RE: VCO relationship with 4 Mhz ocsillator

Posted by Stewart Baker on Nov 26, 2004; 6:58am
URL: http://elecraft.85.s1.nabble.com/VCO-relationship-with-4-Mhz-ocsillator-tp372030p372033.html

On Thu, 25 Nov 2004 20:03:33 -0800, Ron D'Eau Claire wrote:

> That's not the case for the 4 MHz oscillator in the K2, Jean-François.
> First of all, the 4 MHz clock on the control board and the trimmer capacitor
> C22 have nothing directly to do with the PLL. That's why adjusting C22
> doesn't change your frequency. But what you read in the handbook is right.
> Let's look at that first.
> It is true that a PLL requires a reference oscillator, and the output
> frequency is based on the reference oscillator frequency. One problem with
> PLL's is phase noise - jitter in the frequency caused by the loop constantly
> correcting the output frequency to agree with the reference oscillator.
> Elecraft provides superb phase performance by doing something unusual. They
> vary the frequency of the reference oscillator. The K2's reference
> oscillator is varied over a 5 kHz range. The Phase-Locked Loop tunes in 5
> kHz "steps" and the reference oscillator is tuned to provide continuous
> coverage within those 5 kHz steps. If you look in the archives (or have an
> older K2) you'll see that a year or so ago some effort was put into adding a
> temperature compensation circuit to the PLL reference oscillator in the K2.
> That was because of a basic rule in radio: If the frequency of an oscillator
> can be varied, it will try to drift! The K2's reference oscillator sometimes
> showed annoying levels of drift, and the circuit was improved to stop that.
> Let's get back to that 4 MHz oscillator on the control board. The K2 does
> something else a little differently from many radios. The frequency readout
> you see on the LCD display is *not* produced by constantly monitoring the
> oscillator frequency in the K2. Instead, the K2 frequency readout is a lot
> like the old vacuum-tube-day dials, in which you turned a knob, that knob
> varied the frequency of an oscillator, and it also moved a pointer. The
> pointer moved over a scale with numbers on it that showed the frequency. The
> numbers on the scale were produced by hooking up a frequency counter to the
> rig and checking the frequency produced as the tuning knob was turned. The
> correct numbers were then printed on the scale behind the pointer so it
> could be set back to any desired frequency later. Not every frequency was
> recorded - just frequencies at regular intervals. Frequencies in between the
> numbers could be estimated by "interpolation". That is the frequency exactly
> halfway between 7000 kHz and 7010 kHz could be assumed to be 7005 kHz, and
> so on.
> That's how the K2 works, but its all done electronically. First of all,
> there are no variable capacitors or other old-style tuning mechanisms used
> in the K2. The oscillators - the beat frequency oscillator or BFO, and the
> phase locked loop reference oscillator (called the variable frequency
> oscillator or VFO in the K2 literature) are both tuned with voltages.
> Voltage-variable capacitors replace the old big mechanical variable
> capacitors. The voltage-variable capacitors change capacitance and therefore
> the oscillator's frequency in response to the level of a direct-current
> voltage applied to them.
> When you run CAL PLL, a frequency counter built into the K2 measures the PLL
> frequency. The K2 applies a tuning voltage to the PLL reference oscillator
> and notes the frequency measured by the counter. This information is
> recorded in memory so it can be looked up later. The K2 tunes through the
> whole range of frequencies the PLL can cover noting the actual frequency
> measured at regular intervals, and then recording the voltage applied to the
> reference that will produce the frequency.
> After CAL PLL is done, all of that data has been stored.
> Now, in normal operation, you choose a band and spin the frequency dial to a
> specific number on your LCD display. When you do that, the K2 looks up the
> proper voltage from the data that was stored when your ran CAL PLL and
> applies it to the PLL reference oscillator. That will put the reference
> oscillator on the correct frequency, according to the data that is in
> memory. Of course this happens very quickly so it appears that the tuning is
> happening in direct response to your turning the knob.
> Notice that the accuracy of the readout depends upon how accurately the
> frequencies and tuning voltages were stored when you ran CAL PLL. That's
> where the 4 MHz clock and C22 come in. The Control Board 4 MHz clock is the
> time base for the frequency counter! So the closer it is to 4 MHz the more
> accurately the K2 will measure the K2 PLL frequency and the more accurately
> the dial calibration of the K2 will be in use.
> That's why the procedures for setting C22 all remind you to re-run CAL PLL
> (and CAL FIL) after adjusting it, in order to make the K2 store new
> frequency and tuning voltage data in memory. After you do that you will
> notice the change in the dial calibration.
> Wayne published an excellent method for setting C22 on the Elecraft web site
> (www.elecraft.com). Look under Builder's resources for "Adjusting C22 to
> calibrate the K2's Frequency Display".
> Ron AC7AC

>> What an excellent explanation, well done....

73
Stewart  G3RXQ

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of Jean-François Ménard
> Sent: Thursday, November 25, 2004 5:56 PM
> Cc: Elecraft - Maling List
> Subject: Re: [Elecraft] VCO relationship with 4 Mhz ocsillator
>
> Sorry to bother you....
> I read in my ARRL handbook about PLL... and now I understand that the
> PLL must have a fixed, precised, reference to phase lock it's loop....
> the 4 Mhz oscillator clock...
> I will find the place to put my scope probe to see the 4 Mhz oscillator
> also... just for my understanding.
> Thanks and sorry again for your time.
> Le 04-11-25, à 19:49, Jean-François Ménard a écrit :
>> Could someone tell me the short story about the relationship between
>> the PLL reference oscillator and the 4 Mhz oscillator from the control
>> board ???
>> I'm asking this question because when I calibrate the PLL reference
>> oscillator at TP3 using the internal frequency counter of the K2 and
>> also using my external Instek frequency counter also connected at TP3,
>> trying to match the K2 PLL frequency on both display.... Well.... I'm
>> turning C22 on control board, associated to the 4 Mhz oscillator....
>> and that make the PLL oscillator to change.... Why ?!?!?
>> I can see the on the control board that the MCU have an output /PLLCS
>> that go to the PLL synthetizer.... that must be there, but why ?
>> This setup seems to be particular to the K2 concept ?!?!? Right, wrong
>> !?!?
>> Thanks for your time...
>>
>> Sincerly,
>> ===========================
>> Jean-François Ménard / VA2VYZ
>> [hidden email]
>> Elecraft K2 #4130
>> http://homepage.mac.com/jfmenard
>> ===========================
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> ===========================
> Jean-François Ménard / VA2VYZ
> [hidden email]
> Elecraft K2 #4130
> http://homepage.mac.com/jfmenard
> ===========================
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