LF Stability
David W. Borden
dborden@laser.net
Sun, 07 Feb 1999 23:51:22 +0000
Terry,
I think we should try some weak signal FSK (morse is ok). Maybe you read
my missive to Andre earlier. I think On Off Keying (OOK) is bad because while
it is off, your receiver is grabing lots of bad noise. FSK sounds much
better in
terms of narrowing down the receiver and FFT processing on the narrow
bandwidth.
Your WWVhop technique sounds great to insure we are listening (watching,
processing, whatever) in the right place. Remember when we drove the Hal
blue transceiver with Fred? I would like to drive my Kenwood with the Fox.
In
the end I will probably just use a Fox receiver......david,k8mmo
At 17:33 2/7/99 -0500, you wrote:
>In looking at the information going back and forth regarding frequency
>stability, I have a few comments. Let me know if I am all wet.
>
>1. If we are going to communicate over long distances, we need some
>method of determining what frequency to transmit, and therefore to
>receive on. The more accurate the two sides are on exact frequency, the
>narrower the receiver can look. This reduces noise, and can lead to
>additional "processing gain". The problem is how to coordinate the
>frequency between the two sides to an amount that is worthwhile.
>
>2. Frequency stability is also an issue. Both the transmitter and
>receiver WILL drift. They should either drift very. very slowly, or
>one must track the other. This drift can lead to one side drifting out
>of the communications channel of the other, resulting in loss of
>communications.
>
>3. Both of the above are important. It is not good enough that the
>receive LO is stable. Same with the transmitter. It must also be
>accurate, especially if a narrow-bandwidth receiver is used. The above
>is probably obvious, but should be kept in mind if we are to achieve
>success.
>
>4. Milli-hertz reception using FFTs may be a good way of receiving
>slowly modulated signals. The problem is that it takes a finite amount
>of time to collect the samples you will perform the FFT on. As we
>learned several years ago when we did the original Amrad FFT-based audio
>spectrum analyzer on my 56001 DSP board, the hertz-per-bin is directly
>related to the number of FFT points and sample rate. If a signal is
>sampled at 4000HZ, and a 1024-point FFT is run, it will take 1024/4000,
>or roughly 1/4 second to acquire enough samples to perform the FFT.
>Nyquist says your maximum frequency is 2000Hz. Since you only use the
>real results, the FFT final product will be 1024/2, or 512 bins. The
>combination of the above yields a resolution-per-bin of about 2000/512,
>or 4Hz (if I did my math right). Bottom line is you will get number of
>bins equal to half the number of FFT points. Also, you must acquire
>samples equal to the number of FFT points, before you can do the FFT.
>
>As an example, if we want to do a 1024-point FFT, with a resolution of
>0.1 Hz per bin, that means we need to collect roughly 1000 samples, as a
>sampling rate of 100Hz. It will take 10 seconds to collect these 1000
>samples at 100Hz. The numbers are not as clear using other FFT sample
>rates, but the idea is the same. Plently of processing time to do the
>FFT, but the signalling speed must be very slow, or you will end up with
>a bit symbol overlapping multiple FFT bins. It still takes the inverse
>of Hz/bin to get the number of samples to run the FFT (I think).
>
>5. I have been playing with building an LF receiver for the better part
>of a year now. One of the core pieces is that both LOs are based on
>Analog Devices AD9832 DDS oscillilators. To overcome both frequency
>accuracy and frequency stability, I propose to periodically move the
>receiver to a known reference signal (such as WWV with a small frequency
>offset), and perform a large FFT. Using the results of the FFT, actual
>receive frequency can be calculated very accurately (much better than
>aural or scope beating of frequencies). The receiver is now calibrated
>for a while, and can go back to the frequency of interest. Since both
>LOs are using the same master oscillator, and that oscillator error is
>now known, we can assume accurate frequency operation. If this is done
>periodically, drift is also accounted for. This approach does not rely
>on yet another oscillator (or more), to verify accuracy.
>
>6. I love the AD9832 DDS, and it's big brother the AD9850/51, and plan
>to use them for lots of fun stuff. It takes a 25MHz oscillator in, and
>divides that by a 32-bit binary number, for an output from below 1HZ to
>over 9MHz. The down-size is that a low-pass filter is required on the
>output, and the output impedance is 300ohm. I have a 5-pole butterworth
>filter and an AD817 op-amp buffering the output. The AD9832 is
>controlled by a 3-wire serial interface, which at the moment I drive
>from a parallel port. It can move to other frequencies instantly, based
>on how fast you can load the 32 bits (or maybe less). In addition,
>there are two frequency registers, and 4 phase registers that modify the
>output signal. These are controllable from software, or hardware pins.
>Can anyone say frequency hopping? How about FSK or QPSK, or?? Neat
>device. I am making a small PC board for the device, that incorporates
>the oscillator, 9832, and some biasing and bypassing parts. The end
>result is a 24-pin, .6 inch module that will plug into a 24-pin socket.
>
>Well, that some ideas and comments for now. Please comment back if I am
>in error, or to amplify on above. Terry, WB4JFI.
>