[Elecraft] Up-conversion.

[Jack Smith]

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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