Reply by Nicholas Kinar●October 12, 20062006-10-12
Thank you ever so much for this reply, Tom; it is very much appreciated.
Concerning the DAC, I will incorporate one into my design. I had initially
thought of using a VCO for this purpose, but it is easier to know the
frequency at a certain time if I use a DAC. I will also think of increasing
the sweep rate.
Once again, Tom, thank you for replying to my post. Your assistance is
greatly appreciated, and I thank you for your suggestions.
Nicholas
"Tom" wrote:
Nicholas
>
>> However, by digital signal processing, is it possible to determine the
>> distance to the reflector without homodyning the two signals? (This is
>> for
>> lower frequencies of electromagnetic waves, whereby the received wave can
>> be
>> converted by an ADC, and then subsequently homodyned by digital signal
>> processing.)
>
> Yes, providing you accurately know the frequency of the transmitted
> signal over the sweep time (easy if you are using a DAC, harder if you
> are controlling a free-running VCO with a control voltage).
>
> The disadvantage is that the receiver bandwidth must equal the
> transmitter bandwidth. In comparison, for normal FMCW, at least when
> you are considering fairly short range operation, the beat frequencies
> are small, so the receiver bandwidth can be small.
>
> Note high bandwidth ADCs tend to have low bit depth (ie low dynamic
> range), which may be a problem depending on the maximum range, clutter,
> target RCS, etc.
>
>> Is it possible to determine the distance to the reflector without being
>> dependent on the duration T of the frequency-swept wave?
>
> I think you will find that the range resolution (accuracy) is dependent
> on the swept bandwidth B during time the transmitter "on time" T.
> Therefore you could reduce T by increasing the sweep rate B/T.
> Note however that T must be long enough to deal with your maximum
> range, and also that a very short T makes the duration of each received
> signal (or beat signal) short, which reduces the accuracy to which you
> can determine its frequency - whether this is a problem depends on your
> system parameters.
>
> Tom
>
Reply by Tom●October 12, 20062006-10-12
Nicholas
> However, by digital signal processing, is it possible to determine the
> distance to the reflector without homodyning the two signals? (This is for
> lower frequencies of electromagnetic waves, whereby the received wave can be
> converted by an ADC, and then subsequently homodyned by digital signal
> processing.)
Yes, providing you accurately know the frequency of the transmitted
signal over the sweep time (easy if you are using a DAC, harder if you
are controlling a free-running VCO with a control voltage).
The disadvantage is that the receiver bandwidth must equal the
transmitter bandwidth. In comparison, for normal FMCW, at least when
you are considering fairly short range operation, the beat frequencies
are small, so the receiver bandwidth can be small.
Note high bandwidth ADCs tend to have low bit depth (ie low dynamic
range), which may be a problem depending on the maximum range, clutter,
target RCS, etc.
> Is it possible to determine the distance to the reflector without being
> dependent on the duration T of the frequency-swept wave?
I think you will find that the range resolution (accuracy) is dependent
on the swept bandwidth B during time the transmitter "on time" T.
Therefore you could reduce T by increasing the sweep rate B/T.
Note however that T must be long enough to deal with your maximum
range, and also that a very short T makes the duration of each received
signal (or beat signal) short, which reduces the accuracy to which you
can determine its frequency - whether this is a problem depends on your
system parameters.
Tom
Reply by Nicholas Kinar●October 11, 20062006-10-11
I am working on an application which involves a Frequency-Modulated
Continuous Wave (FMCW) system. I understand that the frequency-swept wave
is sent out from an antenna toward a reflector, and the received reflection
is then homodyned (mixed) with the original wave. (I understand that this
mixing is a form of multiplying both signals in the time domain.) This
produces a beat frequency f_b.
However, by digital signal processing, is it possible to determine the
distance to the reflector without homodyning the two signals? (This is for
lower frequencies of electromagnetic waves, whereby the received wave can be
converted by an ADC, and then subsequently homodyned by digital signal
processing.)
Is it possible to determine the distance to the reflector without being
dependent on the duration T of the frequency-swept wave?
Thank you for your time and assistance!