I'm thinking it'd be a rather poor idea to make a reference oscillator for a
DDS (or PLL) equal in frequency to an IF somewhere in your radio... right?
Since you're just asking for it to get into the IF itself?

In the ideal world, would one choose reference oscillator frequencies that
weren't harmonically related to IFs? I've seen those "IF planning"
spreadsheets (that calculator where mixer spurs are going to fall), and such a
choice seems like a good start.

Thanks,
---Joel Koltner

Joel Koltner wrote:
I'm thinking it'd be a rather poor idea to make a reference oscillator for a
DDS (or PLL) equal in frequency to an IF somewhere in your radio... right?
Since you're just asking for it to get into the IF itself?

In the ideal world, would one choose reference oscillator frequencies that
weren't harmonically related to IFs? I've seen those "IF planning"
spreadsheets (that calculator where mixer spurs are going to fall), and such a
choice seems like a good start.

I would certainly avoid it. With thorough enough shielding you can keep
just about any signal out of any circuit, but there's no need to tempt fate.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott wrote:
Joel Koltner wrote:
I'm thinking it'd be a rather poor idea to make a reference oscillator
for a DDS (or PLL) equal in frequency to an IF somewhere in your
radio... right? Since you're just asking for it to get into the IF
itself?

In the ideal world, would one choose reference oscillator frequencies
that weren't harmonically related to IFs? I've seen those "IF
planning" spreadsheets (that calculator where mixer spurs are going to
fall), and such a choice seems like a good start.

I would certainly avoid it. With thorough enough shielding you can keep
just about any signal out of any circuit, but there's no need to tempt
fate.

If you have a decent receiver, you would need over 100db of isolation.
That is very hard to get in a small space. One of the arts of receiver
design is taking care that all of the frequencies and mixing products
don't show up in the passband.

Erich N6FD

Thanks guys, I'll keep my reference at something like 40MHz and my last IF at
something like 21.4MHz...

--Doh!

You could always redesign the IF.


"Joel Koltner" wrote in message
...
I'm thinking it'd be a rather poor idea to make a reference oscillator for
a
DDS (or PLL) equal in frequency to an IF somewhere in your radio... right?
Since you're just asking for it to get into the IF itself?

In the ideal world, would one choose reference oscillator frequencies that
weren't harmonically related to IFs? I've seen those "IF planning"
spreadsheets (that calculator where mixer spurs are going to fall), and
such a
choice seems like a good start.

Thanks,
---Joel Koltner

On Sep 30, 7:37*pm, "Joel Koltner"
wrote:
I'm thinking it'd be a rather poor idea to make a reference oscillator for a
DDS (or PLL) equal in frequency to an IF somewhere in your radio... right?
Since you're just asking for it to get into the IF itself?

In the ideal world, would one choose reference oscillator frequencies that
weren't harmonically related to IFs? *I've seen those "IF planning"
spreadsheets (that calculator where mixer spurs are going to fall), and such a
choice seems like a good start.

Thanks,
---Joel Koltner

Joel -
In fact the BFO for your radio is already in the IF bandwidth, and
there probably is some leakage into the IF amp.

For most applications, you don't notice that leakage unless you
wonder why your AGC is kicking in but you aren't hearing anything.

I have found that keeping the BFO out of the IF, especially for a
diode-mixer-level BFO (+7dBm) requires putting the BFO in a box.

I would expect that putting your DDS in a box will help in general.

Tim.

Erich wrote:
Tim Wescott wrote:
Joel Koltner wrote:
I'm thinking it'd be a rather poor idea to make a reference
oscillator for a DDS (or PLL) equal in frequency to an IF somewhere
in your radio... right? Since you're just asking for it to get into
the IF itself?

In the ideal world, would one choose reference oscillator frequencies
that weren't harmonically related to IFs? I've seen those "IF
planning" spreadsheets (that calculator where mixer spurs are going
to fall), and such a choice seems like a good start.

I would certainly avoid it. With thorough enough shielding you can
keep just about any signal out of any circuit, but there's no need to
tempt fate.

If you have a decent receiver, you would need over 100db of isolation.
That is very hard to get in a small space. One of the arts of receiver
design is taking care that all of the frequencies and mixing products
don't show up in the passband.

Erich N6FD

I was thinking of several layers of milled-from-billet cabinets between
the signal and the receiver. I suppose I should have mentioned just
what I meant by "enough".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

"Tim Wescott" wrote in message
...
I was thinking of several layers of milled-from-billet cabinets between the
signal and the receiver. I suppose I should have mentioned just what I
meant by "enough".

Surely it's easier to take the circuit; wrap it in kapton tape, hold it by the
output leads, and dip the entire thing into molten lead? :-)

Joel Koltner wrote:
"Tim Wescott" wrote in message
...
I was thinking of several layers of milled-from-billet cabinets between the
signal and the receiver. I suppose I should have mentioned just what I
meant by "enough".

Surely it's easier to take the circuit; wrap it in kapton tape, hold it by the
output leads, and dip the entire thing into molten lead? :-)


That's right. We're radio amateurs. I forgot...

What flux makes lead stick to Kapton?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html