Simple narrow  Filters

I required a simple narrow bandwidth filter without large stop-band attenuation to select a mixing product from an output of a  square law mixer , I wanted the selective circuit to be as simple and as inexpensive as possible . As my filter requirements where not that stringent with respect to ultimate stopband , 20 dB would be fine ,that would get my wanted signal well above the crud  where i could use it ,  but I did require a very  narrow bandwidth to select a wanted mixing product that appeared  at a slightly higher output level ~ 6 dB above the other products . The internet is a great place to start to look for  ideas . .
I investigated several filter techniques , the single phasing crystal filter where by you use some anti-phase feedback via a "neutralising" capacitor  to keep the bandwidth symmetrical and  one varies the load on the crystal element to effectively change the bandwidth of the pass-band >
there is lots of information on the net if one searches under phasing crystal filter  with examples and circuitry to achieve this .
I found a good twin crystal derivative on a La 8    website ,  it had great bandwidth performance and very good stop band attenuation but the loss was a little excess BUT that could be due to my poor filter input and output matching ??

I was wanting to extract a relatively weak mixing product from a square law mixing circuit as required in the technique for generating the Local oscillator for a Double sideband  suppressed carrier  demodulator circuit . The square-law mixing  products are abundant at the output of the square law circuit ,and one has to have a narrow filter to select the wanted product from the plethora of mixing products .
With communications audio bandwidth from 300 Hz to 3000 Hz,  one is starting with an advantage  , by the time you have passed the DSB through the squaring circuit, the squaring action places the generated  sideband harmonics plus and minus 600 Hz ( not 300) either side of the  wanted synthesised carrier , This relaxes the requirements of the filter bandwidth . I did a composite sweep build up of the output spectrum of the squaring mixer over 30 seconds and you can see the definite "notches" where the frequency tailored audio tapers away

   I did play around with ceramic filters /resonators at 1 Mhz but gave up as their low Q makes the simple  filter bandwidth far too wide . Quartz crystals are your only real choice for a simple and inexpensive filter with the desirable performance . I chose frequencies that where in common use by  the microprocessor world , these are inexpensive and span a wide range , The lower the frequency the better the filter performance  wrt bandwidth , I.843 MHz  was my frequency of choice .
I wanted to make the filter as simple as possible ,easily reproducible  without specialised test equipment .  I did do some experimentation where by in a simple class A rf amplifier . one replaces the emitter bypass capacitor with a crystal at the wanted frequency and use the phenomenon  of series resonance   produces very low impedance  and effectively lifts the gain of the amplifier at the crystal resonance . hence one sees a peak in amplifier gain at resonance . There is a proviso to this ,  you really need about 10 k emitter resistor for this technique to work well and provide a reasonable stopband attenuation figure along with a narrow  bandwidth . I found that a lot of + 12v amplifier configurations gave good filter performance 20-30 db stopband ,narrow bandwidth 200-400 Hz but an overall negative gain , it was only when the emitter resistor was reduced to the typical 470 ohms or less did gain become positive but stopband attenuation and bandwidth suffered , so I gave up on this technique .


There is a neat little cascade 4 transistor amplifier I found on Harry Lythall - SM0VPO   website it had 3 transistor in totem pole configuration ie the current flow was common to the first 3 active devices and each transistor had its emitter bypassed to earth via 1nf capacitors , This circuit has a lot of gain in a small package and I found that by using a crystal in place of  an emitter  bypass cap an  effective simple filter with gain could be  made.  I experimented with the crystal in the first bypass cap position , The second bypass cap position and twin crystals,   


                                                             Note the difference in the filter response  between  bypass 1 and bypass 2        ??   crystal  loading ?? 


bypass  1  and 2             with closely matched crystals

There is a handy trick one can do using an NE592  video amp  and Im sure it will work with other  IC's which use a feed back network to achieve  fixed  gain  or control . The uA733  and NE 592  frequency response can be tailored by the feedback network between the gain set  pins  ( see the NE592 Application notes for more information on this)
 I came across a  filter circuit on the 'net using a quartz crystal as being the feedback network  and thus controlling the gain of the chip with respect to frequency .  ,     this amplifier has good gain with frequency response and will be my choice of active filter circuit to select the frequency component I require  from the mixer output


            NE592            200 Hz  B.W  at  10 dB level  !!                                                                                typical  swept crystal response 



Suitable narrow response  and with 20 db gain to boot      ! perfect                     cicuit i used ..................    note centre tap of output tank  decoupled to earth

I will try the NE592  as a filter  , my only minor technical concern is that with this active filter,  the high level broadband  signals get access to the potentially corruptible solid-state electronics before the passive filtering of the crystal  can shave down the broadband crud the amplifier sees . this of course is a theoretical potential problem that I feel in my application wont really be a problem .  


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