Tom Rauch wrote:
>>>IM products are not symmetrically dispersed Gary. Where they are USB or
>>
> LSB
>
>>>depends on relative level (and I would guess phase, I need to think
>>
> about
>
>>>that) of the original frequencies.
>>>
>>>73 Tom
>>>
>>>
>>>
>>
>>Hi Tom,
>>
>>If IM products are generated at RF (after filters) I don't think that
>>there is a difference. Each signal should modulate the other equally.
>
>
> There sure is a difference! I have to be very careful doing two-tone
> receiver tests to make sure the levels are equal, otherwise one 3rd order
> product will be much stronger than the other.
>
> The stronger signal generally becomes the local oscillator and is more
> limited (gain compression), while the other is a linear product of the
> mixing. So 2f1-f2 is not the same level as 2f2-f1 when either F2 or F1 is
> stronger.
>
> The (input power to output power) transfer functions of an amplifier or any
> less than perfectly linear stage are complex, and always vary with level.
> I'm sure there are other reasons also.
>
> 73 Tom
>
>
Hi Tom,
I think you are right about the levels of upper and lower products
having the capability of being non-symmetric. I remember reading in
Termans handbook about mixers that says that the "stronger signal acts
as the local oscillator". It said that the local oscillator had to be a
lot stronger in level to be dominant. Otherwise either signal would act
as the local oscillator. I don't know how much difference is required.
It has been some time since I have done any 2 tone testing. I have
usually used a double side band signal with a single tone to generate
the two tone signal. That way the two were always equal in strength no
matter what level it was set at. An added benefit was that the frequency
spacing of the two tones was twice the frequency of the tone used.
Easier to get wide spaced tones which made it easier to see on the
analyzer.
Some notes, I am sure that you are aware of but may be of interest to
others:
When we speak of upper and lower IM products we are talking about the
two combinations of difference frequencies and not the sum and difference.
Each tone takes its turn acting as the local oscillator in a mixer. Of
the products generated there are sum and difference frequencies. The sum
frequencies fall well outside the amplifier tuned circuits and are
attenuated. Only the difference frequencies fall near the wanted signal.
Example: for 3rd order products.
2a-b
2b-a
where
a = 4000
b = 4001
2x4000 = 8000 + 4001 = 12001. The tank gets rid of.
2x4000 = 8000 - 4001 = 3999. We call this the lower side IM product.
2x4001 = 8002 + 4000 = 12002. The tank gets rid of.
2x4001 = 8002 - 4000 = 4002. We call this the upper side IM product.
SPECS:
IM products are often "specked" as being so many db down from PEP of an
amp / transmitter. That is buying false dbs! It can make the spec look
much better than it really is. The proper way to spec it is how many db
down from one of the two tones used for the test. Specing it against PEP
it looks 6 db better than it is from one of the two tones. (each tone of
a 2 tone signal is 6 db below PEP)
The other problem that comes into play with specing it against PEP is
that any distortion products are added in and become part of the PEP
power. PEP does not add linearly. It adds as the square of the product.
So a little bit of distortion adds a lot to the PEP reading. This makes
the IM products to PEP ratio look even greater. The greater the IM
products are the greater the error in true ratio. It can be a several db
error. Comparing IM product levels to one of the two tones eliminates
most of those errors.
As an example, when using a 2 tone test the power in each tone adds
together for the total average power. 10 watts + 10 watts = 20 watts
average power. The PEP power of that signal would be 40 watts.
With a 3 tone signal, each having 10 watts, the average power would be
30 watts but the PEP would be 90 watts.
PEP in this case is 3 times average power rather than 2 times as it is
with two tones.
The more signals present the greater the multiplier for PEP. Four 10
watt tones would have an average power of 40 watts and a PEP of 160
watts. 4 times multiplier.
The point is that as the IM products increase on a signal, that in
effect increases the number of tones. PEP increases at a faster rate
than average power increases and This increases the PEP to IM ratio.
(PEP contains distortion product power)
The power in the IM products is going to be lower than the original tone
signals so things won't work out at exactly the ratios given above,
although they could be that high on a badly distorted amp.
Note that the multipliers given above for PEP are approximate because of
the addition of the distortion products to PEP. The PEP of two tones is
not exactly 2 times because of the distortion products.
From the above it can be noted that the more restricted the bandwidth
of a signal is the higher the average to PEP ratio will be. A very wide
audio bandwidth will give a much lower average power level with a given
PEP level than a restricted bandwidth. That is one of the reasons why
using restricted audio increases the "talk power" of a signal.
Average power is what counts. PEP is a necessary evil.
Reduce the bandwidth to zero (cw signal) and the PEP and average are the
same.
73
Gary K4FMX
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