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Old 14th Feb 2017, 4:47 pm   #1
VHF_BUILDER
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Question 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.
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Old 14th Feb 2017, 5:42 pm   #2
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Default 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.
 
Old 14th Feb 2017, 6:29 pm   #3
m0cemdave
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Default 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.
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Old 14th Feb 2017, 7:08 pm   #4
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Default 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.
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Old 16th Feb 2017, 12:21 am   #5
orbanp1
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Default 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
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Last edited by orbanp1; 16th Feb 2017 at 12:38 am.
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Old 16th Feb 2017, 9:53 am   #6
VHF_BUILDER
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Default 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.
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Old 16th Feb 2017, 6:07 pm   #7
orbanp1
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Default 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.
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Old 16th Feb 2017, 6:16 pm   #8
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Default Re: 450KHz Mechanical Filters

Quote:
I must have been asking Google the wrong questions
on a par with the ultimate answer being 42! One of the best ways I have found for asking Google is a broad question and then look at the images resulting, easy to spot a circuit or graph.
 
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