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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: Filter problem
Date: Thu, 12 Jun 2025 19:29:56 -0700
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On Thu, 12 Jun 2025 17:02:59 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

>john larkin <jl@glen--canyon.com> wrote:
>
>> 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?
>
>Milliwatts at high impedance.
>
>
>> 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=v3lr01
>> 34wjl5dliqdh2aowlpd&raw=1
>> 
>> Just scale that up 100,000:1
>
>That's only one section, I am going to need a lot more than that because
>the wanted and unwanted frequencies are only 5.3% apart.
>
>
>> The arithmetic is tedious.  My NORMA program might help.
>
>I have found some calculations in the RSGB Handbook but my big worry at
>these frequencies is unwanted 'components' in the form of wires (or box
>sides) with inductance and stray capacitance everywhere.  If I make a
>simple tuned circuit with an air-cored coil (or helix) across the width
>fo a small die-cast box, the return currents from the grounding of the
>capacitor at one end will flow back through the box wall to the
>grounding point of the coil at the other end..  These currents will
>spread out and may interact with a similar circuit layout at the other
>end of the box to give unwanted coupling.
>
>I had this happen many years ago with something that worked at 100 Kc/s,
>so the problem could be much worse at 150 Mc/s.  (It also caught out the
>designers of the RA17, hence the hacksaw slot most of the way through
>the chassis.)
>

Heck, 150 MHz is almost DC.

>
>> 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?r
>> lkey=vijxk9kcqw0ve5iplw7p9h390&raw=1
>
>I am avoiding the use of printed circuits, it is all being built on
>tagstrips and standoff pillars - and a lot of the circuit can be
>supported off the valveholder tags (but not the filters).

Retro look.