[NLRS] RF Frequency Counters

[Doug Reed]

Hi John.

Regarding your numbers, you seem to be confusing accuracy and stability. 
  All those 1E-9 or 1E-10 numbers refer to the *stability* of the 
oscillator after it has been adjusted *accurately* to the reference 
frequency. Accuracy is not specified since that depends on who did the 
calibration.

The easiest way to understand this is to think of how much your 10GHz 
transverter drifts during warmup and during the day. That is stability. 
How closely 10368.100 corresponds to 144.100 on your IF radio dial is 
accuracy.

The specific question about accuracy was +- 1 count +- the accuracy of 
the timebase. If the last digit of the display is 6, then +- 1 count 
means the last digit could really be 5,6, or 7, i.e. plus minus 1 count. 
The timebase accuracy of the time base is related to the frequency. For 
a 1E-8 timebase and 10 MHz (1E7) signal, the accuracy of the reading is 
.1 Hz. Therefore, when the counter reads 10.000.006, the counter is 
really saying you are somewhere between 10.000.004.9 and 10000.007.1 in 
frequency. If you bump the counter reading out to a 10 second display, 
it now says 10.000.005.7 which actually means between 10.000.005.5 to 
10.000.005.9 in frequency. So if you want to know your frequency to 
accuracy better than 1 Hertz, you need a timebase that is 10 times more 
accurate than the highest frequency you want to read. i.e. 10GHz = 1E10 
therefore timebase = 1E-11. Or to look at it another way, if you want 
all 12 digits on the display to mean something, try to find a 1E12 or 
1E13 timebase......

30 years ago, my physics teacher explained it this way. If you divide 1 
by 3, the mathematician would say the answer was .3333 repeating. But 
the physicist would say the answer was .3 since one digit of accuracy 
was all that was specified in the problem. In the same way, the 
frequency counter displays more digits than the accuracy of the 
measurement really should permit.

Most commercial frequency counters come with a standard timebase and are 
usually calibrated to 1E-6 or maybe 1E-7 accuracy. i.e. the counter may 
display 12 digits but only the first 6 or 7 really mean anything. The 
high stability timebase option *may* mean it has been accurately 
adjusted to 1E-8 or 10-9, but you can't know that. It does means it will 
stay put better where ever it is actually set.

The key to accuracy is having a known stable frequency standard. For HF 
users, WWV is the frequency standard. You tune your radio to 10MHz WWV 
and adjust for zero beat and say that you are calibrated. If there is a 
residual 100Hz beat note, you are calibrated to 1E-5. If you are 
watching the S-meter waver around 1 per second, that is 1E-7 accuracy.

But if you want 1E-9 accuracy for VHF and above, you need to start 
monitoring LORAN or use GPS to "discipline" a high stability local 
oscillator over a long period of time. That is why a number of us bought 
HP Z3801A frequency standards a few years ago. They automatically 
monitor GPS and adjust an internal high stability oscillator for 10MHz 
at better than 1E-10 accuracy.

If you want to build something, start looking for articles about TACGPS 
or other "disciplined" oscillator techniques. If you want to buy 
something, search for Z3801A and later models in the Internet and Ebay. 
You do mention having a kit, the one from MUD last year? That will work. 
I bought one too. I highly recommend putting the crystal oscillator 
inside a wide mouth glass vacuum thermos jar and then putting the whole 
thing in a block of foam. Or whatever other approach you want, double 
oven arrangements preferred....

I highly recommend you start reading with:
	http://www.realhamradio.com/GPS_Frequency_Standard.htm
	http://www.symmetricom.com/
	http://tf.nist.gov/
If you only start with these, you still have a week of reading and 
research to do. Most of the practical info you need is on the first link.

