[Premium-Rx] Receiver Frequency Display Accuracy & Calibration

[Bob Milne]

Hi Ahmet,

I find the traditional "zero-beat" method gives good accuracy on fully
synthesized receivers where all oscillators are derived from a single
reference oscillator. The trick is to have the receiver in CW mode and
the pass band centered about the received frequency. Also, a set of
headphones with good low-fequency response helps. I tune to the
highest WWV frequency available at the time, but 10 MHz is usually
sufficient. 

Set your receiver display to 10 000.00 (or 10 000.000 if you have 1-Hz
resolution). With the CW pass band centered about the received
frequency (easy to do with the R8 series, RA6790/GM, or MSR5050A) you
will hear a low-frequency "growl" if your reference oscillator is way
off, or a "swishing" sound of several beats/second if you're close.

Then, you adjust the reference oscillator frequency until the
"swishing" sound repetition rate is as low as you can get it. If you
can get one swishing cycle at 10 MHz, your reference oscillator is
accurate to 0.1 ppm.

Be aware that this method is extremely sensitive, and easily shows ANY
short-term or long-term drifts in the oscillator. And more
importantly, the effects of temperature rise and putting the case back
on the receiver. In the case of the R8, I had to fiddle for about an
hour to get it right--finally setting the reference off by about 5
beats/second to achieve 1 beat/second after the case was put on.

But all this begs the question of how close you need to be in the
first place. Look at your receiver's frequency-accuracy and stability
specs. Unless you've got a really stable oscillator (like the
RA6790/GM), you'll find that time and temperature will noticably
affect the accuracy. But also take into account the smallest frequency
step your receiver has. Bearing in mind that not all stations you
listen too are EXACTLY on frequency, the closest you can tune to a
particular station with 10-Hz tuning steps is something within 10
Hz--no matter how accurate your reference oscillator is. So if your
reference oscillator is off a few ppm, it's no big deal.

Regards....
....Bob