Sunday, April 20, 2014

Si570 Programming Continued...

The relationship between the various frequency setting elements is described as follows:

As previously mentioned, the output frequency is the result of the DCO frequency divided by the product of two divisors.  The DCO frequency is the product of a crystal frequency in the 114.25 MHz range and a settable reference frequency and must remain within the range of 4.85 to 5.67 Ghz.  RFREQ has 38 bits of resolution allowing for a theoretical frequency setting resolution of 0.09 ppb at the test frequency of 114.285 MHz.

The HSDIV and N1 values have additional restrictions on them, specifically HSDIV can have the values 4, 5, 6, 7, 9 or 11.  N1 can be 1 or any even number up to 128 inclusive (1, 2, 4, 6, ... 128).

Typically at startup you would determine the fxtal value specifically for your chip:
By interrogating the chip, one can determine the values for HSDIV, N1 and RFREQ.  By knowing the default startup frequency, the crystal frequency can be calculated.  In my case, HSDIV = 11, N1 = 8 and RFREQ is 0x2B6109f08.  Dividing this value by 2^28 gives me 0x2b as the integer portion or 43 decimal.  To figure out the fractional part one would divide 0x6109f08 (the least significant 28 bits of RFREQ) by 2^28.  Working in decimal instead of hex:

  0x6109f08 = 101752584 / 2^28 = 0.3790579140

Adding this to the integer portion gives me a RFREQ value of 43.3790579140.  Plugging this and the other knowns into the formula above gives me:

  fxtal = (56320000 * 11 * 8) / 43.3790579140 = 114252365.964832 or 114.252365 Mhz

The datasheet states that the crystal frequency is only accurate to +/- 2000 ppm at 114.285 Mhz, or about +/- 0.22857 so my device is within the pretty horrible specification.

So, running this the other way, the output frequency is found as:

  fout = (114252365 * 43.3790579140) / 11 * 8 = 56319999.524391 = 56.32 Mhz

A guy could envision a mechanism wherein you output a WWV frequency and tweak RFREQ until zero beat and then store the value for future reference, but the known frequency technique will get you very close.

If the DCO frequency is only moved +/- 3500 ppm, the oscillator output will be phase continuous.  This should make implementing protocols such as WSPR, JT65 and the like much cleaner than my previous work using the Parallax Propeller.  Also, in theory there is .42 Hz frequency resolution capable which would allow for much closer FSK tone shifts used in WSPR than the 1Hz resolution I had with the propeller.

It is unclear from the datasheet what is precisely meant by the +/- 3500 ppm figure.  One interpretation is that once you have moved a total of 3500 ppm from whatever frequency you last set using the "large frequency change technique", you would have to recalculate the RFREQ and divisor values and set them back to the device.  In my experience, most of the Arduino implementations I have reviewed have taken a flavor of this approach.  While this is certainly safe, it is unlikely to be necessary.  The other interpretation I have of the datasheet is that you could move the DCO over its entire range without recalculating parameters and do it in a phase continuous manner as long as no single frequency change is more than 3500 ppm.  My interpretation is that once you have changed the frequency by either of these mechanisms, the new DCO frequency (not the output frequency) becomes the new "center frequency" described in the datasheet.  So, under my theory, you could move the output frequency a total of 5.67 - 4.85 GHz = 0.82 Ghz 3500 ppm at a time.  Of course, this would require a different part than I have which only has a range of 10-160 Mhz.  Under this theory, as long as you didn't jump more than 17.5 Mhz between frequency settings, you only have to recalculate RFREQ and the change in frequency could be done phase continuous.  This theory I have not yet tested.

More to come...

Saturday, April 19, 2014

Si570 Programming

Messing around with the Si570 chip that is used in the Minima transceiver designed by Farhan Akhtar.  This is my lash-up:

After a bit of tweaking, I have the oscillator working.  It is no picnic trying to interpret the Si570 datasheet however.

Interestingly enough most of the code implementations I have looked at for the Arduino make a couple of errors in interpretation of the datasheet.  The Si570 is not really fun to program, but it has a number of benefits going for it.

My particular chip is a 10-160 MHz CMOS model.  When I first power it up, it comes up at 56.320 MHz.  The datasheet is not explicit on this, but supposedly the individual parts are "tuned" at the factory to their default frequency by adjusting the various parameters that are used to set the frequency.  However, the datasheet does not provide very flattering stability numbers and nowhere does it provide a statement of how closely it is calibrated in the factory.

So, in the absence of highly accurate frequency measurement capabilities you are faced with having to calculate the internal crystal frequency based on the default output frequency (from the datasheet) after reading the frequency setting parameters from the chip itself.

