100KHz low pass with constant 50R input impedance

[Clifford Heath]

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.

<https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg>

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?

Clifford Heath.

 

[John Larkin]

On Sat, 15 Jun 2019 12:20:39 +1000, Clifford Heath
<no.spam@please.net> wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

Clifford Heath.

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:

https://www.dropbox.com/s/smxzjcno8ygjpjl/Bessel_CR.pdf?dl=0

https://www.dropbox.com/s/y5hwpcmkutksjlq/Gaussian_CR.pdf?dl=0

https://www.dropbox.com/s/c1nyhqjszqcbh2u/Linphase_CR.pdf?dl=0


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.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

On Sat, 15 Jun 2019 12:20:39 +1000, Clifford Heath
<no.spam@please.net> wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

Do you need a constant impedance up to a few gigahertz or just some
nice 50 ohm load around the 400 MHz (RF+LO image frequency) ?

Use a 100 kHz HPF to feed a 50 ohm dummy load (idler) to dissipate the
unwanted power.

 

[Gerhard Hoffmann]

Am 15.06.19 um 04:20 schrieb Clifford Heath:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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.

Yes, LTspice has its own ideas what to present in the "recently used"
history and what not.

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?

Clifford Heath.

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.

regards, Gerhard

 

[Clifford Heath]

On 15/6/19 4:40 pm, upsidedown@downunder.com wrote:

On Sat, 15 Jun 2019 12:20:39 +1000, Clifford Heath
no.spam@please.net> wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

Do you need a constant impedance up to a few gigahertz or just some
nice 50 ohm load around the 400 MHz (RF+LO image frequency) ?

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.

Clifford Heath.

 

[Clifford Heath]

On 15/6/19 12:44 pm, John Larkin wrote:

On Sat, 15 Jun 2019 12:20:39 +1000, Clifford Heath
no.spam@please.net> wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

Clifford Heath.

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.

To stay resistive, it has to match the order of the-band.

> Your filter looks pretty lossy to me.

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.

> Here are Jeroen's notes:

Filed away for reference thanks, but these are all band-pass filters,
and >=3rd order, which I don't need.

> Do you need wideband termination on both ends of the filter?

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.

Clifford Heath.

 

[piglet]

On 15/06/2019 3:20 am, Clifford Heath wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

Clifford Heath.

What's wrong with the traditional diplexer approach. LPF to IF path and
HPF to resistor?

piglet

 

[John Larkin]

On Sat, 15 Jun 2019 20:24:10 +1000, Clifford Heath
<no.spam@please.net> wrote:

On 15/6/19 12:44 pm, John Larkin wrote:
On Sat, 15 Jun 2019 12:20:39 +1000, Clifford Heath
no.spam@please.net> wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

Clifford Heath.

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.

To stay resistive, it has to match the order of the-band.

Your filter looks pretty lossy to me.

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.

Here are Jeroen's notes:

Filed away for reference thanks, but these are all band-pass filters,
and >=3rd order, which I don't need.

Jeroen's filters are all lowpass and start at 3rd order.

Do you need wideband termination on both ends of the filter?

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.

Clifford Heath.

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

 

[Jeroen Belleman]

On 2019-06-15 15:24, John Larkin wrote:

On Sat, 15 Jun 2019 20:24:10 +1000, Clifford Heath
no.spam@please.net> wrote:

On 15/6/19 12:44 pm, John Larkin wrote:
On Sat, 15 Jun 2019 12:20:39 +1000, Clifford Heath
no.spam@please.net> wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

Clifford Heath.

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.

To stay resistive, it has to match the order of the-band.

Your filter looks pretty lossy to me.

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.

Here are Jeroen's notes:

Filed away for reference thanks, but these are all band-pass filters,
and >=3rd order, which I don't need.

Jeroen's filters are all lowpass and start at 3rd order.


Do you need wideband termination on both ends of the filter?

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.

Clifford Heath.

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.

