Getting real about passband ripple -questions..

[Darktrax]

The scenario is quality microwave kit put in series, typically some waveguide, polarizer, low noise amplifier, down-converter.

Typically, a reasonable specification for a down-converter is (say) +/- 0.25dB for any 40MHz segment, and +/- 1dB over the band, which might be 400MHz (at X-Band). This is probably a worst case. The delivered kit might be better than that. Sometimes the partial band spec is over 30Mhz, sometimes 50MHz.

A reasonable specification for a LNA is (say) +/- 1dB over the band.
Waveguide switching and other extras might have their own ripples.

The way I see it, the passband characteristic will be the sum of the characteristics of the various parts taken together. One might get lucky so that a "bump" or tilt in the response of one might counter some "dip" in another, to give a great overall result, or one can get unlucky, where two "droops" coincide.

Where the modulation is various forms of phase-shifted keying, like 8PSK or DQPSK, it is hard to see small variations in passband response can matter much. Gross variations are obviously not acceptable.

Even if the modulation types involve QAM or any other type of amplitude modulation, the changes are so swift that it has components that spread throughout the modulated spectrum. One cannot really say that a "bump" at some frequency affected a specific data waveform amplitude in the time domain, because it took energy from the entire spectrum.

Hence I ask.
. What is the best that can be expected for passband ripple/tilt using high quality kit?
. Are there any techniques to counter passband ripple? What is the best stuff around?
. An equalizing filter to counter "tilt" sounds feasible, but is it needed/justified? I don't know how I would get hold of such kit.
. Is a specification of +/- 0.5dB, or even +/- 0.75 dB (guaranteed) a common, reasonable, achievable thing? Adding up the specs of the parts would indicate it is not!

 

[HMS1021]

We have a similar system with a W/G filter, W/G limiter and W/G to SMA adapter. These element are 8-12.4 GHz WR90 components and have <.25dB insertion loss each. The ripple over our 500 MHz band is <.2dB. These are followed by a LNA, a image reject high pass ( 2 W/G to SMA adapters back to back), a isolator, and a mixer spec'd from 4-12GHz. Everything combined has 23dB of gain and .3dB total ripple across 500MHz.

When doing the system analysis you RMS all of the component ripples together, this actually works in practice.
To minimize transmission ripple but more importantly group delay select passive component that are much wider than your band. For filters machined parts with maximally flat or Butterworth designs (avoid Chebyshev) are the best but may be too large for some applications. The LNA should be a bit wider than your band to avoid group delay issues at the band edges but avoid a broadband LNA due to the high noise power (KTB) out of it. Also a LNA in X-band with only <1000MHz of bandwidth will be flater and can have better NF and IP2/IP3 performance than a broadband LNA.

Equalizing filters can work but once again group delay has to be watched.

Since group delay is such a driver our RF and IF bandpass filters are fairly wide and are used to mitigate out of band interference and mixer spurs and images. We use CIC and FIR filters in the digital domain (i.e. after the ADC) to set our detection/noise bandwidth since these filter have near zero ripple and group delay.

 

[biff44]

yep, the all add up. but luckily usually not all at one peak.

You would perhaps by the filter to be "max flat" or "max flat group delay" to minimize its ripples. The other parts, try to buy them with as good a VSWR as u can. Keep interconnect lengths small. And in a worst case, u might need a gain or phase equalizer that u can hand trim to get it just inside of the specification limits.

 

[Darktrax]

Firstly - thanks all very much, especially HMS1021 for the replies.

When still putting together the list of equipment, clearly the "guaranteed" specifications are very much conservative.
A number like +/- 1dB can be applied to kit that might well perform to +/- 0.2 dB when actually delivered.

When doing the system analysis you RMS all of the component ripples together, this actually works in practice.

This, for me, is counter-intuitive. If I connect (say) a filter in line with a broader band amplifier, the sweep response is the straightforward sum of the individual responses. Two droops together make a bigger droop.
For design purposes, where one cannot know if the ripples coincide, but are maybe (randomly?) distributed across the band, then I guess the RMS sum of the specifications is a valid estimate provided one does not get unlucky! Again, the data sheet values need to be discounted (carefully!), because they reflect excessive manufacturer caution.

Since group delay is such a driver our RF and IF bandpass filters are fairly wide and are used to mitigate out of band interference and mixer spurs and images.

I take the point that group delay from using filters with too-steep attenuations too close to the band edges, and group delay response from various kit in general likely has more impact on messing up modulated signals than relatively small amplititude variations (ripple) in the passband response. Often the customer will be asking for filters that may include all sorts of tricky sections to make the response steeper on one side, or include a reject notch very close, as if one can make a passband begin directly where a transmit-reject band ended, with no room in between.

One thing I do say is to pay careful attention to the "+/-" in the specification. It somehow easily gets glossed over, or even left out of the client requirement. I can see why, but I am always surprised at how often I have to be very clear on this.