this. AND do that with signals of the extremes separated by a mere 250
Hz. We are so picky. Why does anyone put up with us :>)
> Only to a first approximation are filters linear.
>
> Usually the major source of non-linearity is in the inductors. Even
> powdered iron is non-linear to some degree. Ferrite is worse, of course.
> Air is pretty close to perfect.
>
> Capacitors are also non-linear at some level, as C is not constant with
> applied voltage. NP0/C0G caps are very good, but again not perfect.
>
> Filter generated intermodulation problems are real and measurable. Been
> there and done that and have the Tee-shirt.
>
> Jack K8ZOA
>
>
> On 4/15/2011 1:01 PM, Alan Bloom wrote:
>> Hi Geoff,
>>
>> I guess I don't understand. Filters are linear devices. How can they
>> cause IMD? If the signal generator's harmonics are removed by the
>> receiver filters then it is the same as if the sig gen were "clean", no?
>>
>> 73,
>>
>> Alan N1AL
>>
>>
>> On Fri, 2011-04-15 at 11:54 +0100, Geoffrey Mackenzie-Kennedy wrote:
>>> Hi Alan,
>>>
>>> Indeed, most receivers do have some form of filter "up front" in the front
>>> end, and therein lies one source of IMD measurement error if harmonics are
>>> present in the applied test signals.
>>>
>>> While the MiniCircuits application note AN-00-008 which Jack mentioned
>>> points out that harmonics present in the applied test signals are a
>>> potential source of error when measuring the IMD performance of a device
>>> such as a mixer, for the same reasons error can be introduced by a
>>> receiver's input filter(s). So it is essential that "clean" sources are used
>>> when testing a receiver.
>>>
>>> 73,
>>> Geoff
>>> GM4ESD
>>>
>>>
>>> On April 15, 2011 at 04:26 +0100, Alan Bloom wrote:
>>>
>>>> Hi Geoff,
>>>>
>>>> Most receivers have bandpass and/or low-pass filters in the front end
>>>> that would filter out the harmonics of a square-wave test signal. I
>>>> could dream up some unusual situations where the harmonics could cause a
>>>> problem, but normally it would not be an issue.
>>>>
>>>> Alan N1AL
>>>>
>>>>
>>>> On Fri, 2011-04-15 at 00:37 +0100, Geoffrey Mackenzie-Kennedy wrote:
>>>>> Hi Al,
>>>>>
>>>>> You did say "normally not an issue", but when making even or odd order
>>>>> intermodulation measurements on a receiver the use of RF sources whose
>>>>> harmonic content is significant can and usually will lead to invalid
>>>>> results.
>>>>>
>>>>> 73,
>>>>> Geoff
>>>>> GM4ESD
>>>>>
>>>>>
>>>>> On April 14, 2011 at 22:58 +0100, Alan Bloom wrote:
>>>>>
>>>>>> Hi Jack,
>>>>>>
>>>>>> You're right that when testing a wide-band device like an amplifier you
>>>>>> have to filter out the generator harmonics in order to get a valid
>>>>>> third-order IMD test. But when testing a narrow-band device like a
>>>>>> receiver the harmonics are normally not an issue.
>>>>>>
>>>>>> Those Coilcraft low-pass filters look very nice, especially for he
>>>>>> price. Where can you buy them?
>>>>>>
>>>>>> Alan N1AL
>>>
>>>
>>
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