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

Jeroen Belleman

 

[bitrex]

On 6/15/19 9:24 AM, John Larkin wrote:

On Sat, 15 Jun 2019 20:24:10 +1000, Clifford Heath
no.spam@please.net> wrote:

On 15/6/19 12:44 pm, John Larkin wrote:
On Sat, 15 Jun 2019 12:20:39 +1000, Clifford Heath
no.spam@please.net> wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

Clifford Heath.

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.

To stay resistive, it has to match the order of the-band.

Your filter looks pretty lossy to me.

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.

Here are Jeroen's notes:

Filed away for reference thanks, but these are all band-pass filters,
and >=3rd order, which I don't need.

Jeroen's filters are all lowpass and start at 3rd order.


Do you need wideband termination on both ends of the filter?

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.

Clifford Heath.

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.

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.

<https://en.wikipedia.org/wiki/Lattice_network#Constant_Resistance_Low-Pass_Filters>

 

[John Larkin]

On Sat, 15 Jun 2019 17:33:39 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

On 2019-06-15 15:24, John Larkin wrote:
On Sat, 15 Jun 2019 20:24:10 +1000, Clifford Heath
no.spam@please.net> wrote:

On 15/6/19 12:44 pm, John Larkin wrote:
On Sat, 15 Jun 2019 12:20:39 +1000, Clifford Heath
no.spam@please.net> wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

Clifford Heath.

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.

To stay resistive, it has to match the order of the-band.

Your filter looks pretty lossy to me.

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.

Here are Jeroen's notes:

Filed away for reference thanks, but these are all band-pass filters,
and >=3rd order, which I don't need.

Jeroen's filters are all lowpass and start at 3rd order.


Do you need wideband termination on both ends of the filter?

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.

Clifford Heath.

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.

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.

I'd like to see that, if it's not too much trouble. We are doing some
work with distributed amplifiers and it may relate.

Mini-Circuits has some fancy tiny constant-R filters.

https://www.minicircuits.com/WebStore/RF-Filters.html

but only down to 150 MHz.

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

Jeroen Belleman

--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[bitrex]

On 6/14/19 10:20 PM, Clifford Heath wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

Clifford Heath.

Conversion to a bridged-T network including the 50 load to help define
the wideband input impedance will help reduce the loss, consider:

<https://en.wikipedia.org/wiki/Lattice_network#Results_derived_by_network_analysis>

How Zin is not dependent on the source impedance, and:

<https://en.wikipedia.org/wiki/Lattice_network#Constant_Resistance_Low-Pass_Filters>

For how the much easier-to-analyze lattice structure can be mechanically
converted to an unbalanced equivalent.

 

[Clifford Heath]

On 15/6/19 11:24 pm, John Larkin wrote:

On Sat, 15 Jun 2019 20:24:10 +1000, Clifford Heath
no.spam@please.net> wrote:

On 15/6/19 12:44 pm, John Larkin wrote:
On Sat, 15 Jun 2019 12:20:39 +1000, Clifford Heath
no.spam@please.net> wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

Clifford Heath.

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.

To stay resistive, it has to match the order of the-band.

Your filter looks pretty lossy to me.

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.

Here are Jeroen's notes:

Filed away for reference thanks, but these are all band-pass filters,
and >=3rd order, which I don't need.

Jeroen's filters are all lowpass and start at 3rd order.

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.

So the best front-end correction network for a lowpass might be a
terminated highpass.

Yes, that's the basic idea. There's a few different ways to do it.

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.

That might work. It must have been too obvious for me to see it

Clifford Heath

 

[Clifford Heath]

On 16/6/19 2:06 am, bitrex wrote:

On 6/15/19 9:24 AM, John Larkin wrote:
On Sat, 15 Jun 2019 20:24:10 +1000, Clifford Heath
no.spam@please.net> wrote:

On 15/6/19 12:44 pm, John Larkin wrote:
On Sat, 15 Jun 2019 12:20:39 +1000, Clifford Heath
no.spam@please.net> wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

Clifford Heath.

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.

To stay resistive, it has to match the order of the-band.

Your filter looks pretty lossy to me.

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.

