A down-conversion mixer from up to 200MHz down to 50KHz low IF likes to see a solid 50R load, and not see signals reflected from a following low-IF filter. It should not be necessary to use a really fast op-amp (that can deal with image frequencies) in the LPF, if a passive LPF is used first. So that's what this filter is intended for.
The filter sketched has 100KHz corner frequency, and a very constant 50R input impedance. I got this working nicely in LTSpice, but can't find the ASC file now.
It seems it should be possible to do away with or reduce some of the resistors (at the expense of greater dependence on the 50R nature of the load) and in the process to reduce the loss... but I can't see how to calculate that.
Any thoughts on reducing the loss while staying close to 50R in and out of the passband?
Jeroen here posted some designs for constant-input-impedance filters. I did some simplified versions, not as constant as his.
The idea is to make an LC filter, and hang a network across the input to absorb the stopband drive that the filter wants to reflect. The simplest such network is a series RC to ground.
Your filter looks pretty lossy to me. A pure LC filter has theoretically no losses in the passband.
Here are Jeroen's notes:
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Do you need wideband termination on both ends of the filter? Nothing outside of the passband makes it through the filter, so the output side may not need to be wideband matched.
My versions were lopsided too, only wideband 50 ohms on one end, and not so good a match as Jeroen's. I only needed to do a pretty good job of absorbing cable reflections.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
Yes, LTspice has its own ideas what to present in the "recently used" history and what not.
Back in the time when shortwave receivers still were of some interest,they had crystal roofing filters with truly weird impedances. The ring mixers did not like that: collapse of IP3. The solution was a large JFET in CG between the mixer and the filter that could be set up for 50 Ohm impedance, wideband.
TI P8000 / P8002 or Teledyne Crystalonics CP643. (RIP) That also made up for the 6-8 dB loss of the ring mixer. The FET ran at 55 mA and was quite linear, unimpressed by the filter Zin.
Well, up to GHz really, since this is for instrumentation use I cannot know what antenna might be used, just that I don't want rectified spuria upsetting things. We plan to digitise I & Q from the output of the LPF, so can reduce the resolution BW below 200KHz in DSP. Can't get rid of something odd that aliases into that band though, so the LPF should be clean and flat-looking.
To stay resistive, it has to match the order of the-band.
It's necessary to "lose" the stop-band signal somewhere, so that has to be resistive. The question is how not to lose so much pass-band.
Filed away for reference thanks, but these are all band-pass filters, and >=3rd order, which I don't need.
I don't think the kind of LPF I will follow this with (100KHz) will show much impedance variation from 1MHz up, so no, I don't think I do. If I needed matching at the passive filter output I would use a buffer stage.
I just want to stop most of the RF from reaching the steep op-amp filter after the passive filter.
Jeroen's filters are all lowpass and start at 3rd order.
I was thinking that an ideal LC lowpass can be paralleled at its input with an ideal LC highpass, both filters load-end terminated. The paralleled input end must look like a perfect wideband 50 ohms.
(derived by the maximum power transfer theorem and conservation of energy)
So the best front-end correction network for a lowpass might be a terminated highpass.
For the first-order case, the lowpass is just series L into the 50r load, and the absorber is C+50r to ground. If L/50 = C*50, the input looks like 50 ohms wideband.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
Yes, you can do that for Butterworth filters. For other filters, there will be some ripple around the cross-over frequency. The trick is to minimize that.
There are also constant-resistance bridged T or L sections. These have first-order frequency responses. Let me know if you're interested.
For examples and more, see N. Balabian, T.A. Bickart, Electrical Network Theory, Wiley 1969, ISBN 0471-04567-4 and G. Amsel, R. Bosshard, R. Rausch, C. Zajde, Time domain compensation of cable induced distortions using passive filters for the transmission of fast pulses, Rev. Sci. Instr. Vol. 42, No. 8, August 1971, pp. 1237-1246
The low-stress "LTSpice-accelerated" design procedure could be to design a lattice filter lowpass with a constant input impedance (Za and Zb conjugate duals of each other.)
Then use the transformation equations to transform to the equivalent single-ended bridged-T network which will have the same properties. tweak that form of it in LTSpice using the available off-the-shelf inductance values as parameters.
Ahh, right you are, I saw series LCs and only glanced at the curve, thinking it was S21. Still, 3rd order is unnecessary as I will follow it with a steep op-amp filter. Just want to keep RF out of the op-amp.
Yes, that's the basic idea. There's a few different ways to do it.
That might work. It must have been too obvious for me to see it :)
That's almost exactly the filter I showed, but with the shunt R across the capacity instead of the output. It's pretty lossy. Maybe that's unavoidable for constant-R.
Thanks, that helped. I found my LTSpice file, and modified it to use the exact topology from that page and to calculate the resistors for a given
0 < k < 1. What I had before was a result of a fixed k=0.25, and I didn't know the formulae for varying k. The result is here for anyone else to use.
With k=0.98 you get close to the minimum 6dB loss. At that point, you can short the 1R Rseries, remove RShunt, and Zin still stays within an ohm of 50, which is good enough for the mixer.
Clifford Heath.
---- Cut Here for ConstZ_LPF.plt ---- [AC Analysis] { Npanes: 2 { traces: 1 {2,0,"-V(Input)/I(Rsource)"} X: ('M',2,10000,100000,1e+06) Y[0]: (' ',4,49.9998,0.0001,50.001)
Of incidental interest here is a paper from back in the day (proceedings of the journal of network analysis-something-something, 1964) that shows it's impossible to build a variable attenuator with a constant output impedance out of any number of linear elements and linear pots ganged on a shaft:
the key concept is that an ideal LC filter alone creates loss in the stop b and and transition band by REFLECTING energy back to the input and thus CAN NOT present broadband 50 Ohms. It has no other option. Ideal L C componen ts cannot dissipate energy. So to have a wideband 50 Ohm input you NEED to use a diplexer configuration of HPF with LPF with a dummy load or a pad.
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