The Si570 requires mathematical calculations with 38 bits of precision in order to accurately set it's frequency.  Most Arduino implementations rely on double precision floating point math functionality.  However, the Arduino implementation of double precision is in fact identical to its single precision floating point.  Namely double = float data types silently behind the scenes.  So, most of these implementations give up on the number of digits of precision required to control the Si570 even down to the 1 Hz level.  Theoretically you should be able to set the frequency to within .42 Hz, but the unfortunate floating point double precision implementation on Arduino prevents even 1 Hz resolution.

The Si570 stores a reference frequency used in the output frequency calculation as a 38 bit values where 10 bits are use as a decimal multiplier.  The remaining 28 bits are a fractional multiplier stored as an integer value scaled by 2^28.

The internal DCO is constrained to 4.85 to 5.67 GHz determined by the internal crystal frequency multiplied by a reference frequency.  According to the settings in my chip, I calculate my crystal frequency to be 114.252365 MHz.  Secondarily, frequency excursions of the DCO can be done in a phase continuous manner if the change in frequency is no more than +/- 3500 PPM.  Many implementations however are calculating this based on the output frequency rather than the DCO frequency in error.  Frequency excursions larger than this require the DCO to be stopped and restarted in a non-phase continuous manner that can take upwards of 10 ms to complete.  Circuitry sensitive to runt pulses may require resetting after this operation.

The output frequency is the DCO frequency divided by the product of two divisors.  There are additional restrictions on these two divisors.

More to follow...

Monday, March 10, 2014

20 metre listening

Seems that 20 metres continues to be active with DX this evening.  Currently listening to OD5ZZ from Tripoli, Lebanon.  The band is much more quiet, but then again the weekend contest has no doubt ended.  UT7UJ is active from Kiev, Ukraine towards the upper end of the band.  It is sure nice to hear DX stations again on the bands after so much lack of propagation.  ZS3CX is on from South Africa, T88XT from Palau, VK4LJ from Australia, and ZL2BQ from New Zealand are all quite loud.

20 metre activity picking up

Managed to get enough of the shack cleared out that I could do some listening on 20 metres last evening.  I was pleasantly surprised how active the bands were, including well into the evening.  The contest weekends raise the question in my mind just how dead the bands are as opposed to just how "inactive" the bands are.  It seems that just about every contest weekend I can hear all sorts of DX but otherwise the bands have the appearance of being dead.

I listened to a number of Kuwait stations, New Zealand and mid-Atlantic stations working pile-ups.  After listening to one particular pileup for about an hour, there was momentary lull and I tossed my call sign out to 9K2WA using QRP (5 watts) on SSB and he came right back to me and gave me the obligatory 59 signal report.  (Yeah right...)  But he was about 20 over S9, so who knows how strong I really was on his end.

Monday, February 3, 2014

SWL tonight

Listening to AM broadcast band tonight and am hearing Canadian stations, quite strongly.  31 and 49 metre bands are quite active this evening as well.  Crisp and clear evenings can be fun listening.

Friday, September 20, 2013

Fun with diodes...

Been playing around with crystal radios lately.  I have a loose coupled radio that consists of a pre-selector tuned circuit, a detector with a 1N34 germanium detector than can be switched out to allow external detectors to be used with the tuning circuits.  Lastly, I have a wave trap tuned circuit that can be used to null out strong adjacent signals that might otherwise swamp the desired signal.  These three modules are all built up in separate boxes and just placed near each other to couple together as strongly or weekly as desired by just varying the distance between them.

Now I am playing around with point contact semiconductors.  This one is galena though I have some iron pyrite I would like to try as well.

I have built a few "fox-hole" receivers using the old blue double edge razor blades and a pencil but never got a chance to play around with point contact rectifiers made from minerals until now.  Should be fun.

Saturday, August 24, 2013

What do you get...

...when you combine a dog bumper launcher...

...with a 22 caliber blank cartridge loaded in the handle of this rather innocuous looking device?

Well...  Tie a bit of kite string around the bumper (the white bit on the end), make some big loops on the ground with the line, insert ear plugs, stand behind and well clear of the line, pull back the spring loaded plunger on the bottom and it's bye-bye-bumper...

I used it today to launch over my house (which has a 45 degree (12/12 pitch) metal roof) a belaying line so I could get up there and clean some moss off the skylights and inspect the seals.

The bumper flew about 200' in the air before the trailing line slowed it down and dropped it in the front yard.  You will get the neighbor's attention as the sound of a gun-shot rings through the neighborhood...  I live on some acreage out of the city limits, so no boys in blue arrived on my doorstep.  Guess I will fail the GSR test at the airport security checkpoint, eh?

I am not sure I can be very accurate with this thing to carefully place antenna lifting lines, but it certainly worked a treat for my get a line over the house to anchor to the pickup truck activity.

We of course had to turn a pack of golden retrievers loose to chase an untethered bumper which was a bit of a riot on a sunny Saturday in the northwest.