Here are Jeroen's notes:

Filed away for reference thanks, but these are all band-pass filters,
and >=3rd order, which I don't need.

Jeroen's filters are all lowpass and start at 3rd order.


Do you need wideband termination on both ends of the filter?

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.

Clifford Heath.

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.



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.

https://en.wikipedia.org/wiki/Lattice_network#Constant_Resistance_Low-Pass_Filters

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.

Clifford Heath.

 

[Clifford Heath]

On 16/6/19 11:45 am, Clifford Heath wrote:

On 16/6/19 2:06 am, bitrex wrote:
On 6/15/19 9:24 AM, John Larkin wrote:
On Sat, 15 Jun 2019 20:24:10 +1000, Clifford Heath
The filter sketched has 100KHz corner frequency, and a very
constant 50R input impedance. >>>>>> Any thoughts on reducing the loss while staying close to 50R in
and out of the passband?
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.)
https://en.wikipedia.org/wiki/Lattice_network#Constant_Resistance_Low-Pass_Filters
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)
Y[1]: ('Âľ',0,-0.00066,6e-05,-0)
Log: 0 0 0
PltReal: 1
PltImag: 1
Representation: 2
NeyeDiagPeriods: 0
},
{
traces: 1 {524291,0,"V(output)"}
X: ('M',1,10000,0,1e+06)
Y[0]: (' ',0,0.0398107170553497,2,0.501187233627272)
Y[1]: (' ',0,-88,8,-0)
Log: 1 2 0
PltMag: 1
PltPhi: 1 0
NeyeDiagPeriods: 0
}
}
---- Cut here for ConstZ_LPF.asc ----
Version 4
SHEET 1 1204 808
WIRE 144 112 80 112
WIRE 496 112 224 112
WIRE 640 112 576 112
WIRE -48 208 -96 208
WIRE 80 208 80 112
WIRE 80 208 32 208
WIRE 128 208 80 208
WIRE 288 208 208 208
WIRE 448 208 288 208
WIRE 496 208 448 208
WIRE 640 208 640 112
WIRE 640 208 576 208
WIRE 800 208 640 208
WIRE 288 304 288 208
WIRE 448 304 448 208
WIRE 640 304 640 208
WIRE 800 304 800 208
WIRE -96 352 -96 208
WIRE -96 496 -96 432
WIRE 288 496 288 368
WIRE 288 496 -96 496
WIRE 448 496 448 384
WIRE 448 496 288 496
WIRE 640 496 640 368
WIRE 640 496 448 496
WIRE 800 496 800 384
WIRE 800 496 640 496
WIRE 640 528 640 496
FLAG 640 528 0
FLAG 80 208 Input
FLAG 800 208 Output
SYMBOL voltage -96 336 R0
SYMATTR InstName V1
SYMATTR Value SINE(0 1 3Meg 0)
SYMATTR Value2 AC 1 0
SYMATTR SpiceLine Rser=0
SYMBOL ind 128 128 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L1
SYMATTR Value {L}
SYMBOL cap 304 368 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C1
SYMATTR Value {C}
SYMBOL res 224 192 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value {Z0}
SYMBOL res 592 192 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value {Z0}
SYMBOL res 464 400 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R4
SYMATTR Value {Rshunt}
SYMBOL res 48 192 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName Rsource
SYMATTR Value {Z0}
SYMBOL res 784 288 R0
SYMATTR InstName Rload
SYMATTR Value {Z0}
SYMBOL res 592 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R3
SYMATTR Value {Rseries}
SYMBOL cap 624 304 R0
SYMATTR InstName C3
SYMATTR Value {Xload}
TEXT -128 792 Left 2 !.AC DEC 20 {F0/10} {F0*10}
TEXT -32 8 Left 2 ;Constant input impedance low-pass filter, e.g. for IF
output from a mixer
TEXT -128 584 Left 2 !.PARAM F0=100kHz Z0=50 k=0.98 a=1
TEXT -128 560 Left 2 ;Edit this to set the cutoff frequency,
characteristic impedance, passband loss:
TEXT -128 768 Left 2 ;Edit this to change the simulation frequency range:
TEXT -128 648 Left 2 ;Edit this to experiment with varying load reactance:
TEXT -128 672 Left 2 !; .STEP PARAM Xload LIST -100p 0p 100p\n.PARAM
Xload 0p
TEXT 488 656 Left 2 !.PARAM w0=2*pi*F0\n.PARAM L=Z0/(w0*k)
C=1/(w0*Z0*k)\n.PARAM Rseries=Z0*(a-k)/k Rshunt=Z0*k/(a-k)\n.measure L
param {L}\n.measure C param {C}\n.measure Rseries param
{Rseries}\n.measure Rshunt param {Rshunt}
TEXT 488 616 Left 2 ;Calculations and measurements:

