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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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14th Feb 2017, 4:47 pm | #1 |
Diode
Join Date: Feb 2017
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 9
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450KHz Mechanical Filters
Good afternoon and thanks for letting me join your forum.
I am currently building a homebrew dual-conversion SSB transceiver and have managed to get some new old stock mechanical filters from Hungary, sealed boxes but have discontinued for many years. I have the full spec for these filters however I have a question about how to correctly terminate the filters as I have never worked with mechanical filters before so please excuse my ignorance. Having measured the input and output pins they are reading about 15 ohms on a multi-meter however there is no connection to ground on either of these pins so my guess is that they have an internal transformer which is isolated from ground (or at least the metal case). So my question how would I go about feeding and terminating the filters, in the past I have used a simple L-Match or toroid transformer for matching crystal filters however they had a clearly defined ground connection whereas these mechanical filters do not. Here is the link to the filters in question: http://www.ebay.co.uk/itm/1919018588...%3AMEBIDX%3AIT Any help and guidance will be greatly appreciated. Best Regards Rob. |
14th Feb 2017, 5:42 pm | #2 |
Guest
Posts: n/a
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Re: 450KHz Mechanical Filters
Reading the specs. on the listing, they need 2k7 input and output resistance/load (sounds about right to me). The reason there is no ground connexion is that the input and output go directly to the transducers (coil type in this case I assume from the low resistance)) grounding either pin wouldn't make any difference. As these are at a low frequency I would suggest using modern op-amps for both feed and output. then you can define the impedance accurately.
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14th Feb 2017, 6:29 pm | #3 |
Octode
Join Date: Feb 2009
Location: Bletchley, Buckinghamshire, UK.
Posts: 1,223
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Re: 450KHz Mechanical Filters
From the spec, it looks like they need 950pF capacitors to resonate the coils.
Also, the input and output circuits probably need to be capacitively coupled to keep DC off them. |
14th Feb 2017, 7:08 pm | #4 |
Diode
Join Date: Feb 2017
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 9
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Re: 450KHz Mechanical Filters
Thanks for the replies, I will hook up a test circuit to my analyser and have a play around to see what gives me the best match result, I can use trimmer capacitors to set the optimum resonance I guess.
Cheers Rob. |
16th Feb 2017, 12:21 am | #5 |
Heptode
Join Date: Nov 2009
Location: London, Ontario, Canada
Posts: 675
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Re: 450KHz Mechanical Filters
Hi Rob,
Some of the other data that did not make it to the eBay specs are: Insertion loss <= 10dB max coil DC current < 6mA max coil RF voltage < 6VRMS max voltage between coils and house < 120V case needs to be RF grounded The filter shapes are given in Fig. 2. ("2. abra", in diagram, e.t.c.) Filter parameters are measured as in Fig. 4. In case of serial tuning 100 Ohm resistance gives the optimal power transfer. The coils can be tuned out in the parallel mode as well, as in Fig. 5. The Cn tuning capacitors are 950pF. Good luck, Peter Last edited by orbanp1; 16th Feb 2017 at 12:38 am. |
16th Feb 2017, 9:53 am | #6 |
Diode
Join Date: Feb 2017
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 9
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Re: 450KHz Mechanical Filters
Thanks Peter you are a wizard - where on earth did you find that data, I spent best part of a week trawling the web?
I must have been asking Google the wrong questions, anyway this is very helpful and should get me started - hopefully I can return the favour in the near future. Cheers and best 73 Rob. |
16th Feb 2017, 6:07 pm | #7 |
Heptode
Join Date: Nov 2009
Location: London, Ontario, Canada
Posts: 675
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Re: 450KHz Mechanical Filters
Hi Rob,
The data is from a series of articles about using these (and other) Gamma mechanical filters, from a Hungarian radio amateur magazine. It is from the articles titled "The application of mechanical filters in SSB transceivers, part I and II; by J. Nemeth; Radiotechnika, Vol. XVI; April, 1966, pp. 127-129; May, 1966, pp. 169-171; (In Hungarian) Peter Last edited by orbanp1; 16th Feb 2017 at 6:13 pm. |
16th Feb 2017, 6:16 pm | #8 | |
Guest
Posts: n/a
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Re: 450KHz Mechanical Filters
Quote:
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