Measuring TCXO frequency?  - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 26, 2015, 09:30:26 pm
content: I bought this 28.8mhz TCXO about a year agohttp://www.ebay.com/itm/1pcs-TCXO-28-8MHz-28-800M-Ultra-precision-Crystal-Oscillators-DIP-0-5ppm-NT-/171907199482?hash=item280677b1faI have tried to measue its frequency with my UniT that is suppose to measure up to 200mhz, and a bus pirate which is suppose to measure up to 40mhz. Neither seem to be giving me any correct numbers if they give numbers at all. I recently found out this is a square wave 3.3v TCXO. Ive used it at 5v and hoping that hasnt had a major effect on it.The thing is I know its working becuase when I connect it to an RTLSDR it works properly and when I use an SDR dongle as a spectrum analyzer near its output pin I get a huge spike a few PPM from the dongles crystal controlled IF. But a volt meter only shows very lttel voltage coming out like .005v or something and no frequency meter will count it. What exactly is happening here?

author: Matje
author_rank: Regular Contributor
timestamp: August 26, 2015, 10:44:53 pm
content: Quote from: rwgast_lowlevellogicdesin on August 26, 2015, 09:30:26 pmNo ordinary volt meter will measure properly at this high a frequency. See the manual for it, it should be in there, a few dozen kHz is what can be expected.A proper frequency counter (with the frequency range *and* a real high frequency input stage, IOW not a DMM) will certainly measure the frequency, provided the input impedance of the counter and the output impedance of the TCXO are halfway suitable. Again, see the manual of the DMM and of the TCXO to see whether it can work. Not that DMM manuals usually specify anything useful in that regard - because there is nothing approaching useful to specify... ;-)

author: xrunner
author_rank: Super Contributor
timestamp: August 26, 2015, 10:54:37 pm
content: Quote from: rwgast_lowlevellogicdesin link=topic=53117.msg741120#msg741120Really? What is the exact model number of this Uni-T meter? I'd like to see the specs.

author: Lightages
author_rank: Unknown
timestamp: August 26, 2015, 10:56:36 pm
content: Most multimeters will not measure an only positive going logic level signal. They usually want to see a zero crossing point as in a AC signal. Try putting a capacitor in series and see if it will measure it.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 26, 2015, 11:36:23 pm
content: UniT61E manual says 200mhz, didnt realize it had to be a 0 centered frequency thats about useless

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 26, 2015, 11:48:17 pm
content: Hmmm AC coupling didnt help im only gettin .08***mV on the output pin at 4.5v input, shouldnt I be at least seeing 2.25 out?

author: xrunner
author_rank: Super Contributor
timestamp: August 26, 2015, 11:52:51 pm
content: Quote from: rwgast_lowlevellogicdesin link=topic=53117.msg741120#msg741120The manual for the meter says it can measure up to 220 MHz as you stated. I'm beginning to wonder if the output of the TCXO is a whole lot lower than you think it is. If it's way, way down that would explain how a radio receiver dongle can pick it up and the meter cannot measure it. Can you verify the output amplitude with a scope?

author: miguelvp
author_rank: Super Contributor
timestamp: August 27, 2015, 12:23:39 am
content: My UT71B has no problem with a 26MHz OCXO no zero crossing requiredHere are some screen captures of my scope as well from a previous test:

author: xrunner
author_rank: Super Contributor
timestamp: August 27, 2015, 12:48:55 am
content: Quote from: miguelvp on August 27, 2015, 12:23:39 amYep - I bet his TCXO output is extremely low ...

author: miguelvp
author_rank: Super Contributor
timestamp: August 27, 2015, 12:58:50 am
content: I just noticed that the OP mentioned the TXCO in question is a 3.3V but he fed it 5.0VThis other listing (probably not the same as the OP but similar) has more specification details:http://www.ebay.com/itm/1pcs-TCXO-28-8MHz-28-800M-Ultra-precision-Crystal-Oscillators-DIP-0-5ppm-NT-/131129276731No output level on the specs though. Maybe the OP is using pin 1 to supply the voltage or maybe by feeding it 5V the thing is toast since it can only take +-5% of 3.3VEdit: maybe that fried the output amplifier or the voltage regulator.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 27, 2015, 04:01:44 pm
content: Yes Ive seen that listing and wondered, at the time of buying this TCXO it was the ONLY one being sold on ebay. Ive fed it 5 volts a few times never for a sustained period and it has never gotten hot, but like I said it seems to do its job perfectly when connected to an RTLSDR.....Why would the output be so low? Unfourtantely I do not have a scope faster than 2mhz, but its frequency counter shows it counting at 5mhz or more when set on 100mV per division.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 27, 2015, 04:44:03 pm
content: The SDR is an indication that the oscillator is running. Such an oscillator usually needs local decoupling to work properly. Also a long cable diretly at the output can upset it due to reflections or capacitive loading.To test the amplitude one could buid a simple demodulating probe - a HF detector with a small schotky or Ge diode. This should give a rough indication of the amplitude.The secont possible test would be adding a 74HC... divider to bring the frequency the scope can handle. 74HC... schould be fast enough for 25 MHz even at 3.3 V supply, but check the DS first.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 27, 2015, 07:09:57 pm
content: So I used mini coax out and this is my PCB it is decoupled with a 1uf surface mount cap under all that goopMy intention is to sheild it inside a copper clad enclosure for shielding as a separate module for a bigger system, also enclosed in copper.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 27, 2015, 07:43:16 pm
content: Decoupling with 1 µF is plenty or to much capacitance. At such high frequencies its usually more like 10-100nF, possibly an additinal eletrolytic and maybe a ferrite bead. The larger the capacitor, the lower the resonance, and impedance increases beyound that.The cable looks like a 20-30 pF capacitor to the source. This might be all the oscillator can drive.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 28, 2015, 06:25:18 pm
content: I acually decouled with a .1uf and a 1uf my mistake. the 4th pin was labled as NC/VConn I have no idea what vconn means?

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 31, 2015, 12:34:05 am
content: Ok so for anyone what was wondering what was happening here, this thing does have a CML output I hooked it up to 3.3v and then used a 50ohm pullup measured 28.80 exactly on my meter