 

[bitrex]

On 6/15/19 10:41 PM, Clifford Heath wrote:

On 16/6/19 11:45 am, Clifford Heath wrote:
On 16/6/19 2:06 am, bitrex wrote:
On 6/15/19 9:24 AM, John Larkin wrote:
On Sat, 15 Jun 2019 20:24:10 +1000, Clifford Heath
The filter sketched has 100KHz corner frequency, and a very
constant 50R input impedance. >>>>>> Any thoughts on reducing the
loss while staying close to 50R in
and out of the passband?
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.)
https://en.wikipedia.org/wiki/Lattice_network#Constant_Resistance_Low-Pass_Filters

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.

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:

<https://www.dropbox.com/s/2t697szqrn6ocqv/Black-%20Constant%20Impedance%20Attenuators.pdf?dl=0>

 

[bitrex]

On 6/15/19 10:41 PM, Clifford Heath wrote:

On 16/6/19 11:45 am, Clifford Heath wrote:
On 16/6/19 2:06 am, bitrex wrote:
On 6/15/19 9:24 AM, John Larkin wrote:
On Sat, 15 Jun 2019 20:24:10 +1000, Clifford Heath
The filter sketched has 100KHz corner frequency, and a very
constant 50R input impedance. >>>>>> Any thoughts on reducing the
loss while staying close to 50R in
and out of the passband?
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.)
https://en.wikipedia.org/wiki/Lattice_network#Constant_Resistance_Low-Pass_Filters

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.

Glad that worked out. Yeah the bridged T should not be that lossy, the
150 ohm doesn't need to be there.

 

[bitrex]

On 6/16/19 1:34 AM, bitrex wrote:

On 6/15/19 10:41 PM, Clifford Heath wrote:
On 16/6/19 11:45 am, Clifford Heath wrote:
On 16/6/19 2:06 am, bitrex wrote:
On 6/15/19 9:24 AM, John Larkin wrote:
On Sat, 15 Jun 2019 20:24:10 +1000, Clifford Heath
The filter sketched has 100KHz corner frequency, and a very
constant 50R input impedance. >>>>>> Any thoughts on reducing
the loss while staying close to 50R in
and out of the passband?
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.)
https://en.wikipedia.org/wiki/Lattice_network#Constant_Resistance_Low-Pass_Filters


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.


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:

https://www.dropbox.com/s/2t697szqrn6ocqv/Black-%20Constant%20Impedance%20Attenuators.pdf?dl=0

The next paper in line looks interesting too "An elementary method for
obtaining network responses to periodic, non-sinusoidal sources."

Maybe I gotta bribe my academic contact and see if I can get my hands on
the full set.

 

[Michael Terrell]

On Friday, June 14, 2019 at 10:20:47 PM UTC-4, Clifford Heath wrote:

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.

https://www.dropbox.com/s/ixzbczde1bqo91m/100KHz50R_LPF.jpg

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?

A common method at Microdyne was to use a 3dB pad between the filter and a mismatch source or load.

 

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?



A common method at Microdyne was to use a 3dB pad between the filter and a mismatch source or load.

yes,

the key concept is that an ideal LC filter alone creates loss in the stop band and transition band by REFLECTING energy back to the input and thus CANNOT present broadband 50 Ohms. It has no other option. Ideal L C components 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.

m