Jean-Daniel HB9AKQ ) writes:
On 22 mar, 16:50, (Michael Black) wrote:
Jean-Daniel HB9AKQ ) writes:
Hello,

I am in the process of choosing a technology for a crystal filter in
that frequency range. I am facing the choice of buying a filter and
paying the price, or to build a filter myself.

Are you hoping/expecting to get a narrow filter out of this, or will
it be a roofing filter, with the real selectivity at a different frequency=
?

I thought there were limitations on selectivity at those frequencies.
I can't recall ever seeing a narrow filter up there (and while it's been a=

long time since I read that QST article, which is the only article in
the ham magazines that I've seen about making crystal filters that high
up, I thought it addressed that issue).

If it's just a roofing filter you need, you can pull one out of an older
cellphone. =A0The big clunky kind have parts that are recognizable.

=A0 =A0Michael =A0VE2BVW



Hello, I need a roofing filter with a few kHz wide that can withstand
1dBm or a little more.
I am not sure that the MQF you find on the market or in older
cellphones can do that. The frequency should be between 45 and 80 MHz
and the price should be right. What is your experience on this?

But then it's not really a roofing filter.

That term applies to a filter used after the first mixer to limit
the passband but which doesn't provide ultimate selectivity.

My point was that I wonder if you can get a few KHz of selectivity
up there. Otherwise, we'd surely see it in commercial equipment
and even home made. All the design books decades ago would say the
best design is to put the ultimate selectivity right after the first
mixer of a receiver, yet as receiver design changed to put the first
IF in the 45 to 70MHz range, I've never seen an instance of a narrow
filter right after the first mixer.

I don't know whether it can be done, but the fact that it isn't done
suggests there are limitations.

The filters in a cellphone would be wide enough for FM, and given that
all the cellphones I've seen convert to a lower frequency where there
is another filter, they probably are relatively loose filters up at
45MHz or so.

MIchael VE2BVW