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

[Eric Brombaugh]

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