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

[Eric Brombaugh]

It's a pretty general topic in communications. A quick google turns up 
this page:

http://www.avren.com/Courses/TX_RX_Architectures_plain.htm

that has some explanations. In particular look at the topics of

* Complex modulation
* Image Reject Mixer
* Vector Modulator
* direct conversion

All discuss aspects of the fundamental operation of using multiplication 
by a complex exponential to shift frequency while suppressing unwanted 
images.

Eric

On 10/04/2010 11:02 PM, cheater cheater wrote:
> Thanks!
> So is there an easy explanation of what a quadrature mixer is? I'm
> happy for heavy maths, but heavy electronics scare me a bit ...
> I'm afraid the only things I can find arr involved radio-world descriptions.
>
> Cheers,
> D.
>
> On Tue, Oct 5, 2010 at 00:10, Eric Brombaugh<ebrombaugh1 at cox.net>  wrote:
>> 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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>>>
>>
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