Reply by Vladimir Vassilevsky●July 19, 20072007-07-19
Steve Underwood wrote:
>>> I am using a DDS to generate
>>> phase-continuous, 4-level FSK. I'm then "multiplying" the output of
>>> the DDS (using a PLL/VCO module) by 11, creating an RF carrier. The
>>> output of the VCO is the RF carrier (~929MHz).
>>
>> I did a very similar design for the professional paging transmitter
>> somewhat 14 years ago. This is a good approach although you should be
>> carefull about all kinds of noise and spurs from PLL and DDS.
>
>
> We did this about 16 years ago for an Ermes type signal, but at 928MHz -
> Ermes never took off, so the transmitter never went into production.
Our primary goals were POCSAG and FLEX standards. From the RF
prospective, ERMES and FLEX are very similar. We built about 100
transmitters. At that time it became apparent that paging as a whole
goes to the decline.
> With the cheaper DDS chips at that time, and the available DACs, we
> could only generate our digitally crafted signal at a few MHz.
Likewise. However the main limitation for me was the PLL chip. It was
very problematic to use the phase comparator at higher frequencies.
Also, with the higher DDS clock the frequency resolution is lower; the
32bit phase accumulator is not very big at all.
> By the
> time we multiplied that to 928MHz the noise was troublesome. We ended up
> with a mixed approach, which gave clean results more easily. These days
> the DDS can be much faster, and I guess multplying by only 11 will work
> well.
It was possible to get a clean signal by the direct multiplication by
PLL even at that time. However it took a lot of work on optimization of
everything.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by Tim Wescott●July 18, 20072007-07-18
Jon Mcleod wrote:
> Vladimir Vassilevsky wrote:
>
>>> Doing this, I can actually create a carrier that meets the
>>> transmission mask, but the modulation doesn't really work. I'm using
>>> the PLL loop filter to replace the premodulation filter that might
>>> have been in a commercial paging transmitter 15 years ago. My
>>> problem is that the PLL is either unstable, or the loop filter
>>> flattens out the symbols. I can't seem to get it right.
>>
>> This is not right. The PLL filter should be relatively wide; that
>> reduces the noise of VCO. The main filtering should be done in the
>> digital domain prior to the DDS modulation. The PLL filer should have
>> just enough of attenuation to kill the digital aliases and spurs.
>
> I see. The uC talking to the DDS should interpolate and then filter the
> base band "square wave," and then pass the result to the DDS as a string
> of frequency words. I don't think the uC I was using would have had
> nearly enough horsepower to do this .. I'd need a real DSP. Actually,
> the DDS has a "linear sweep" function that maybe could have helped,
> sweeping continuously between the 4 levels? I think there was also a
> version of this part with RAM inside of it, so I could have
> precalculated the waveforms and loaded them in.
>
>>> For premodulation, the protocol specifies a "10th order bessel with
>>> fc=3.9KHz".
>>
>> Bessel-10 is the heritage of the old time analog transmitters.
>> Actually, any linear phase lowpass filter with the 60dB/oct rolloff at
>> 3.9kHz will do the job.
>
> This would have been a "FIR" filter?
>
>>> Generally speaking, is there any analytical way to "convert" a
>>> baseband premodulation filter into the correct loop filter for my setup?
>>
>> Yes, you can do it that way however it would not be the optimal
>> solution for the spectral purity of the transmitter.
>>
>>> [At this point, the project is basically moot. A guy here found an
>>> old paging transmitter somewhere that still works. But I would like
>>> to see my setup work before I scrap it. I'd like to walk away from
>>> this learning SOMETHING. Any thoughts are appreciated].
>>
>> Well, now you know that it is possible to make it work :)
>
> Yes, but I think I did have one very weak link -- my uC was not powerful
> enough to do the DSP required!
>
> Thanks for the answers!
Depending on what you need it for you may be able to just use the square
frequency output -- the pagers will probably still be able to decode the
result, but your transmitted bandwidth will be wider than spec. As long
as you're doing it on a bench you'll be OK. Your linear phase change
idea may also be close enough, and the sidebands would be lower.
I'd be tempted to generate my signal then run it through a series of
doublers and/or triplers to multiply by 9 or 12. No, this isn't as cool
as a PLL, but I could probably design the circuit in my head and
assemble it dead-bug on a ground plane, all without recourse to a pad
and pencil, much less a computer, for the design.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by Jon Mcleod●July 18, 20072007-07-18
Vladimir Vassilevsky wrote:
>> Doing this, I can actually create a carrier that meets the
>> transmission mask, but the modulation doesn't really work. I'm using
>> the PLL loop filter to replace the premodulation filter that might
>> have been in a commercial paging transmitter 15 years ago. My problem
>> is that the PLL is either unstable, or the loop filter flattens out
>> the symbols. I can't seem to get it right.
>
> This is not right. The PLL filter should be relatively wide; that
> reduces the noise of VCO. The main filtering should be done in the
> digital domain prior to the DDS modulation. The PLL filer should have
> just enough of attenuation to kill the digital aliases and spurs.
I see. The uC talking to the DDS should interpolate and then filter the
base band "square wave," and then pass the result to the DDS as a string
of frequency words. I don't think the uC I was using would have had
nearly enough horsepower to do this .. I'd need a real DSP. Actually,
the DDS has a "linear sweep" function that maybe could have helped,
sweeping continuously between the 4 levels? I think there was also a
version of this part with RAM inside of it, so I could have
precalculated the waveforms and loaded them in.
