source --- amplifier --- relay coil.

When source is present the amplified is detected and drives a relay coil
to keep it energized.
When source fails, relay opens switching to backup source.
When source is again active, relay closes reverting back to primary source.
A 10 MHz xtal or other 10 MHz filter in series with amplifier input will
ensure that amplifier will respond to source going off frequency.

Glenn

On 2/4/2019 9:26 PM, Jeff Blaine wrote:

--

Glenn Little                ARRL Technical Specialist  QCWA  LM 28417
Amateur Callsign:  WB4UIV            wb4uiv@arrl.net    AMSAT LM 2178
QTH:  Goose Creek, SC USA (EM92xx)  USSVI LM  NRA LM  SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"

Do you have any phase continuity requirements?  Do you need to failover if the input is 9MHz or 9.999999MHz instead of 10MHz? If not then any carrier-operated relay will do it.

Tim N3QE

Hi

The thing you will (rarely) find out there are devices that act like a GPSDO only the input is
10 MHz, or 1 pps or something else entirely (T1 , E1 …). They watch a set of inputs and
(based on a list of rules) decide what is the best source to use. The output 10 MHz gets
locked to that until something better comes along or it goes away.

If all your “stuff” needs a continuous 10 MHz, this is the sort of box you would want to have.
It keeps your radio on the air, your frequency counter counting, and all those breadboards
doing whatever it is they do.

Since they aren’t real common, finding one is a bit of a chore. Finding one with inputs
(and outputs) that are useful to you is a bit harder still. Figuring out the programming interface
to set up rules that make sense to you ….usually not very easy at all.

Regardless of how you do it, isolation is an issue. Having a 10 MHz standard line with a very
close in spur is not a real good thing. It can mess up a lot of stuff and it passes through the
cleanup loops in most gear. Is 120 db far enough down? I’ve certainly seen problems from
stuff at that level at a small offset …. Simply having a switch with this or that db isolation is
just a starting point. Grounding and shielding are very much on the “to do” list as well.

Lots of fun !!!

Bob

On Tue, Feb 5, 2019, 8:00 AM Bob kb8tq <kb8tq@n1k.org wrote:

I was going to mention that. I modified my Perseus SDR to use an external
oscillator and just removing some SMT resistors to disconnect the original
oscillator output was insufficient. Still coupled in a spur from the
original oscillator and bad close in phase noise. I had to disable the
original oscillator by removing power. I didn't spend the time to try to do
that automatically. Maintaining phase continuity in the changeover would
also be hard.

Do you have any examples of something small that does this well?

Regards,

Mark

Hi

Nothing that I have seen on the surplus market. Some of the things companies I’ve worked for over the
years did a pretty good job ( at least to the specs the customer requested …).

Bob

There is a plug-in circuit built for the HPSDR radios that detects
presence of an external 10 MHz and when it's present shuts off power to
the onboard oscillator.  It would probably be easy to adapt to other
environments.  It's at http://www.k9ivb.net/Hermes/index.htm

John

On 2/5/19 2:09 PM, Bob kb8tq wrote: