On 05/23/11 10:44 PM, Jim Lux wrote:
-----Original Message-----
From: WB6BNQ<wb6...@cox.net>
Sent: May 23, 2011 2:17 PM
To: Discussion of precise time and frequency measurement<time-nuts@febo.com>
Subject: Re: [time-nuts] smallest rubidium
David,
You should have said so in the first place. Unless you are sure that the
reference
oscillator is the base for all the generated frequencies, it would not make
sense to
install a Rb source in the radio. You are correct concerning most of the
"so-called" higher stability options in that they are barely worth the price
asked
for them and only meet specs in a tightly controlled environment.
I believe the reference is the best for all the frequencies that are used, so
there would be some advantage in a rubidium, but I think they are going to be
too big. I'd hard to know what size this is, but I'm guess very roughly I have
about 25 x 25 mm. But that's only from looking at manual and comparing it to the
size of a tuning knob!!
The spec of the TCXO, that is a rare option (SO-1) is:
Oscillator frequency: 20 MHz
Frequency stability long term: +/- 10^-6 / year
Temperature stability +/- 10^-7 (-10 to +50 deg C)
Adjustment range +/- 60 Hz.
Weight 25 g
Output: more than 0 dBm into 50 Ohms.
There's no way this will be used anywhere near -50 deg C. I should think the
coldest it would ever get would be +10 deg C, and that would be unlikely.
Looking at the service manual for this transceiver
http://www.g8wrb.org/data/Kenwood/TS-940S/TS-940S_Service_Manual_revised_edition.pdf
it appears one needs to remove about a dozen components from a PCB board if
installing this option, then solder the TCXO in the place of those components.
For a lot of HF radios, the TCXO performance required is such that the actual frequency
be within 20 Hz of the displayed frequency (that's the NTIA standard). 20Hz comes from
empirical tests of how close the frequency needs to be to not require a
"clarifier" for intelligible speech on SSB. (probably moderated, too, by
what's easy and practical to do in a portable transceiver)
(http://www.ntia.doc.gov/osmhome/redbook/ed200801rev201009/M_9_10.pdf)
bear in mind that that there's two radios in this whole stackup, because the
transmitter has comparable frequency accuracy as the receiver. So the overall
frequency uncertainty is on the order of 30 Hz (sqrt(2)*20 Hz).
Hitting a 0.5 ppm accuracy (15Hz out of 30 MHz) is a fairly challenging spec to
meet over a wide temperature range.
Bearing in mind this is going to be used at room temperature and since inside a
piece of equipment it will be above room temperature, do you think achieving
better than the above specs will be easy? I guess it should be. But whilst I'm
putting one osciallator, I might as well put the best I can fit. I guess its
only a matter of time before a 20 MHz oscillator with sine wave output comes up
on eBay.
Dave
--
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