[sdiy] Can you call the low pass filter in a frequency shifter an antialiasing filter?

[Eric Brombaugh]

Sorry - Local Oscillator. It's an RF systems term that refers to the 
shift frequency that's applied to a mixer in a heterodyne system. For 
what it's worth, a Bode Frequency Shifter is basically an audio 
implementation of a quadrature mixer - something that's used throughout 
wireless communications, albeit at higher frequencies.

Eric

On 10/04/2010 01:53 PM, cheater cheater wrote:
> Hi Eric,
> sorry, what's an LO?
>
> Thanks,
> D.
>
> On Mon, Oct 4, 2010 at 18:38, Eric Brombaugh<ebrombaugh1 at cox.net>  wrote:
>> On 10/04/2010 02:12 AM, cheater cheater wrote:
>>>
>>> Hi,
>>> I have come across this question today and I don't know what the
>>> filter is called exactly, or if it can be specifically identified with
>>> a special name for it. Any ideas?
>>
>> There are a number of ways to implement a frequency shifter, but I'll assume
>> you're referring to the analog Bode-style shifter such as is described in
>> this Moog patent:
>>
>> http://www.pat2pdf.org/pat2pdf/foo.pl?number=3,800,088
>>
>> There are two types of LPF in this architecture, both used in the generation
>> of the local oscillator. At block 17 is a 20kHz LPF that is used primarily
>> to clean up the 20kHz osc and ensure that there are no higher harmonics. As
>> Tim noted, the filters at blocks 23 and 24 are image filters which are used
>> to remove unwanted mixing products. The +/-5kHz LO is created by mixing a
>> 15-25kHz oscillator with a 20kHz oscillator. This will produce products
>> simultaneously at +/-5kHz and 35-45kHz - the image filters remove the
>> unwanted tone a 35-45kHz, leaving the desired +/-5kHz tone.
>>
>> There are alternative ways to implement the Bode shifter that don't require
>> this however. Some modern shifters generate the LO directly using digital
>> techniques while still using the analog dome filter&  mixer approach. Other
>> shifters operate entirely in the digital domain, digitizing the input signal
>> and doing all the phase shifting and mixing numerically before outputting
>> thru a DAC. Neither of these approaches requires the image filter, but they
>> will require post-DAC reconstruction filters.
>>
>> Eric
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>