I've just built a 1.5 GHz synthesizer using a 10 MHz CFPT 9301 TCVCXO from IQD (Farnell order code 127 2080) as the reference. I've just powered it up for the first time and was disappointed to see 100 Hz sidebands at around -40 dBc on the 1.5 GHz output. That equates to a peak deviation of 2 Hz at 1.5 GHz or 13 mHz on the reference. I have lots of low noise LDOs on the board powering all the different parts independently, so I was surprised that 100 Hz was getting in there; and I was about to try powering off batteries when, whilst viewing the output in FM Demod mode on my SA, I realised the XOSC was light-sensitive! I turned off the (halogen bulb) room light in my lab - and no more 100 Hz sidebands! Anyone heard of that: a light-sensitive XOSC !?
I've just built a 1.5 GHz synthesizer using a 10 MHz CFPT 9301 TCVCXO from IQD (Farnell order code 127 2080) as the reference. I've just powered it up for the first time and was disappointed to see 100 Hz sidebands at around -40 dBc on the 1.5 GHz output. That equates to a peak deviation of 2 Hz at 1.5 GHz or 13 mHz on the reference. I have lots of low noise LDOs on the board powering all the different parts independently, so I was surprised that 100 Hz was getting in there; and I was about to try powering off batteries when, whilst viewing the output in FM Demod mode on my SA, I realised the XOSC was light-sensitive! I turned off the (halogen bulb) room light in my lab - and no more 100 Hz sidebands! Anyone heard of that: a light-sensitive XOSC !?
TIA
More info: it's not sensitive to visible light, only infra-red! And I can detect the TV remote!
I recall that older-style ceramic-package transistors and op amps could definitely be light sensitive. Even the ceramic frit around a metal can's leads could admit enough light to cause odd effects, line-noise hum and harmonics being among them.
This doesn't usually seem to be a problem with plastic-package parts, but I suppose it's possible that there might be enough IR leakage through some encapsulation plastics to read the semiconductor junctions and have some effect.
Before you blame light exposure, you might want to run some further tests. Halogen lamps of some sorts can put a lot of "hash" back onto the AC power mains, and you might just possibly be getting enough through-the-air (radiated) pickup somewhere to sneak into your signal path.
Might be a good idea to rig up a simple oscillator (555, battery, LED) operating at 100-200 Hz, light it up, and shine it all over your circuit. You *might* find that some component other than the XO is the light-sensitive culprit... glass-pack diodes are notorious for this.
Also (and this is a bit of a long shot) check for "tunable hum"... a classic problem with direct-conversion radio receivers which "leak" their LO signal back out into the antenna. If your RF carrier is getting out into the room, it could be picked up by powerline-operated devices and generate intermodulation products, which can then be re-radiated (or conducted) and picked up by the receiver. Preventing this usually requires shielding the LO, and having some sort of high-reverse-isolation RF input stage between antenna and mixer in order to keep the LO from blasting all over the place.
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If you can detect the TV remote, then use that to find the light sensitive component. Transistors, diodes, and ICs are light sensitive if the die is exposed.
Indeed. I noticed the carrier frequency shifting as I moved around the bench. Then I turned the room light on/off and saw a ~160 Hz shift at 1.5 GHz. Then I got my (LED) bicycle light and found that had no effect. Then I got the TV remote. Wit the SA in FM Demod mode, I can actually see the TV remote data bits! My PLL loop BW is about 10 KHz.
This slight quirk aside, I must say the CFPT is a remarkably stable little TCVCXO with very low phase noise. Not cheap though.
did you try to block the emission from the halogen bulb to the XOSC? Is there any change of the supply voltage of the XOSC when switching the bulb on and off?
An incadescent bulb will show the AC component of the supply as slight undulations in the light output.
50 years ago, I built a speech transfer system using a car bulb with DC feed and the output transformer of a radio in series as the transmitter, and a scraped OC71 with an amplifier as the receiver. To my surprise, the audio came through pretty well.
Without the DC bias, the output contains a full-wave rectified copy of the line voltage.
I used those diodes for limiting the input voltage range from 0 to Vcc in a very high impedance circuit. Moving my hand around the circuit caused a huge output swing due to those protecting diodes not shadowed by my hand.
On an other case, a microwave source using a low frequency (
Also some pretty dramatic lowpass, I would think -- part of the reason incandescents work surprisingly well at that is because the filament cools off by radiation so much faster at high temperatures (thermal conductivity due to radiation goes as (T_abs)^3!). At yellow hot (say, 1200K), the temp change per watt is 8 times greater than at invisible heat (say,
600K).
Tim
--
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Website: http://seventransistorlabs.com
To my surprise, it did not sound as muffled as a low-pass should make it. Maybe we were so far in the skirt that all the frequencies were equally attenuated.
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