"John Miles" wrote in message
...
Why would you need specs like that?

Obscenely wideband (500MHz) software-defined radios. There are people out
there asking for such things to be built -- hence the question! (And
wondering whether or they're just having pipe dreams and technology just
isn't there yet...)

Joel Kolstad wrote:

"John Miles" wrote in message
...

Why would you need specs like that?


Obscenely wideband (500MHz) software-defined radios. There are people out
there asking for such things to be built -- hence the question! (And
wondering whether or they're just having pipe dreams and technology just
isn't there yet...)



You have a digital system, which can presumably fix this in software,
and you want to go sticking _trimmers_ in your circuit?!?

--

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

Hi Tim,

"Tim Wescott" wrote in message
...
You have a digital system, which can presumably fix this in software, and
you want to go sticking _trimmers_ in your circuit?!?

The amps are well before the ADCs -- ADCs that sample at anything approaching
1Gsps have atrocious dynamic range compared to an analog signal processing
chain with a decent noise floor (e.g., 6 bits giving about 40dB dynamic range
vs. 80-100dB dynamic range back in the analog domain).

Wouldn't it be a lot easier to just use a part that's inherently
flatter? How about a Sirenza SBB-4089, for example.

If you really want to compensate a part that has some rolloff, you
could try, along your 50 ohm transmission line, putting in series a
shunt combination of an R and a C. The R adds some attenuation, and
the C shorts it out at high frequencies. You need to use tiny parts to
avoid problems with parasitic C and L. 0603 or 0402 should work OK.
You need to take into account the driving amplifier's output impedance,
and the load's impedance, and their variation with frequency. If you
use multiple sections like that, you can tailor the response better
than you can do with a single section. You could add in some
inductance in series with resistance, shunt to ground, if you could
find some inductance that behaves nicely at 2.5GHz. Maybe a shorted
stub? That would let you keep a more constant load impedance versus
frequency for the driving amplifier.

And what use would a 6 bit wideband system be anyway? What are you
receiving with it that doesn't require more dynamic range than you'll
get with 6 bits? And if you're only sampling at 1GHz, why are you
worried about anything more than about 400MHz bandwidth?

Much more interesting to be looking at 14 or more bits...I keep telling
the ADC manufacturers that I'd like 18 bits, with noise and distortion
to match. Haven't been demanding a 5Gs/s rate yet though. Wouldn't
quite know what to do with the data stream.

Cheers,
Tom

Further to the frequency response compensation:

In a world of ideal R, L and C components, you can insert an attenuator
into a transmission line, matched to the line's characteristic
impedance, to get frequency-independent attenuation while maintaining
good return-loss from both input and output ports. If you then modify
that circuit by shunting a capacitor across the attenuator, and lifting
the ground-return of the attenuator and inserting an inductor in series
to ground, and the inductor and capacitor each have reactances equal in
magnitude to the line's impedance at some frequency F (assuming a
non-reactive, constant impedance line here), the system will still have
good return loss at all frequencies, and will have attenuation equal to
that of the attenuator alone at DC, and no attenuation at frequencies
far enough above F. In the transition region, the maximum slope of the
attenuation will be 6dB/octave. That maximum will not be reached for
low values of attenuation. If you cascade such sections, with each
section having a different frequency F and perhaps differing
attenuations, you can make a tailored response adjustment. Cascading
identical sections of high enough ultimate attenuation results in
maximum slopes in excess of 6dB/octave. Obviously trading the places
of the inductor and capacitor give you the opposite shape: more
attenuation at higher frequencies.

I have a little RFSim99 circuit that illustrates this idea--email if
you'd like the file.

It's going to get really difficult to make this work right at high
frequencies, because of the size of the parts and the parasitics
involved. But with modern parts, you might be able to make it work
well out to a couple GHz.

Cheers,
Tom

Thanks for the hints, Tom, I'll look into them.

"K7ITM" wrote in message
oups.com...
And what use would a 6 bit wideband system be anyway? What are you
receiving with it that doesn't require more dynamic range than you'll
get with 6 bits?

As far as I'm aware, pretty much all high data rate digital modulation
schemes can be recovered with 6 bits (or less).

And if you're only sampling at 1GHz, why are you
worried about anything more than about 400MHz bandwidth?

OK, 1.25Gsps? :-) I'm just talking ballpark figures.

---Joel

"radio" implied to me that you are dealing with a multitude of signals
in that bandwidth, not just one, and my practical experience tells me
that you will have to deal with a wide dynamic range. What do you do
with the interfering signal that's 20dB above your signal of interest?
If you're wired between the source and the receiver with no outside
signals getting in, I wonder if you're engaging in overkill for the
receiver...

OK, so 1.25Gsps gets you to maybe 500MHz bandwidth. That's still a far
cry from 2GHz. Why would you be worried about the flatness over 2GHz
when your bandwidth is "only" 500MHz?

Of course, there are 'scopes available these days that sample a lot
faster than that.

Cheers,
Tom

Hi Tom,

"K7ITM" wrote in message
oups.com...
"radio" implied to me that you are dealing with a multitude of signals
in that bandwidth, not just one, and my practical experience tells me
that you will have to deal with a wide dynamic range. What do you do
with the interfering signal that's 20dB above your signal of interest?

Ah, the classic wideband SDR problem! You filter it out, of course, with an
adjustable notch filter (or two or three or...). Or at least that's all I
know to do about them...

OK, so 1.25Gsps gets you to maybe 500MHz bandwidth. That's still a far
cry from 2GHz. Why would you be worried about the flatness over 2GHz
when your bandwidth is "only" 500MHz?

So that you can tune around over 2 or 3GHz and everything's still flat. I
realize that's probably not really absolutely necessary, but as I say,
people are asking for these levels of flatness, and if it's readily doable
sometimes it's easier just to do what's ask rather than try to educate them
about why their request isn't really needed. :-)

---Joel