Re: Up-conversion.

Posted by Jack Smith-6 on Aug 12, 2006; 1:02pm
URL: http://elecraft.85.s1.nabble.com/Up-conversion-tp392867p392907.html

Yes, exactly. The minimum required Q scales proportionally to the
fractional bandwidth. Hence at a given frequency the required 400 Hz
filter demands resonator Q's 12.5 times higher than for a 5 KHz filter,
assuming a bunch of other factors remain constant. They don't, but this
is a good enough approximation for our purpose of a back-of-the-envelope
discussion.

Hence, the resonator Q requirements for a 5 KHz filter at 40 MHz are not
grossly different than for a 500 Hz filter at 4 MHz, which is quite
achievable. But for a 400 Hz filter at 45 MHz, the required resonator
Q's get into the million range.

I have in my junkbox a 20 KHz wide crystal filter with a center
frequency of 157 MHz used as a front end filter to improve VHF FM
receiver interference rejection from nearby paging transmitters. (Nearby
in both the geographical and frequency senses.)

Jack



Geoffrey Mackenzie-Kennedy wrote:

>
>> There are some many practical problems with the holder capacitance,
>> stray capacitance and the like that would make such a filter
>> challenging, even if someone were to deliver a box of 45 MHz crystals
>> with measured Qs of 2 million to my doorstep. And if  the box of
>> crystals arrive, to obtain frequency stability might require
>> stabilizing the filter assembly in a temperature controlled oven.
>>
>> The typical roofing filters at 45 MHz have a bandwidth of 20 KHz or
>> so. Thus the fractional bandwidth is 50 times larger and the Qx is
>> down into the 100K range, making these filters relatively easy to
>> realize.
>
> --------------------------------------------------------------------------
>
>
> An interesting 42.5 MHz filter design appeared several years ago using
> just four crystals in ladder configuration. The designer was not
> looking for narrow bandwidth nor good shape factor, but illustrating a
> "method". The bandwidth was 5 kHz, the shape factor was 4, stopband
> better than 80db and the response was symmetrical. In addition to the
> usual inductors across each crystal to take care of holder
> capacitance, as inspired by Zobel, two other other modifications were
> incorporated. (1) Parallel tuned circuits were placed at each end to
> prevent degeneration into a poor Cauer lowpass filter. (2) Parallel
> tuned circuits were used as coupling elements, presumably to tweak
> mesh frequencies. Insertion loss unknown, but a fair number of
> elements to compensate for temperature change.
>
> 73,
> Geoff
> GM4ESD
>
>
>
>
>
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