From what I understand so far the bias should should not necessarily be
the same and the capacitors will not scale as suggested.

The ESR of the crystal would be different for different overtone
crystals so therefore the bias should change to provide correct
drive and C to ground on the C-Tap would also need to change. According
to Matthy the Cap to ground should be 0.5 to 1 x ESR and emitter drive
impedance less than ESR. Or have I misinterpreted something here ?

I am also unsure of whether the tank should be tuned above or below
resonance. Rhea's book suggests the tank should be capacitive at Fo
indicating the tank itself would be tuned above the operating frequency.

I would like to learn more about the Butler emitter follower crystal
oscillator.

If I ask the specific questions about this typology and someone gives me
an answer, where did they obtain this information. Have they learned by
just playing with the circuit in a hit and miss fashion or have they
first obtained fundamental information from some text and using what
they have learnt then played with the circuit. If so, what would this
text be ?


If Ro = Rsource/(beta+1), is Rsource in the oscillator case of the
emitter follower Rp of the coil in parallel with the bias resistance
or does it also include (Rphi+RE) x Beta. Is Beta used here an
approximation calculate by Ft/F ?




wrote:
On Mon, 28 Aug 2006 13:25:40 GMT, David
wrote:

Thanks for the feedback.

I do not have a ready supply of ferrite cores and do not really want to
wind coils.
I have surface mountable air fixed coils that provide Q 100 that I
expect should be suitable for most applications and use of trimmer
capacitors.

The only high frequency crystal oscillator I could find in the EMRFD
(100MHz) is page 4.15 which is a common base butler. I understand these
suffer parasitic effects, the tank is only effective for a limited range
of inductance. The butler emitter follower is offered as a more superior
typology but this circuit is not found in this book.
The circuit values in his example are incorrect as the 25nH inductor in
the tank requires over 100pF to resonate at 100MHz. The circuit shown
has a net capacitance of around 40pF ?

Built as described, it works. However the 25nH is an approximation
as distributed C and also lead lengths add significantly. Mine tuned
with 33ohms substituted for the crystal from 93 to 122mhz.

There is very little design information given with respect to ratio of
the C-Tap or emitter bias.

Read the test as the concepts are outlined rather than how to design
xyz circuit in cookbook fashon. If you need help, the easy way is
to calculate the reactances at the shown frequency and then using
those numbers scale for the desired frequency. The bias point would
be the same at any frequency for a given transistor and power.

I also have his book "Introduction to Radio Frequency design". This does
go into more depth but is centred mainly around colpitts oscillators
(not good for harmonic crystal oscillators).

In general oscillators in the 20mhz range are harmonic or overtone
designs and more subject to parasitic effects. I've bult the 100mhz
design and it worked fine for me within the limits of the crystal used
though after a x4 multiplication the thermal drift was unacceptable
and the crystal was the first order contributor. The oscillator
otherwise behaved well.

My solution for the whole mess was a lower frequency osc
and using low order harmonics. The lower frequency crystale proved
both less fussy to oscilate and more stable in fundemental mode.
Note: I was interested in sufficient stability to copy SSB at ~1296mhz
so even a few dozen Hz drift is noticeable..

Allison




wrote:
On Mon, 28 Aug 2006 05:59:39 GMT, David
wrote:

Dave,

I do have EMIRFD but find almost all the circuits are based around rf
transformers for matching and most of them are low frequency (3MHz etc).
Up to around 200mhz thats not a big issue if properly scaled and the
right ferrite used.

I've used a lot of ideas from that book at 6 and 2m. The book list
presented is one I'd have suggested and added the venerable handbook
both current and a few older copies.

Allison


Regards

David

Dave Platt wrote:
Anyone recommend a "decent" book that would cover oscillators, small
signal RF amps and matching techniques sufficiently to enable a novice
to start experimenting with circuits and have enough detail in the text
to tweak the circuits to get them running properly ?
"Experimental Methods in RF Design" by Hayward, Campbell, and Larkin,
available from the ARRL, would be a good place to start. It's the
successor to the classic "Solid-State Design for the Radio Amateur"
from back in 1977.

If you can find copies of Doug Demaw's "Design notebook" and "QRP
notebook", there's some good reading there as well. Not as advanced
as "Experimental Methods" but perhaps a bit more accessible.

Buying a bunch of back issues of QRPp might also be helpful. In a lot
of the articles which discuss QRP receiver and transmitters, the
authors go into detail about their own particular circuit preferences,
tweaks, construction and debugging techniques, etc.

For a slightly deeper background, I'd recommend "Troubleshooting
Analog Circuits" by Robert Pease. It's not specific to RF but is full
of useful tricks and ideas.