[minima] Re: si570 Arduino Interface
- From: Ashhar Farhan <farhanbox@xxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Fri, 24 Jan 2014 06:13:07 +0530
mark,
the si570 output should have been a square wave. it shows a sinewave in the
screen capture. i am not sure what happened. Probably there was an LPF used
somewhere?
the desirable characteristic of a local oscillator is low phase noise. the
scope will not tell you that. nor will a regular spec an tell you that.
that's because we are looking at how high the 'grass' is right next to the
oscillator frequency. we are looking at something around -150dbc : very few
spectrum analyzers will show that straight-off.
there are a few ways to measure it though. one is that you use a long-ish
low pass filter chain like the one we use for 14 mhz (only double in
length), and then, instead of our series traps, we insert crystals matched
within 100 hz. these will provide a very deep, very sharp notch at one
frequency, then feed the local oscillator such that if falls right into
that 'hole', leaving the grass around it high enough to be noticed by the
spectrum analyzer.
distorted waveforms from the local oscillator are not necessarily bad. for
instance, the Si570's square wave's spectrum is a result very strong odd
harmonics (you can generate a square wave by adding up odd harmonics of a
fundamental frequency). those harmonics could produce signals (when mixed
with the 20 Mhz IF), but they so high in the frequency that they are easily
stripped off by the low pass filters.
homebrewers who have loving produced 'perfect' sinewaves from their VFOs
often feel dejected when they connect their VFOs to diode mixers! the
'terribly distorted waveform' on their scopes are not at all a cause of
worry, as long as each wave has exactly exactly the same shape as the
previous one : something that a VFO's Q, energy levels, etc. will determine.
- f
On Fri, Jan 24, 2014 at 5:26 AM, Steve VK2SJA <vk2sja@xxxxxxxxxxx> wrote:
> Hi Mark,
>
> I suspect that this might be a case of "The Devil hiding in the digital
> details".
>
> Your running the SA at 150Mhz span with 100kHz resolution bandwidth. Can
> you try drop the span to say 50kHz centred on the frequency of interest
> and make sure your resolution bandwidth is set as low as it can go? 100Hz
> I think for the Rigol DSA-815? Essentially setup the analyzer as if you
> were going to sweep a crystal filter. Then take another snapshot of both
> Si570 and AD9851 outputs. Might be worth while trying a couple different
> output frequencies as well.
>
> Great blog BTW. Keep posting.
>
> 73, Steve
>
>
> > Hi Gang
> >
> > I have my si570 working with the Minima code on my Mega2560 board - I've
> > grabbed some images on the 'scope and SA of the output at 34.something
> > MHz. For a comparison I have also done the same for a AD9851 based DDS I
> > have on my bench at the same frequency.
> >
> > I would be interested to learn from those more wise than me what the
> > differences are, please remember that the si570 is straight to the
> > instruments whereas the DDS has passed through a 70MHz low pass filter
> > on board the eBay sourced module.
> >
> >
> http://g0mgx.blogspot.co.uk/2014/01/minima-arduino-si570-interfacing.html
> >
> > Mark
> > G0MGX
> >
> >
>
>
>
>
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