Frequency Divider
 

Hello Gang,

I've got a crystal (7.155 mhz) that I need to use at half frequency. I
decided to pump it into a flip flop, but I'm having an awful time using
2n3904 transistors. The triggering seems to be a very challenging design
issue.

Anybody have any thoughts, (or other ideas for halving frequency)?

Do you have a trigger before the flip-flop???

"Peter Barbella" wrote in message
news:_VkYd.142$qN3.78@trndny01...
Hello Gang,

I've got a crystal (7.155 mhz) that I need to use at half frequency. I
decided to pump it into a flip flop, but I'm having an awful time using
2n3904 transistors. The triggering seems to be a very challenging design
issue.

Anybody have any thoughts, (or other ideas for halving frequency)?

On Fri, 11 Mar 2005 17:52:58 GMT, "Peter Barbella"
wrote:

Hello Gang,

I've got a crystal (7.155 mhz) that I need to use at half frequency. I
decided to pump it into a flip flop, but I'm having an awful time using
2n3904 transistors. The triggering seems to be a very challenging design
issue.

Anybody have any thoughts, (or other ideas for halving frequency)?


Lack of info. What kind of OSC? What kind of FF? Is there an
amp/buffer between the OSC and FF?

Generally this is usually a non problem as at 7mhz most flops are easy
to drive. Typical line up I use for this is a 2n3904 osc (any type),
a capactor coupled 2n3904 common emitter amp with a 1k collector load
and about 100 to 300K resistor between the collector and base to bias
it and couple that into the clock input of a 74LS74/74HC74 wired as
/2 or other config as needed. I also try to bias the flops clock
input to about half the VCC for better results. I've never had this
fail and it works fine to about 30-50mhz depending on FF used.


Allison
KB1GMX

Yes, there is a trigger. Here is the schematic.
http://mysite.verizon.net/nonobar/circuit.jpg

There's actually a capacitor coupling the trigger pulse into the diodes (I
forgot to show it).

The two bias resistors going to -15 are critical as are the commutating
capacitors (not shown) across the 3.3k resistors.

It wants to work, but in a very unstable manner. I think I simply may be
taxing the rise times of the 3904s.

Pete


"Barrie Hiern" wrote in message
news:owlYd.52687$xt.52461@fed1read07...
Do you have a trigger before the flip-flop???

"Peter Barbella" wrote in message
news:_VkYd.142$qN3.78@trndny01...
Hello Gang,

I've got a crystal (7.155 mhz) that I need to use at half frequency. I
decided to pump it into a flip flop, but I'm having an awful time using
2n3904 transistors. The triggering seems to be a very challenging design
issue.

Anybody have any thoughts, (or other ideas for halving frequency)?

pump it into a flip flop, but I'm having an awful time using
2n3904 transistors. The triggering seems to be a very
challenging design issue.

You built the divider out of discrete transistors and steering diodes?
Running this sort of divider at 7 MHz will be marginal... some "speedup
caps" in parallel with the base drive resistors may help.

or other ideas for halving frequency?

74HC74.

Tim.

From: "Peter Barbella" on Fri, Mar 11 2005 7:40 pm

Yes, there is a trigger. Here is the schematic.
http://mysite.verizon.net/nonobar/circuit.jpg

There's actually a capacitor coupling the trigger pulse into the
diodes (I
forgot to show it).

The two bias resistors going to -15 are critical as are the
commutating
capacitors (not shown) across the 3.3k resistors.

It wants to work, but in a very unstable manner. I think I simply may
be
taxing the rise times of the 3904s.

The legacy transistor 2N3904 (and 2N3906) aren't "taxed"
any rise times. They are old workhorse transistors
which will work on up to 30 MHz (f_t is 150 MHz or
better)...provided the biasing AND the rather large
base-emitter capacitance is compensated. A good text
(also a "legacy" one) is Millman and Taub's "Pulse,
Digital, and Switching Circuits" which has much good
basic information on discrete device non-linear-
operation circuits.

The -15 VDC supply isn't necessary nor is it "sensitive"
if the trigger-switchover voltages are selected with
reasonable care. Ten percent tolerance resistors can
do the job with a single-rail supply of +5 VDC.

The "commutating" capacitors (common term is just
"speed-up") across the collector-to-base resistors
MAY be critical since those compensate for the base
to emitter capacitance and thus set the high frequency
response in that part of the circuit.

Since +5 VDC supply is there, one or two Schmitt
trigger inverters or gates from the oscillator output
(74HC family, for example) to a 74HC74 dual D F-F
will work fine. A quarter for each device and there
will be gates and a flip-flop left over to use
somewhere else. No sweating out component values
for discrete device circuits, response of the 74HC
devices known and dependable.

For that matter, the crystal oscillator itself can
use the remaining inverters of a Schmitt trigger hex
inverter. An ordinary non-Schmitt inverter (74HC04)
can do the oscillator and squaring-up task as well.
No sweat on that.

On Fri, 11 Mar 2005 17:52:58 GMT, "Peter Barbella"
wrote:

Try CD4060 or CD4024, oscillator and multiple divide all done for you
for 69 cents.

Hello Gang,

I've got a crystal (7.155 mhz) that I need to use at half frequency. I
decided to pump it into a flip flop, but I'm having an awful time using
2n3904 transistors. The triggering seems to be a very challenging design
issue.

Anybody have any thoughts, (or other ideas for halving frequency)?

VE3ELQ ) writes:
On Fri, 11 Mar 2005 17:52:58 GMT, "Peter Barbella"
wrote:

Try CD4060 or CD4024, oscillator and multiple divide all done for you
for 69 cents.

I wouldn't use one of those, unless it was a lower frequency. At 7MHz,
straight versions of those ICs are pushing their limits. And it's
not as if better CMOS devices haven't come along in the 35 years or
so since that series was introduced.

Michael VE2BVW



Hello Gang,

I've got a crystal (7.155 mhz) that I need to use at half frequency. I
decided to pump it into a flip flop, but I'm having an awful time using
2n3904 transistors. The triggering seems to be a very challenging design
issue.

Anybody have any thoughts, (or other ideas for halving frequency)?

WhAT ABOUT A HEF4060?
"Tim Shoppa" schreef in bericht
ups.com...
pump it into a flip flop, but I'm having an awful time using
2n3904 transistors. The triggering seems to be a very
challenging design issue.

You built the divider out of discrete transistors and steering diodes?
Running this sort of divider at 7 MHz will be marginal... some "speedup
caps" in parallel with the base drive resistors may help.

or other ideas for halving frequency?

74HC74.

Tim.

On Tue, 15 Mar 2005 18:45:33 +0100, "Fam J.Hoekstra"
wrote:

WhAT ABOUT A HEF4060?
"Tim Shoppa" schreef in bericht
oups.com...
pump it into a flip flop, but I'm having an awful time using
2n3904 transistors. The triggering seems to be a very
challenging design issue.

You built the divider out of discrete transistors and steering diodes?
Running this sort of divider at 7 MHz will be marginal... some "speedup
caps" in parallel with the base drive resistors may help.

or other ideas for halving frequency?

74HC74.

Tim.



you are asking difficult questions, not everybody over there know
Philips devices, but they usually go twice as high as other standard
devices, so it might work - provided you have it

LA8AK
---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm