Hi experts,

So I have a National Instrument MyDAQ which can generate up to 20 KHz signal. And I have some questions/ideas that I wanna ask the experts here......

1) is it possible use a signal generator such as a MyDAQ in place of PLL as a LO to a RF system for testing purpose?

2) if yes, and I am wondering, if it's possible to cascade two 10x frequency multiplier to get the frequency up to 2MHz? sort to make it a poor man's function generator.

3) if 1 and 2 are both yes, then that really got me to wonder, why do we ever need a PLL in a RF system at all if there's a way to vary the crystal oscillator and simply multiply the frequency by multipliers?

Probably dumb questions to the experts, but I haven't seen any discussion on this.

Tom

 

1) Only if the signal generator has a control voltage input.

2) 20 kHz is too low a frequency to be practical for RF work. DDS or a crystal oscillator is much more common.

 

Hi experts,

So I have a National Instrument MyDAQ which can generate up to 20 KHz signal. And I have some questions/ideas that I wanna ask the experts here......

1) is it possible use a signal generator such as a MyDAQ in place of PLL as a LO to a RF system for testing purpose?

2) if yes, and I am wondering, if it's possible to cascade two 10x frequency multiplier to get the frequency up to 2MHz? sort to make it a poor man's function generator.

3) if 1 and 2 are both yes, then that really got me to wonder, why do we ever need a PLL in a RF system at all if there's a way to vary the crystal oscillator and simply multiply the frequency by multipliers?

Probably dumb questions to the experts, but I haven't seen any discussion on this.

Tom

(1)Yes,it is,but the MyDAQ is likely to be disappointing,as its frequency stability is probably not good enough--the on line specs don't say what it is.
The other point here is that the fundamental frequency range of the MyDAQ is inadequate in the practical case,which brings us to the next question!

(2) Yes, & this method was used extensively for many years---BUT!
Frequency multiplication is not a wideband process like frequency division.

The normal way is to distort the fundamental frequency,then feed it into a circuit resonant at some harmonic of that frequency.

Multiplying by more than 4 in one stage becomes quite tricky,so higher factors are achieved using multiple stages.

Normally multiplication is performed in multiples of 2 or 3,which tends to leave your X10 out in the cold!

Another major problem is that outside a quite small range,the multiplication stages need to be retuned with each change in frequency.

Amplitude modulation modes,such as full carrier AM,SSB,etc cannot be multiplied in this manner,as the modulation is distorted by the non linear stage needed to create harmonics.

Such signals are normally shifted in frequency by the use of converters (mixers) with the output from a multiplier or PLL used as a local oscillator signal.
Frequency modulation can be multiplied,however.

(3) From the above,it may be seen that frequency multiplication,although a simple concept,becomes quite complex in execution.
PLL systems are frequency agile,so although they appear more complex,they make life a lot easier if you need to shift a modulated signal up or down in frequency,while retaining the ability to vary the ultimate frequency.

 

(1)Yes,it is,but the MyDAQ is likely to be disappointing,as its frequency stability is probably not good enough--the on line specs don't say what it is.
The other point here is that the fundamental frequency range of the MyDAQ is inadequate in the practical case,which brings us to the next question!

(2) Yes, & this method was used extensively for many years---BUT!
Frequency multiplication is not a wideband process like frequency division.

The normal way is to distort the fundamental frequency,then feed it into a circuit resonant at some harmonic of that frequency.

Multiplying by more than 4 in one stage becomes quite tricky,so higher factors are achieved using multiple stages.

Normally multiplication is performed in multiples of 2 or 3,which tends to leave your X10 out in the cold!

Another major problem is that outside a quite small range,the multiplication stages need to be retuned with each change in frequency.

Amplitude modulation modes,such as full carrier AM,SSB,etc cannot be multiplied in this manner,as the modulation is distorted by the non linear stage needed to create harmonics.

Such signals are normally shifted in frequency by the use of converters (mixers) with the output from a multiplier or PLL used as a local oscillator signal.
Frequency modulation can be multiplied,however.

(3) From the above,it may be seen that frequency multiplication,although a simple concept,becomes quite complex in execution.
PLL systems are frequency agile,so although they appear more complex,they make life a lot easier if you need to shift a modulated signal up or down in frequency,while retaining the ability to vary the ultimate frequency.

Thanks.

1) How would I measure frequency stability on a MyDAQ? Can you direct me to a link?


2) I see there's a 12x multiplier on Minicircuit. What you think of it?

http://www.minicircuits.com/pdfs/ZX90-12-63+.pdf
I don’t understand what you mean by “multiplication stages need to be retuned with each change in frequency.” Can you elaborate?

I also don’t understand the part when you said AM can’t work with an LO like this. How’s the data in amplitude get distorted by the frequency harmonics?

3) So if I understand the concept correctly, frequency multiplication is done by harnessing the harmonics of the fundamental frequency, which means at the output of these multipliers I would get not just get the desired multiplied frequency but also the harmonics, which makes it hard to use as a variable LO. Right?

 

I don’t understand what you mean by “multiplication stages need to be retuned with each change in frequency.” Can you elaborate?

If you have a x12 multiplier and you change the input by 1 MHz., the output changes by 12 MHz. For a given GBW product, the circuits of the next stage will have to sacrifice one or the other.

 

Thanks.

1) How would I measure frequency stability on a MyDAQ? Can you direct me to a link?
If you have,or can borrow, a frequency counter,feed the MyDAQ output into the counter,turn both on.
Leave them about an hour to stabilise,then about every 10 minutes or so,observe the frequency displayed on the counter,write it down.
Do this for several hours,then make an Excel graph of frequency variation versus time.
You should be able to determine the frequency stability of the MYDAQ from the list of observations & the graph.


2) I see there's a 12x multiplier on Minicircuit. What you think of it?

http://www.minicircuits.com/pdfs/ZX90-12-63+.pdf

It is designed for use at UHF,where the required tuned circuits are built into the device--no good for your purpose.

I don’t understand what you mean by “multiplication stages need to be retuned with each change in frequency.” Can you elaborate?

Yes, frequency multiplication is performed by distorting the fundamental signal so that it produces multiple harmonics.
We pick off which harmonic we require by passing the distorted signal through a resonant circuit tuned to the required frequency.

If we change the fundamental frequency,the harmonic frequencies also change,so that they will no longer be within the resonance "peak" of the tuned circuit,& will be attenuated.

I also don’t understand the part when you said AM can’t work with an LO like this. How’s the data in amplitude get distorted by the frequency harmonics?
I was actually talking about increasing the frequency of an already modulated signal using frequency multiplication.
Of course,a frequency converter stage doesn't care a hoot where its LO signal came from.

I will include the following,however,as it gives an idea of what happens if we do put an AM signal through a frequency multiplier.

An AM signal actually consists of a carrier fc,the upper sideband (USB) fc+fmod,& the lower sideband fc-fmod.,where fmod is the modulating signal.

You can now see this is a complex signal.

To make things worse,fmod isn't a single frequency,but a band of frequencies which must be transmitted through the system & correctly reproduced at the far end.
When the sidebands are distorted,they produce extra signals which interact to produce other undesired in band products which in turn distort the demodulated signal at the receiver.

3) So if I understand the concept correctly, frequency multiplication is done by harnessing the harmonics of the fundamental frequency, which means at the output of these multipliers I would get not just get the desired multiplied frequency but also the harmonics, which makes it hard to use as a variable LO. Right?

For (3),Yes,but the resonant circuit is tuned to the harmonic you require.

This does bring up a further point,in that you could,if you need to tune a lot of multipliers to get to your final frequency,mistune one,pick up an incorrect harmonic,happily tune all the others up,& be on the wrong frequency at your ultimate output.

Many years ago,I worked at a Radio Broadcasting site with large HF Transmitters,which needed to operate on multiple frequencies so as to get the best day & night coverage.

To this end,they employed several switchable crystal oscillators,followed by tuned multiplier stages,& then RF amplifiers,up to the large output stages which were high level modulated by equally big modulators.

It was quite possible to inadvertently select the wrong crystal,& tune the things up on the wrong output frequency,if you weren't paying attention.
Usually,the tuning "just didn't feel right",which would alert you to the problem,but there was a story that one of these had been tuned in the middle of the "20 metre" Ham band on one occasion,& sat there for about an hour before anyone noticed.

You really need to read up on this stuff from the RF point of view.
If you can get your hands on an older ARRL Handbook or similar for the information on multipliers.
The newer ones don't quite cover it as well,but have some good stuff on mixer/converter stages.