Mechanical filters for intermediate frequency (IF) applications - pictures - Page 1
Forum section: electronics / projects

author: mister_rf
author_rank: Contributor
timestamp: January 18, 2023, 09:22:13 am
content: MF200+E-0310/3 mechanical filters for intermediate frequency (IF) applications.Center frequency: 200KHz; Bandwidth: 3100Hz;Manufactured by RFT (RFT was not a manufacturer but a consortium of state-owned companies (VEB) in the field of radio and television in the former DDR (East Germany)).

author: Kleinstein
author_rank: Super Contributor
timestamp: January 18, 2023, 12:18:56 pm
content: The construction is odd, with so many elements. Nomally mechanical resonances are rather high Q (e.g. Q > 1000 is common an Q<100 more like an exception). So the extra elements may be there to make the bandpass wider, not for an even sharper response. So ideally this would be a filter with flat pass band and not a rounded top like a simple 2nd order filter.The way the elements a mounted and coupled to the input and output, it looks like they use a kind of torsional mode.Those mechanical elements also have quite a lot of resonances at higher frequencies. I would have expected at least 2 different shapes (e.g. larger diameter but shorter) to make it less likely that 2 of the higher resonances also fall on top of each other.

author: mawyatt
author_rank: Super Contributor
timestamp: January 18, 2023, 02:36:10 pm
content: Beautiful images, nice macro work, thanks!!Do you know details about the cylinders, how is the electrical to mechanical vibration created, and visa-versa?Likely these are Synchronously Tuned Elements, which was common back then for IF use.Rockwell-Collins made mechanical IF filters long ago, would be interesting to "see" what these look like inside if anyone has access to such.Best,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 18, 2023, 03:26:20 pm
content: Looks like there's several zeroes in there (bond wires hovering over several positions), which in part explains the extra elements. Should be nice and sharp, at least close to the passband.Not familiar with mechanical filters; is the resonant mode a property of the cylinder, or the cylinder-bondwire system (lumped equivalent)? Feels like 200kHz should be high enough for some radial mode at that size. Not sure how the bondwires work out though, are they resonant lengths as well?Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: January 18, 2023, 04:36:23 pm
content: I would expect 200 kHz to be one of the lowest resonances. This is from faint memory of using similar size and shape cylinders for resonant ultrasound - this is measuring the electic constants from the set of resonant frequencies and dimenions. The set of frequenies can be used to get even more complicated sets with some anisotropy (e.g. 3 or 4 independent constants - more gets tricky but is possibly).The resonance would be inside each cylinder and the bond wires for a relatively weak coupling between them. The coupling could also widen the resonances by adding damping.Resoances with the cylinders as mass and the wire as a kind of spring would be way lower ( < 1 kHz and way lower for the torsional case).These cylinders have quite a lot of resonances of different shapes and frequency. A few at low frequency and than increasingly denser spaced higher up. From the way the excitation and detection is connected I would guess a kind of torsional mode, twisting the ends of the cylinder relaive to each other. The position could minimize the couling for some of the unwanted higher modes, by placing them in zeros of these modes (e.g. at about 1/4 and 3/4 of the length for the next torsional mode).For the lower frequencies there are only few (e.g. the center for the lowest torsional mode) zeros (points that don't move).The response curve could be interesting and would likely show some leakage at other resonances. One may have to combine such a filter with a simple electical one for those.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 18, 2023, 05:28:43 pm
content: Yes, in general you want to use resonators (like this, or quartz, ceramic, etc.) with electrical filters to catch the spurs. You'll still have some close-in peaks, but usually -60dB or lower, giving good adjacent station or image rejection.Tim

author: mansaxel
author_rank: Super Contributor
timestamp: January 19, 2023, 03:25:33 pm
content: The Swedish subsidiary of ITT, Standard Radio & Telefon, built a couple nice SW receivers using mechanical filters from one of the RFT-Werke. Hopefully, that was the closest someone or something from the DDR got to get into the Very Special locations those were used, as the military and the SIGINT agency were one of the major users.

author: mawyatt
author_rank: Super Contributor
timestamp: January 19, 2023, 04:31:41 pm
content: Digital Radio Frequency Memory, or DRFM has been an important component in Electronic Warfare for some time, so suspect these mechanical filters were employed prior to the ability to use digital memory for such.Steep shape factor filters with high dynamic range are also an important ingredient in "finding" tiny signals hidden from normal view in the skirts of much larger signals, kind of hiding in plain sight so to speak.Best,

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 19, 2023, 04:53:59 pm
content: Collins mechanical filters have been the most common ones here. Great macro shots.

author: mister_rf
author_rank: Contributor
timestamp: February 01, 2023, 01:11:23 pm
content: ФЭМ2-018-500-3В-1 (FEM2-018-500-3V-1);Mechanical frequency filterCenter frequency: 500KHz;Bandwidth: 3000Hz;Manufactured in the former Soviet Union, Alchevsk plant "KRATON", 1983.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 01, 2023, 01:33:11 pm
content: Beautiful!