[Premium-Rx] Receiver Frequency Display Accuracy & Calibration

[w3jn]

You hafta be sure that there's no offset on the BFO to do this.  While the 
BFO *may* be derived from the same oscillator, there are radios that offset 
the BFO a KHZ or 1.5 KHZ in USB, LSB, and CW.  An ICOM R-71 is one example 
of this, IIRC, and I believe other hammy hambone radios have the same 
arrangement.

Also, as BOb noted below, it's VERY important that if close-in frequency 
measurement is important that you have a temperature stable ref oscillator. 
A crystal oven is an absolute necessity, and even then, the temp in the oven 
will vary enough to change the ref oscillator somewhat.  And then, to adjust 
the ref oscillator, you hafta be sure to warm up the radio for an hour or 
so - and don't try and make any subsequent measurements when the receiver is 
cold.   I've found that the Cubic R-3030 ref oscillator will drift 10 hz 
from cold.

73 John

----- Original Message ----- 
From: "Bob Milne" <rmilne at cfl.rr.com>
To: "Ahmet Gundes" <ahmet-m at usa.com>
Cc: "Premium-RX" <premium-rx at ml.skirrow.org>
Sent: Monday, October 04, 2004 2:34 PM
Subject: Re: [Premium-Rx] Receiver Frequency Display Accuracy & Calibration


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


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