Frequency Planning

Dear list,

Once I know I will be in the 900 MHz ISM band and the bandwidth of my
signal will be about 10 MHz, how then do I go about selecting RF
center frequency, crystal frequency, intermediate frequency, and
sample rate? Is it good enough to say:

RF: 900 MHz
Crystal: 100 MHz
IF: 25 MHz
Sample rate: 100 MHz

Am I missing something?

Brian wrote:

> Dear list,
> 
> Once I know I will be in the 900 MHz ISM band and the bandwidth of my
> signal will be about 10 MHz, how then do I go about selecting RF
> center frequency, crystal frequency, intermediate frequency, and
> sample rate? Is it good enough to say:
> 
> RF: 900 MHz

Nobody would allow that, especially with 10 MHz spectrum.

> Crystal: 100 MHz

No such crystal.

> IF: 25 MHz

Bad choice. Besides, there is no such IF filter.

> Sample rate: 100 MHz

Check the price of ADC.

> Am I missing something?

Stupident.
Instead of casting arbitrary numbers, you've got to choose from what is 
available, what are the goals and what are the constraints.




Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

> > IF: 25 MHz
>
> Bad choice. Besides, there is no such IF filter.

Sorry for the lack of research before posting, but could you explain
this point? Why is it a bad choice?

Thanks

Brian wrote:

>>>IF: 25 MHz
>>
>>Bad choice. Besides, there is no such IF filter.
> 
> 
> Sorry for the lack of research before posting, but could you explain
> this point? Why is it a bad choice?

The 10 MHz wide image falls straight into GSM band. Good luck filtering 
that out with input selectivity.

VLV

On Oct 23, 7:38=A0am, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
> Brian wrote:
> >>>IF: 25 MHz
>
> >>Bad choice. Besides, there is no such IF filter.
>
> > Sorry for the lack of research before posting, but could you explain
> > this point? Why is it a bad choice?
>
> The 10 MHz wide image falls straight into GSM band. Good luck filtering
> that out with input selectivity.
>
> VLV


Is this a receiver?

If so, the short answer is you can make it anything you want.

But with an analysis bw of 10MHz, you=92ve got to plan well or you will
be fighting a whole range of issues.  A 10 MHz analysis bandwidth is
not a trivial exercise.  Especially if you=92re after 70 dB of dynamic
range.

Your topology will be driven by your spec=92s for the receiver.  It=92s
not just about the RF center frequency.

I suggest your plan should incorporate as many off the shelf parts as
possible or you=92ll end up with a bunch of custom work.

If a Tx is involved this becomes a whole different issue and you
really need to understand the regs.

Based on your question I suggest that you research a little more.

Yes they do make over tone crystals and osc specifically for ref use.
And there are reasons that you=92d want to use a system ref for the
vco=92s with as high a Freq as possible.  Is a 100 MHz a wise choice in
your case =85 who would know without seeing a spec.


Are you wanting to design a SA of some sort?

On Oct 23, 8:00=A0am, jim <fiscalconsoc...@gmail.com> wrote:
> On Oct 23, 7:38=A0am, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
>
> > Brian wrote:
> > >>>IF: 25 MHz
>
> > >>Bad choice. Besides, there is no such IF filter.
>
> > > Sorry for the lack of research before posting, but could you explain
> > > this point? Why is it a bad choice?
>
> > The 10 MHz wide image falls straight into GSM band. Good luck filtering
> > that out with input selectivity.
>
> > VLV
>
> Is this a receiver?
>
> If so, the short answer is you can make it anything you want.
>
> But with an analysis bw of 10MHz, you=92ve got to plan well or you will
> be fighting a whole range of issues. =A0A 10 MHz analysis bandwidth is
> not a trivial exercise. =A0Especially if you=92re after 70 dB of dynamic
> range.
>
> Your topology will be driven by your spec=92s for the receiver. =A0It=92s
> not just about the RF center frequency.
>
> I suggest your plan should incorporate as many off the shelf parts as
> possible or you=92ll end up with a bunch of custom work.
>
> If a Tx is involved this becomes a whole different issue and you
> really need to understand the regs.
>
> Based on your question I suggest that you research a little more.
>
> Yes they do make over tone crystals and osc specifically for ref use.
> And there are reasons that you=92d want to use a system ref for the
> vco=92s with as high a Freq as possible. =A0Is a 100 MHz a wise choice in
> your case =85 who would know without seeing a spec.
>
> Are you wanting to design a SA of some sort?


One other question: is this tunable or a block convert?

On 10/23/2010 06:59 AM, Vladimir Vassilevsky wrote:
>
>
> Brian wrote:
>
>> Dear list,
>>
>> Once I know I will be in the 900 MHz ISM band and the bandwidth of my
>> signal will be about 10 MHz, how then do I go about selecting RF
>> center frequency, crystal frequency, intermediate frequency, and
>> sample rate? Is it good enough to say:
>>
>> RF: 900 MHz
>
> Nobody would allow that, especially with 10 MHz spectrum.
>
>> Crystal: 100 MHz
>
> No such crystal.
>
>> IF: 25 MHz
>
> Bad choice. Besides, there is no such IF filter.

Not to criticize the rest of your critique, but at a 10MHz signal 
bandwidth, a 25MHz IF bandwidth can be realized fairly easily with a 
home-brew LC filter.  Depending again on which of the other unrealistic 
goals aren't, it could maybe even be made to sacrifice skirt performance 
but use off the shelf parts.

Now, if it needed exceptionally sharp skirts or something, then it'd be 
a pain.

