Re: Filter problem

On Thu, 12 Jun 2025 10:06:26 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

I am using a common-cathode double triode as a balanced mixer to
generate a frequency in the range 144 to 146 Mc/s fo a 2-metre
transmitter.
>
The inputs are a 150 Mc/s signal from a crystal oscillator and 6 to 4
Mc/s from a VFO.  (The crystal oscillator is modulated with narrow-band
FM by injecting a 90-degrees phase-shifted current from a reactance
valve.)
>
Both inputs are applied to the two grids in push-pull with the anodes
connected in parallel.  The150 Mc/s input can be accurately balanced so
that very little 150 Mc/s signal appears in the output.  The VFO signal
frequency is so far removed from the output frequency that accurate
balancing isn't needed.
>
Coming out of the anodes we have:
144 to 146 Mc/s wanted signal
150 Mc/s unwanted but at a low level
156 to 154 Mc/s unwanted, at the same level as the wanted signal.
>
I need to select for the 144-146 signal and reduce the 156-154 signal by
about 60dB.  Some of this selection will take place in subsequent tuned
stages but it really needs a filter to reduce the unwanted signals
sufficiently.  Some reduction at 150 Mc/s would also be desirable.
>
There are four possibilities which I have considered so far:
>
1)  Use a sharply-tuned circuit to select a single wanted frequency and
re-tune it every time the VFO is altered.  This means an extra operating
burden unless the two controls can be ganged, which is going to be a lot
of trouble to get right.
>
2)  Use a band-pass filter to select 144 - 146 Mc/s.
>
3)  Use a band-stop filter to remove 156 - 154 Mc/s (with possibly a
sharp rejector circuit to attenuate the residual 150 Mc/s).
>
4)  Use a low-pass filter, the 'skirts' of  which may also reduce the
residual 150 Mc/s sufficiently .
>
The second question concerns the physical form of the filter.  It could
be a ladder network  of coils and trimming capacitors in a die-cast box
or it could be made up of resonant lines or lengths of co-ax.  I don't
know of a resonant-line low-pass filter but someone might have come
across one.  There might be room in the enclosure for loosely coiled-up
co-axial cable resonators but trough-lines might be a bit too long
unless they are heavily capacitively loaded.
>
I have some ferrite toroids that could be used to match the valve output
impedance to the filter characteristic impedance.
>
>
Does anyone with experience of filter design have any recommendtions
that don't involve custom-made components or semiconductors?

What's the power level?

You might make a bandpass filter out of commercial inductors and caps
and a few padders or trimmer caps. Build it on dremel'ed FR4 and bolt
it into the box for good grounding. One narrow deep movable notch
might help a lot.

https://www.dropbox.com/scl/fi/d7x21xc720gq0ztl8am5e/Z452.JPG?rlkey=v3lr0134wjl5dliqdh2aowlpd&raw=1

Just scale that up 100,000:1

Or do one of those SSB phasing tricks to kill the image sideband. Yes,
I know it's FM.

Wiliams has the stuff you need to design a passive bandpass filter.
The arithmetic is tedious.  My NORMA program might help.

https://www.dropbox.com/scl/fi/ouuia53uajl99t1nic31u/Norma.jpg?rlkey=g75y8p4glz2ewbepk1l3h8jkg&raw=1

If you are running low power, you could build a bp filter on one of
these:

https://www.dropbox.com/scl/fi/pym7yn95rqopxlkt7x6wk/Z368_BP_Filters.jpg?rlkey=vijxk9kcqw0ve5iplw7p9h390&raw=1

I should still have a few around.