a smattering of VHF and UHF
transistors from Mouser with marginally similar ratings to
those Tom provided. I do wonder about the fT of 5GHz
though. The oscillator's range is from 230-550MHz, so I
wonder why the extreme frequency overhead margin. I went
ahead and ordered a dozen or so potential replacements
having at least the Vcbo of 30v, Vceo of 20v, Vebo of 4v
and fT of over 600MHz with several in the 1-5GHz range. A
couple are TO92 packages, which I envision just folding
the base lead across the top to make contact with the
grounding hex cap nut/cover, but most are SOT-23, which
I'll have to come up with an adapter of some sort for. Any
ideas on this are more than welcome.?
Well, if I'm lucky I'll have some time in the next few
weekends to experiment with possible replacements for the
rare and venerable HP 5086-7082.
I'll let you know what I find.
-Scott?
?
?
