Tektronix 2753P Spectrum Analyzer - Page 1
Forum section: electronics / testgear

author: torquil
author_rank: Regular Contributor
timestamp: October 25, 2019, 10:26:13 am
content: Hi!I got hold of a Tektronix 2753P spectrum analyzer. It seems to work fine, but I have a few questions regarding the restrictions on the input signal. I would hate to damage it.It is apparently very similar to the 495P. The bandwidth is 100 Hz - 1.8 GHz. I could not get hold of an electronic operators of service manual, although I did manage to find documents for many of its "sibling instruments", i.e. 492, 494, 494A, 495 and 2754.In the 495P operators manual it says that the restrictions on the maximum input level are "30 dBm (1W) continuous" and "75W peak, pulse width 10^(-6) s with duty factor 0.001 or less (attenuator limit)". It also says in capital letters "DO NOT APPLY DC TO THE RF INPUT". These limits are stated under the title "Maximum safe input (With 0 dB RF attenuation)".This leaves me with the following questions:1) Will measuring the input resistance using a DMM damage the input due the applied DC voltage?2) If the input restrictions are violated, what exactly is at risk of being damaged?3) What do they mean by DC in the manual when they say it must not be more than 0V? Less than 1 Hz? Less than 0.1 Hz? Less than 0.01 Hz? Less than 0.001 Hz? You get the idea. Wouldn't a more precise statement be to put an upper limit on the amount of signal power below a certain frequency. The input can apparently support an continuous input power of 1 W, i.e. around 7 Vrms at 50 Ohms, but only above some unspecified frequency.4) As mentioned above, the input limits are stated along with "With 0 dB RF attenuation". Does this mean that at e.g. 20 dB attenuation, I can input 30 dBm + 20 dBm = 50 dBm at the input? And 30 dBm + 60 dBm = 90 dBm with the attenuator at 60 dB. If not, what are the limits when the attenuator is nonzero.Just trying to make sure I don't damage the instrument, because it is a lovely thing!Best regards,Torquil Sørensen, Norway

author: kipp
author_rank: Contributor
timestamp: July 03, 2020, 05:17:24 am
content: Hi torquilHope you haven't blown it since nobody replied :-). The restriction that there not be a DC voltage applied to the input is due to the way the first mixer works. The mixer's job is to multiply the incident RF signal by a sine wave generated by the 1st local oscillator, but an exact, bi-linear, multiplication operation of the two signals is difficult to achieve at high frequencies. There aren't circuits that can do it, so a trick is used. By Taylor's theorem, functions can be written as a sum of powers of the independent variables: f(x, y) = a + b x + c y + d x2+ e x y + f y2+ ... and so on, i.e., a constant, plus two terms proportional to the variables, plus three terms proportional to the squares and product of the variables, plus ... (to higher and higher powers). We want a circuit that computes that "x * y" term (input x LO), but as you can see Taylor's theorem tells us that *any* circuit at all that is non linear (and that's the key) will generate an output that contains that term that we want as one component (among other, undesirable things). What this has to do with spectrum analyzers is that the 1st mixer is usually implemented with a pair of diodes, where the non-linear response the diodes have to a voltage applied across them is used to create, in essence, a mess, but a mess that contains within it the mixed signal that is desired. The desired product term can be isolated from the others using band-pass filters, either before or after the mixer, or both.So ... your spectrum analyzer's input is connected straight across a diode to ground, and that diode will be biased into a *nearly* on state so that it's just on the edge of conduction so that the analyzer can respond to weak signals. That means, any DC voltage will see a nearly perfect short circuit to ground. That diode is easily blown, it's not a power rectifier, it is a tiny delicate flower that operates at microwave frequencies. Static discharge from your finger into the input port can blow it easily. Plugging in an antenna and then touching the antenna can blow the mixer. The input must be treated very carefully.But, yes, strictly speaking you can allow some amount of DC power to enter the input port, that diode can safely dissipate some amount of power within itself. What the warning stickers are saying is "don't try to be clever, assume that number is 0 W".The mixer, if blown, cannot be fixed. Well, it *can*, some unlucky and ambitious hobbiests have reported being able to rebuild blown mixers with off-the-shelf diodes, but it's probably more of an interesting experiment to try rather than a satisfactory repair of the instrument.As you say there is an attenuator between the input port and the mixer, and if you have the attenuator turned on it should provide a DC resistance and can protect the diodes in the mixer from accidents. You can also obtain a "DC block" (a capacitor) and install it on the input port, but this will reduce the sensitivity at low frequencies and isn't fool proof. Some spectrum analyzers include these and will tolerate a DC voltage on their input. The capacitor won't allow current to flow indefinitely, but if the capacitance is large and you suddenly apply a large voltage you can still push enough Coulombs out into your mixer to burn out the diode. A related device is a "bias tee" which is a DC blocking capacitor but with the option of connecting something to the "with DC" side through an inductor (to allow only DC). These can be used to supply power to one side while the other is isolated from the DC voltage, so, for example, you can connect the spectrum analyzer to an antenna that needs power supplied to it on the cable, like a satellite TV antenna or an active GPS antenna.You should check the manual to see what the limits for input power are, but the attenuator in my 2794 is not a power dissipation system. You can't put 100s of W of power into the input and set the input attenuator to 60 dB and go ahead and make measurements, the attenuator will be incinerated. My 2794's attenuator has been destroyed this way. The previous owner (not me) had an accident, and the 20 dB and 30 dB dissipator elements in the attenuator are now burnt black crispy chips. The 10 dB still works, and the pass-through (0 dB) setting still works, but that's all I've got (replacements from ebay are hundreds of dollars, so I live with it the way it is). The mixer survived whatever accident happened because the attenuator absorbed the power and sacrificed itself. So ... yeah ... keep some attenuation on at all times, just in case. The spectrum analyzer will work without an input attenuator, but it will not work without a mixer.Measuring the input resistance with a DMM? If I had to guess I would say it is probably safe because DMMs don't normally destroy diodes in Ohms mode, that's normally a safe measurement to make. But the cost of being wrong is high. I wouldn't do it. I never have, even out of curiosity, but I *have* taken DC voltage measurements on the input port with a DMM, to make sure the spectrum analyzer itself wasn't putting a voltage on its input. Follow normal static safety procedures when doing it.You should buy the service manual from QService. It's easy, you give them your credit card and almost immediately they'll e-mail you a link for downloading the PDFs. They are high quality scans, and they've been OCR'ed so they're searchable with a text search (searching for a component identifier in an instrument with 10s of thousands of parts is wonderful). Don't expect to find free copies online, I'm so happy with the service they provide I would never rip them off, I want other people to buy their own copies, too, to keep that service in business, and I suspect all their customers feel the same. The service manuals for this generation of Tektronix analyzers are very good component-level documents. Mine has 500 pages of schematics, hundreds of pages explaining how the instruments work, how to test them to confirm they are operating within specifications, how to calibrate and adjust them. There are some errors in the manuals, but what can you do.You need the service manual, because you need to recap it. You need to replace every single --- *every single* --- electrolytic capacitor in the whole entire instrument. There are hundreds, it will take a lot of time, but it won't cost much and the original ones are destroying the instrument. You can save it if you replace them, you can make it as good as new, even better than new, if you catch it in time. I mean every single capacitor, I don't care how non-critical the component seems to be, I don't care if it "tests good", replace it. My 2794 was saved from certain doom by a re-capping. There was dozens and dozens of surface-mount electrolytics that were destroying the circuit boards they were on, which was an obvious intervention that was needed, but in the end I had to do every single last capacitor, every one, before it was finally 100%. I wasted a lot of time looking for faults, trying to debug it, paying for shipping over and over for a few capacitors at a time, when in the end it was as simple as replacing every single one. You need the service manual for this because (i) somebody might have already replaced some with the wrong components and you need the service manual to see what should be there, and (ii) some capacitors are unobtainable in the capacitance, voltage, and package format they used originally and you need the schematics to see what the actual working voltages are so you can select suitable replacements. Thanks to the progress of technology, most can be replaced economically today with MLCC or tantalum capacitors, which won't leak or out-gass or dry out and that compared to the original aluminum electrolytics have tiny tiny ESRs, so the power supply ripples will be much lower than stock, it'll perform better than new.