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 Post subject: Band Pass Filters
PostPosted: Jun Tue 30, 2020 1:11 pm 
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Joined: Aug Wed 31, 2011 1:38 am
Posts: 147
Hi,

There are these 10.7 MHz Band Pass filters sold on e Bay , made in China they have 2 cans and 2 crystals between the cans. they state that they are 7.5 KHz BW they state they are for AM. Does anyone know if they could be moved to a center frequency of 9 MHz ? I have a scope and a Farnell RF generator.

Thank you , Jim.


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 Post subject: Re: Band Pass Filters
PostPosted: Jun Tue 30, 2020 3:40 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
What's the frequency on the crystals? 10.7MHz?

I guess changing the crystals to 9MHz would work for you. The can transformers look like they are providing impedance match to and from 50ohm to the crystal ladder filter.


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 Post subject: Re: Band Pass Filters
PostPosted: Jun Tue 30, 2020 3:55 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
Try 9MHz HC-49u crystals.


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 Post subject: Re: Band Pass Filters
PostPosted: Jun Tue 30, 2020 7:52 pm 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 283
Motorola1 wrote:
Does anyone know if they could be moved to a center frequency of 9 MHz ? I have a scope and a Farnell RF generator.

I had originally thought that retuning the transformers by adding an extra external padding capacitor would be enough, but I now realize that the crystals woud also have to be changed.


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 Post subject: Re: Band Pass Filters
PostPosted: Jun Tue 30, 2020 10:56 pm 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 624
Location: Montreal, Quebec
I assume it's a ladder filter. But it might be a lattice filter, the two crystals on slightly different frequencies.

But if it's a ladder filter, not only do you need to change crystals (at which point you might consider other options), but some capacitors reflect both bandwidth and center frequency. So just changing crystals (and the coils if they don't cover the new frequency) may not give you the desired bandwidth.


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 Post subject: Re: Band Pass Filters
PostPosted: Jul Wed 01, 2020 12:00 am 
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Joined: Aug Wed 31, 2011 1:38 am
Posts: 147
Hi,

Thank you for the info so far. I saw these on e bay and thought about them but I am looking into other ways to build my project.
I am researching into building at first a receiver section for a 29 MHz Am Transceiver. I guess I could also use 10.7 MHz cans without crystals maybe with 2N5179 transistors then re-tune for 9 MHz. I would build a front-end filter for 29 MHz followed by a transistor or FET.

I thought maybe use a 15 MHz clock oscillator mixed with a 5.0 MHz VFO to produce 20 MHz then mix that with the front end to end up with 9 MHz. At first I may try using a AM detector and see how that works.

Any help on this is highly appreciated.

Thank you, Jim.


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 Post subject: Re: Band Pass Filters
PostPosted: Jul Wed 01, 2020 12:18 am 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 283
Motorola1 wrote:
I thought maybe use a 15 MHz clock oscillator mixed with a 5.0 MHz VFO to produce 20 MHz then mix that with the front end to end up with 9 MHz. At first I may try using a AM detector and see how that works.

Any help on this is highly appreciated.

I was curious why it is necessary to have a 9 MHz IF. I remember seeing a circuit somewhere for a transceiver with frequencies similar to those suggested by you.

There are likely a multitude of frequency combinations that could result in a more standard 10.7 MHz IF.

For example: You could use your 15 MHz clock mixed with a 3.3 MHz VFO to get 18.3 MHz.

Or a 12 MHz clock oscillator mixed with a 6.3 MHz VFO to get 18.3 MHz.

Mixing the 18.3 MHz with the incoming 29 MHz signal would yield an IF frequency of 10.7 MHz. and you could use your existing filter.


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 Post subject: Re: Band Pass Filters
PostPosted: Jul Wed 01, 2020 12:34 am 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 624
Location: Montreal, Quebec
There are a lot of FM bandwidth filters at 10.7MHz. Not just ceramic filters for FM broadcast, but three terminal filters (in cases like crystals) for narrow deviation. Kind of wide for AM, but maybe not at 10M with few stations.

In Communications Quarterly about thirty years ago there was an AM transceiver project (I think for 144MHz, but perhaps 50MHz) using 10.7MHz crystal filters meant for FM.


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 Post subject: Re: Band Pass Filters
PostPosted: Jul Wed 01, 2020 1:56 am 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 624
Location: Montreal, Quebec
I'm not sure aboutbandwidth, but one trick is to replace the bypass capacitor on the cathode or emitter of an amp!ifier stage with a ceramic resonator or crystal.

Cascading a few stages gives a better shape factor.

It's simple and all you need is some crystals at the IF frequency.


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 Post subject: Re: Band Pass Filters
PostPosted: Jul Sun 05, 2020 4:23 pm 
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Be sure that the bandpass filter you intend to use is rated at the voltage of your circuit. Tube radio circuits often have the bandpass filter in the plate circuit of the first mixer. If your filter isn’t rated at the proper voltage, you will need a DC blocking capacitor ahead of the filter.

Norm

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 Post subject: Re: Band Pass Filters
PostPosted: Jul Sun 05, 2020 4:45 pm 
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Location: Sanford Fla 32771 (USA)
Motorola1 wrote:
Hi,

There are these 10.7 MHz Band Pass filters sold on e Bay , made in China they have 2 cans and 2 crystals between the cans. they state that they are 7.5 KHz BW they state they are for AM. Does anyone know if they could be moved to a center frequency of 9 MHz ? I have a scope and a Farnell RF generator.

Thank you , Jim.

I am guessing here, but don't the crystal have to be offset selected to give the desired frequency bandwidth ? To give 7.5 Khz passband one would be low 3.75Khz, the other high 3.75Khz.

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 Post subject: Re: Band Pass Filters
PostPosted: Jul Sun 05, 2020 11:24 pm 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 624
Location: Montreal, Quebec
It depends on the circuit used d.

Lattice filters needed crystals a few KHz apart. The bandwidth didn't quite match that spread (I think I.remember that the bandwidth was a bit wider than the crystal spacing).

But ladder filters have become popular in recent years, I'm assuming the commercial board mentioned is a ladder filter. With those, the crystals are all on the same frequency, the more exact the better. Bandwidth is a combination of centre frequency, crystal specs, and !oading crystals.


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