-- 

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

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

On 10/23/2010 06:36 AM, Brian wrote:
> Dear list,
>
> Once I know I will be in the 900 MHz ISM band and the bandwidth of my
> signal will be about 10 MHz, how then do I go about selecting RF
> center frequency, crystal frequency, intermediate frequency, and
> sample rate? Is it good enough to say:
>
> RF: 900 MHz

Can you legally transmit a 10MHz wide signal centered on 900MHz?

> Crystal: 100 MHz

As Vladimir said, this is unobtanium.  You could use a lower frequency 
crystal and a multiplier chain, or you could make a synthesizer.  These 
days, for all but the highest volume production, it's probably cheaper 
to use a synthesizer running off of a standard frequency crystal.

> IF: 25 MHz
> Sample rate: 100 MHz

100MHz ADCs are pretty expensive, unless you're doing some wierd 
spread-spectrum thing that lets you use 1-bit ADCs.  Even then, a 100MHz 
comparator that's up to the task isn't cheap.  So you have a trade-off 
between your IF filter and your sampling rate -- use an IF filter with 
good steep skirts and you can sample at a lower rate -- there are 50MHz 
ADCs out there that are pretty good, for example.

You need to do an end to end analysis of the whole radio.  You need to 
check for higher-order spurious products out of your mixer, you need to 
check for price and availability of off-the-shelf components (as 
Vladimir touched on, with his comments about crystal and the IF filter), 
you need to give thought to the performance of the various pieces.  If 
your radio needs to change frequency then you need to do it over the 
range of frequencies that it'll be receiving.

Do you have a book or reference on receiver design?  There's a lot of 
subtleties to making a good receiver.  The ARRL Handbook, even though it 
is an "amateur" publication, is on the desks of most pro RF engineers in 
the US.  Going beyond that, I'd suggest Hayward's "Introduction to Radio 
Frequency Design" and "Experimental Methods in RF Design".

http://www.powells.com/partner/30696/biblio/9780872591394
http://www.powells.com/partner/30696/biblio/9780872599239
http://www.powells.com/partner/30696/biblio/9780872594920

-- 

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

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

Tim Wescott wrote:

> On 10/23/2010 06:59 AM, Vladimir Vassilevsky wrote:
> 
>>
>>
>> Brian wrote:
>>
>>> Dear list,
>>>
>>> Once I know I will be in the 900 MHz ISM band and the bandwidth of my
>>> signal will be about 10 MHz, how then do I go about selecting RF
>>> center frequency, crystal frequency, intermediate frequency, and
>>> sample rate? Is it good enough to say:
>>>
>>> RF: 900 MHz
>>
>>
>> Nobody would allow that, especially with 10 MHz spectrum.
>>
>>> Crystal: 100 MHz
>>
>>
>> No such crystal.
>>
>>> IF: 25 MHz
>>
>>
>> Bad choice. Besides, there is no such IF filter.
> 
> 
> Not to criticize the rest of your critique, but at a 10MHz signal 
> bandwidth, a 25MHz IF bandwidth can be realized fairly easily with a 
> home-brew LC filter.  Depending again on which of the other unrealistic 
> goals aren't, it could maybe even be made to sacrifice skirt performance 
> but use off the shelf parts.
> 
> Now, if it needed exceptionally sharp skirts or something, then it'd be 
> a pain.

A filter that needs tuning is a pain in production and a potential 
problem with reliability. But, this is an academic exersize with no 
sense or value whatsoever.

There used to be a legend in our school about the successfully defended 
master thesis which had the following phrase somewhere in the middle:
"As nobody is going to read this anyway, we will make the conductors out 
of wood".

VLV

On Oct 23, 8:59=A0am, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
> Tim Wescott wrote:
> > On 10/23/2010 06:59 AM, Vladimir Vassilevsky wrote:
>
> >> Brian wrote:
>
> >>> Dear list,
>
> >>> Once I know I will be in the 900 MHz ISM band and the bandwidth of my
> >>> signal will be about 10 MHz, how then do I go about selecting RF
> >>> center frequency, crystal frequency, intermediate frequency, and
> >>> sample rate? Is it good enough to say:
>
> >>> RF: 900 MHz
>
> >> Nobody would allow that, especially with 10 MHz spectrum.
>
> >>> Crystal: 100 MHz
>
> >> No such crystal.
>
> >>> IF: 25 MHz
>
> >> Bad choice. Besides, there is no such IF filter.
>
> > Not to criticize the rest of your critique, but at a 10MHz signal
> > bandwidth, a 25MHz IF bandwidth can be realized fairly easily with a
> > home-brew LC filter. =A0Depending again on which of the other unrealist=
ic
> > goals aren't, it could maybe even be made to sacrifice skirt performanc=
e
> > but use off the shelf parts.
>
> > Now, if it needed exceptionally sharp skirts or something, then it'd be
> > a pain.
>
> A filter that needs tuning is a pain in production and a potential
> problem with reliability. But, this is an academic exersize with no
> sense or value whatsoever.
>
> There used to be a legend in our school about the successfully defended
> master thesis which had the following phrase somewhere in the middle:
> "As nobody is going to read this anyway, we will make the conductors out
> of wood".
>
> VLV- Hide quoted text -
>
> - Show quoted text -

If his analysis bandwidth is truly 10 MHz, and he=92s looking at
anything around 70 dB of dynamic range he=92s pretty much forced into at
least a 14 bit sub sampling system for an A/D.  Something like a AD
9245 series would do the job.  They're only 12 or so bucks in 100 lot
sizes.  You can add dither to really get a clean spectrum.

I assuming, but sounds like his ref of 100MHz is to a system clk.
Plenty of choices for that.  The question is what=92s his spec ..
that=92ll be the cost driver.

Somebody has to tune the filters.  If there bought or done in house.
He=92ll have a requirement for pre-selection as well =85 buy=92em.

Then again, he could always do wha V suggest ... pay somebody who
knows what their doing.