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#1
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Compensating frequency response of wideband amplifiers
Could someone point me in the direction of how you might go about flattening
the frequency response of a wideband amplifier MMIC? I'm looking at something like the Watkin-Johnson ECG003 (http://www.wj.com/pdf/ECG003.pdf) and noticing that the response drops a little over 2dB from 500MHz to 2.5GHz. Assuming that I've got plenty of SNR, component count isn't a big deal, and I don't mind tweaking trimmers, how might I go about flattening that respone to, say, 0.5dB or better across that range? Or is it just not very practical? Thanks, ---Joel Kolstad |
#3
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Compensating frequency response of wideband amplifiers
"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...) |
#4
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Compensating frequency response of wideband amplifiers
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 |
#5
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Compensating frequency response of wideband amplifiers
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). |
#6
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Compensating frequency response of wideband amplifiers
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 |
#7
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Compensating frequency response of wideband amplifiers
Joel Kolstad wrote:
Could someone point me in the direction of how you might go about flattening the frequency response of a wideband amplifier MMIC? I'm looking at something like the Watkin-Johnson ECG003 (http://www.wj.com/pdf/ECG003.pdf) and noticing that the response drops a little over 2dB from 500MHz to 2.5GHz. Assuming that I've got plenty of SNR, component count isn't a big deal, and I don't mind tweaking trimmers, how might I go about flattening that respone to, say, 0.5dB or better across that range? Or is it just not very practical? Thanks, ---Joel Kolstad A lead-lag filter with the pole out beyond 2.5GHz may do it for you, but I have no idea how you'd actually implement it. Ditto for cleverly placed stubs. And don't call me when the next batch has slightly different characteristics, or when the one you have varies with temperature. I recall someone writing about this in QEX quite a while back -- it was his first time designing an IF for a radar, and it had to be WIDE band. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
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