[sdiy] Can you call the low pass filter in a frequency shifter an antialiasing filter?
cheater00 at gmail.com
Mon Oct 4 22:53:00 CEST 2010
sorry, what's an LO?
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:
>> 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:
> 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.
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