>> For premodulation, the protocol specifies a "10th order bessel with
>> fc=3.9KHz".
>
> Bessel-10 is the heritage of the old time analog transmitters. Actually,
> any linear phase lowpass filter with the 60dB/oct rolloff at 3.9kHz will
> do the job.
This would have been a "FIR" filter?
>> Generally speaking, is there any analytical way to "convert" a
>> baseband premodulation filter into the correct loop filter for my setup?
>
> Yes, you can do it that way however it would not be the optimal solution
> for the spectral purity of the transmitter.
>
>> [At this point, the project is basically moot. A guy here found an
>> old paging transmitter somewhere that still works. But I would like
>> to see my setup work before I scrap it. I'd like to walk away from
>> this learning SOMETHING. Any thoughts are appreciated].
>
> Well, now you know that it is possible to make it work :)
Yes, but I think I did have one very weak link -- my uC was not powerful
enough to do the DSP required!
Thanks for the answers!
Reply by Steve Underwood●July 18, 20072007-07-18
Vladimir Vassilevsky wrote:
>
>
> Jon Mcleod wrote:
>
>> Mayby not the right group, but..
>
> Why not? Here you can find the best specialists. They are very nice
> people, too.
>
>> On my bench, I have created an ad-hoc signal generator to drive some
>> old paging receiver boards. I am using a DDS to generate
>> phase-continuous, 4-level FSK. I'm then "multiplying" the output of
>> the DDS (using a PLL/VCO module) by 11, creating an RF carrier. The
>> output of the VCO is the RF carrier (~929MHz).
>
>
> I did a very similar design for the professional paging transmitter
> somewhat 14 years ago. This is a good approach although you should be
> carefull about all kinds of noise and spurs from PLL and DDS.
We did this about 16 years ago for an Ermes type signal, but at 928MHz -
Ermes never took off, so the transmitter never went into production.
With the cheaper DDS chips at that time, and the available DACs, we
could only generate our digitally crafted signal at a few MHz. By the
time we multiplied that to 928MHz the noise was troublesome. We ended up
with a mixed approach, which gave clean results more easily. These days
the DDS can be much faster, and I guess multplying by only 11 will work
well.
Steve
Reply by Vladimir Vassilevsky●July 17, 20072007-07-17
Jon Mcleod wrote:
> Mayby not the right group, but..
Why not? Here you can find the best specialists. They are very nice
people, too.
> On my bench, I have created an ad-hoc signal generator to drive some old
> paging receiver boards. I am using a DDS to generate phase-continuous,
> 4-level FSK. I'm then "multiplying" the output of the DDS (using a
> PLL/VCO module) by 11, creating an RF carrier. The output of the VCO is
> the RF carrier (~929MHz).
I did a very similar design for the professional paging transmitter
somewhat 14 years ago. This is a good approach although you should be
carefull about all kinds of noise and spurs from PLL and DDS.
> Doing this, I can actually create a carrier that meets the transmission
> mask, but the modulation doesn't really work. I'm using the PLL loop
> filter to replace the premodulation filter that might have been in a
> commercial paging transmitter 15 years ago. My problem is that the PLL
> is either unstable, or the loop filter flattens out the symbols. I
> can't seem to get it right.
This is not right. The PLL filter should be relatively wide; that
reduces the noise of VCO. The main filtering should be done in the
digital domain prior to the DDS modulation. The PLL filer should have
just enough of attenuation to kill the digital aliases and spurs.
> For premodulation, the protocol specifies a "10th order bessel with
> fc=3.9KHz".
Bessel-10 is the heritage of the old time analog transmitters. Actually,
any linear phase lowpass filter with the 60dB/oct rolloff at 3.9kHz will
do the job.
> Generally speaking, is there any analytical way to
> "convert" a baseband premodulation filter into the correct loop filter
> for my setup?
Yes, you can do it that way however it would not be the optimal solution
for the spectral purity of the transmitter.
> [At this point, the project is basically moot. A guy here found an old
> paging transmitter somewhere that still works. But I would like to see
> my setup work before I scrap it. I'd like to walk away from this
> learning SOMETHING. Any thoughts are appreciated].
Well, now you know that it is possible to make it work :)
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by Jon Mcleod●July 17, 20072007-07-17
Mayby not the right group, but..
On my bench, I have created an ad-hoc signal generator to drive some old
paging receiver boards. I am using a DDS to generate phase-continuous,
4-level FSK. I'm then "multiplying" the output of the DDS (using a
PLL/VCO module) by 11, creating an RF carrier. The output of the VCO is
the RF carrier (~929MHz).
Doing this, I can actually create a carrier that meets the transmission
mask, but the modulation doesn't really work. I'm using the PLL loop
filter to replace the premodulation filter that might have been in a
commercial paging transmitter 15 years ago. My problem is that the PLL
is either unstable, or the loop filter flattens out the symbols. I
can't seem to get it right.
For premodulation, the protocol specifies a "10th order bessel with
fc=3.9KHz". Generally speaking, is there any analytical way to
"convert" a baseband premodulation filter into the correct loop filter
for my setup?
[At this point, the project is basically moot. A guy here found an old
paging transmitter somewhere that still works. But I would like to see
my setup work before I scrap it. I'd like to walk away from this
learning SOMETHING. Any thoughts are appreciated].
Thanks.