[time-nuts] About heterodyne method?

[Bob Camp]

Hi

Quick summary:

Computing / period counters give you a constant number of digits of
resolution regardless of input frequency. Back when this stuff was developed
a counter that gave you nine digits a second was pretty common. 

Typical setup:

1) Take two oscillators and tune them 1 to 10 Hz apart. The technique only
works if at least one of the oscillators can be tuned.

2) Run the oscillators into a double balanced mixer. Normally levels near
the maximum are used on both inputs. 

3) Low pass filter the output. You want to keep the RF out of the 1 to 10 Hz
beat note. Various terminations seem to help the sensitivity of various
mixers (high impedance at audio, reactive termination at RF etc).

4) Run the audio beat note into a limiter. The design of the limiter can be
fairly simple or quite elaborate. You need the limiter because the counters
input channels rarely do well with audio sine waves.

5) Count the frequency of the audio on your counter.

If you start off with 10 MHz and set them 1 Hz apart, you get a 1x10^7
"amplification" of the frequency error. If you count that to nine digits,
the resulting resolution would be 1x10^-15. 

Of course resolution and accuracy are not the same thing. Measurement noise
will dominate the readings past a certain point. A typical setup should get
you below 1x10^-11 at one second without a lot of effort. A good setup can
get you to 5x10^-13 at one second with common parts. A fancy setup with a
complex limiter can go below 1x10^-13 at one second.

Yes indeed there's a bit more than that to it, but that should get you
started.

Bob 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Hui Zhang
Sent: Thursday, July 26, 2012 10:44 AM
To: time-nuts at febo.com
Subject: [time-nuts] About heterodyne method?

Hello everyone:
    
    I hnow 'heterodyne method' is very useful method of pricision frequency
measurement (use DBM and LPF), but I only found very few infomation when I
searched in early docments,  Can someone give me more information about this
'heterodyne method'? The block diagram is best. I will very appreciate of
that.


Hui
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