Re: [time-nuts] Switching transistors, current sources, nonidealties and noise

[Tom Van Baak]

Alex is having email trouble so I'll post on his behalf.
He adds this useful resource to the discussion:

> http://audioworkshop.org/downloads/AMPLIFIERS_OSCILLATION_BJT_CIRCUITS.pdf 
>
> 73 
> KJ6UHN  Alex

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Re: [time-nuts] Switching transistors, current sources, nonidealties and noise

[Scott Stobbe]

The transition frequency of a transistor is more of a ballpark figure of
merit than a device specific constant, as it is also a function of how the
device is biased. Since most amplifiers and oscillators need power gain to
operate, current gain is usually needed (though you could use voltage gain
at the expensive of bandwidth) making ft a useful figure. The transition
frequency is also a good proxy for the effective junction capacitances and
forward transit time of the device.

For a current source, the desired outcome is for the circuit to output one
specific current and no other. So, having a large forward ac-current
bandwidth is not of great importance, as you don't want it to react at all,
you want it to keep sinking/sourcing one current value.

The issue you are having with your circuit is in biasing and loop
compensation. The standard spice model for a transistor does not include
lead inductance (most manufactures will provide a subcircuit model for
their device which includes package parasitics). Referencing the schematic
you previously attached, once your servo amplifier runs out of steam, if Q7
were a 2N3904 looking into the emitter of the device would look like a
10-15 Ohm resistor (1/gm + Rs/beta) in series with a 200 - 250 nH inductor
(tf Rs). When you changed to a BFU520 you lowered the impedance seen at the
base of Q16.

As a test for yourself, If you drive the base of Q16 with an appropriately
chosen voltage source (roughly 3.2 to 3.3 V) you will see a 2n3904 perform
dramatically better.



On Sat, Jul 2, 2016 at 1:04 PM, Attila Kinali  wrote:

> On Fri, 1 Jul 2016 15:45:36 -0400
> Scott Stobbe  wrote:
>
> > If you need higher output resistance you will have to move to a FET based
> > approach. If you need less than 1 pF of output capacitance you will need
> a
> > better transistor and care in how you physically construct your circuit.
>
> I do not think that the output resistance is really the limiting effect
> here, but rather the speed at which the transistor reacts, ie its f_t.
> But I have to admit that I have not fully understood the circuits
> behaviour yet. I still lack good understanding of how a transistor
> behaves under high frequency conditions :-/
>
> Attila Kinali
>
> --
> Malek's Law:
> Any simple idea will be worded in the most complicated way.
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Re: [time-nuts] Good GPSDO?

[Bryan _]

You will find most if all of these units are using a Trimble 63090/73090/65256. 
You will find a lot of good information on this EEVBLOG forum board
http://www.eevblog.com/forum/testgear/bg7tbl-gpsdo-master-reference/
if by chance it is a Symmetricom then
http://www.eevblog.com/forum/projects/a-look-at-my-symmetricom-gpsdo-(ocxo-furuno-receiver)/
As for spec and how good these are I own a the Symmetricom but don't have the 
equipment and more importantly the knowledge to test or give advice to others. 
Will defer that to the experts on Time-Nuts.
p.s. if you search eBay for the Trimble with the mfg. numbers as I mentioned 
you will find them a lot cheaper if you want to roll out your own.

-=Bryan=-

> From: rch...@earthlink.net
> Date: Sat, 2 Jul 2016 21:28:12 -0700
> To: time-nuts@febo.com
> Subject: [time-nuts] Good GPSDO?
> 
> I am just getting into the precise time and frequency stuff.  I thought I 
> should buy a GPSDO.
> 
> There are quite a few similar-looking ones on eBay, such as eBay number 
> 281802105734
> 
> There is also a Trimble at 252162780444
> 
> Would either of these be OK for a newbie like me?
> 
> Richard
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Re: [time-nuts] Good GPSDO?

[David J Taylor]

I am just getting into the precise time and frequency stuff.  I thought I 
should buy a GPSDO.


There are quite a few similar-looking ones on eBay, such as eBay number 
281802105734


There is also a Trimble at 252162780444

Would either of these be OK for a newbie like me?

Richard
___

Richard,

You might also look at a rather more versatile product - Leo Bodnar's 
Precision Frequency Reference (GPS Clock):


 http://www.leobodnar.com/shop/index.php?main_page=product_info_id=234

with multiple programmable frequency outputs.  I have one of these and find 
it quite handy.  For a fixed frequency board-level product, the LTE-Lite 
Evaluation Board:


 http://www.jackson-labs.com/index.php/products/lte_lite

The Trimble stuff is rather dated now, and uses fewer satellites than the 
more recent units.


Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 


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