But in the short term, the quick and dirty answer to your quest is no 
farther away than W0AUS. When you get your counter, you need to check it 
out. Not only does Bob have the signal generators to test it to 26GHz, 
he also has a GPS disciplined 10MHz reference and has all the tools 
needed to accurately adjust your counter's timebase to 10.000000000 MHz. 
  Plan to give the unit at least an hour to warm up and stabilize before 
adjusting it. Do the other tests during that time... To really be sure 
about the accuracy of calibration, plan to leave it over night so it can 
be checked when completely warm and after variations from the adjustment 
settle out. But in practical terms the last bit isn't that necessary. 
Just ask Bob nicely....

I didn't read your whole email first... If the counter has been 
calibrated, that should get you down around the 1E-8 or 1E-9 range. But 
the only way to stay there is to get your own standard built. In 
practical terms, even if the counter is disconnected from your local 
frequency standard and is "only" 1E-8 accurate, you will only be 100Hz 
off frequency at 10GHz and that is good enough to hear the signal. 
Chances are good your IF radio isn't that accurate at 2M.....

73, Doug Reed, N0NAS.

John P. Toscano wrote:
> 
> 
> Hello out there.  I'm looking for an opinion (or several) from the group 
> on RF frequency counters.
> 
> What I have now is a real inexpensive Radio Shack model that tops out at 
> about 1300 MHz (practically DC for some of you folks).  I am interested 
> in something that will work at least up through the 10 GHz ham band, 
> with good accuracy.
> 
> I'm looking at a used HP 5342A microwave frequency counter with Option 
> 001 (high-stability timebase) and Option 002 (amplitude measurement) 
> installed.  It has two inputs, Input 1 rated at 10 Hz to 520 MHz, and 
> Input 2 rated at 500 MHz to 18 GHz.
> 
> If anyone has experience with this model (good or bad), or has better 
> suggestions, I'd like to hear them.
> 
> Also, I'm wondering about accuracy, i.e. what is reasonable resolution 
> at 10368.1 MHz?  The specs on the Option 001 say:
>       Aging rage: < 5e-10 per day after 24-hour warmup
>      Temperature: < 7e-09 over the range of 0C to 50C
>       Short term: < 1e-11 for 1 second average time
>   Line variation:   1e-10 for 10% change from nominal
>          Warm-Up: < 5e-09 of final value 20 min after turn on, at 25C
> 
> The specs say that ACCURACY is +/- 1 count, +/- time base error.  The 
> time base (internal or external) is 10 MHz.  I can't believe that they 
> mean what (I think) this says, i.e. if the time base is accurate to 
> 1e-10 (for example), times 1e6 Hz, that the time base is accurate to 
> 1e-4 Hz and therefore the display at 10 GHz is also accurate to 1e-4 Hz, 
> plus or minus 1 display digit.  I would think, that at best, if the time 
> base was accurate to 1e-10 (for example), that 1e-10 x 1.3681e11 would 
> be 1.3681e1 HZ or 13 Hz, plus or minus 1 Hz on the display, or +/- 14 
> Hz.  The problem is that they don't seem to state the ACCURACY of the 
> timebase anywhere, only its stability.
> 
> The meter has 11 digits, and can DISPLAY 10 GHz to a resolution of 1 Hz, 
> but I wonder what the true ACCURACY is.  The specs above only talk about 
> stability, but who cares if it is stable to 1e-11 if it is 10 MHz off 
> frequency?  Obviously, my need to measure 10 GHz to 1 Hz accuracy is 
> limited, but I want to do a whole lot better than being 10's of KHz off 
> frequency like I am now, based on the IF rig readout.  (Yeah, I do know 
> how to subtract, but the apparent offset is so odd that it can be a 
> challenge to do in my head under pressure, and I'd rather not have to!)
> 
> Supposedly, the instrument I'm looking at has been calibrated against a 
> GPS-locked OCXO.  It also has an external timebase input, and I have 
> (but have not yet put together) a GPS-disciplined OCXO that could 
> potentially be attached.  Is it likely to be worth the effort?
> 
> Thanks in advance for your collective wisdom.
> 
> John (W0JT)
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