[Elecraft] 4 Mhz oscillator

Sun Dec 29 10:43:00 2002

Hi

You cannot accurately measure the 4.000 MHz oscillator directly with a
frequency counter. Most counters have a 1 meg ohm paralleled with about 50
Pf on the input plus the added capacitance of the cable and you may have
more than 100 pf in capacitance. This loading will upset the frequency of
the oscillator. If you use a X10 probe on the counter the probe capacitance
is still around 10-12 pf more than enough to pull the oscillator off
frequency (remember C22 is at most 50 pf and paralleling 10 pf makes quite a
change). The manual has you measure the 12 MHz reference oscillator with the
internal and external counter and adjusting the internal to agree with the
external. This will get you much closer but you are still depending on the
accuracy of the external counter's timebase. The probe loading is why the
manual has you connect both probes at the same time so the frequency shift
is the same for both probes. This is why the method is more accurate than
simply connecting the probe to C22

I prefer using spectrogram to align the 500 and 600 Hz audio tones
broadcasted by WWV so they line up with a 500 and 600 marker on the
spectrogram display. Then moving the indicated 12 MHz internal oscillator by
the same amount with C22. Then re-run CAL PLL and recheck, after a couple of
tries I can put The K2 dead on frequency. WWV is much more accurate than any
counter, in fact all counters are calibrated to WWV so why not use the most
accurate standard?

The beating of the 5th harmonic of the 4.000 MHz oscillator with WWV at 20
MHz is also a good method if you have another radio that will tune 20 MHz
and will put the K2 dead on frequency. I have tried both methods but I like
the Spectrogram method best because you can do the CAL PLL and filter
alignment with the same setup.

Don Brown
KD5NDB

----- Original Message -----
From: "Mike McCoy" <[email protected]>
To: "Helmut Usbeck" <[email protected]>; <[email protected]>
Sent: Sunday, December 29, 2002 8:58 AM
Subject: Re: [Elecraft] 4 Mhz oscillator

> Yes... After an hour warmup I initially used a HP freq counter to set the
> oscillator directly to *exactly* 4 Mhz (measuring right off left side of
> C22) and after PLL found the display to be off. I then used the
recommended
> method of matching the display to the freq counter and it is still off.
I'm
> 99.44% positive my freq counter is in calibration so I don't know what the
> problem is.
>
> As soon as I can hear WWV on 20Mhz I'm going to try the zero beat method.
>
> 73,
> Mike K5PU
>
> >
> > -Always noticed that my display reading of stations being received
> > seemed 50 Hz high.  I figured that the 4Mhz oscillator needed a tweak.
> > I originally aligned it by the frequency counter technique described in
> > the K2 manual, page 57.  Upon checking it I found it to be right on and
> > hadn't changed since I originally did it. So just for kicks I checked
the
> > 4 Mhz oscillator and found it sitting at 3.85 Mhz. That seems to explain
I
> > think my high display reading.  Problem I see is if I used a counter and
> > did it by the book, how come the low 4Mhz reading?  Anyone used a
counter
> > and have this problem?
>
>
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