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Old 25th Nov 2022, 4:05 pm   #267
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by G0HZU_JMR View Post
The diagram below shows how I would normally set up an IP3 measurement. I'd start with 3dB attenuators as indicated and then increase them if required.

The setup below is usually good enough, unless trying for a measurement with very low IMD levels. I'd be happy to use the setup below to carry out an initial check on a mixer. With some mixers the LO drive level can make a big difference. It is also worth playing around with the RF tone levels and the RF and LO frequencies to see how the result changes.

A modern (strong) lab analyser won't need a diplexer at the IF port, but the internal attenuator needs to be set such that the analyser's internal IMD is at least 20dB lower than the mixer IMD. A good margin is needed here because the analyser also sees the image test tones as well as the wanted tones.

Cheers. I assume the use of 3db attenuators is to improve port impedance matching rather than cross-talk isolation between two sign gen? I am kinda of thinking of using 10db attenuators. Secondly, this set-up is for amplifier measurement only? You can't measure a mixer like that, as the LO have to switch the ring diodes on.

I am making a comparison between HP8566B and SSA3021X Plus:

HP8566B:
TOI > +7dbm (5M-5.8G) . DANL < -134dbm (0 atten, 10Hz RBW, 1M-20Ghz) . phase noise -90dbc/Hz at 10KHz offset.

SSA3021X +:
TOP: +10 to + 18dbm. DANL, -161dbm/hz (0 atten at RBW 1 Hz). Phase noise, -98dbc/Hz at 10KHz

Unlike phase noise measurements, it is possible to measure the DUT having a TOI significantly higher than that of the spectrum analyzer. My understanding is that the measured 3rd-order IM products are relative to the noise floor level at a particular RBW instead of absolute measurements. Since the calculation of IP3 involves the difference between the fundamental and the average of the key troublemaking anti-social neighbours; (2f1-f2)-LO, (2f2-f1)-LO (or upconversion LO-(2f1-f2), LO-(2f2-F1) ), the contribution of IM products of the first-stage mixer of the spectrum analyzer is subtracted out. The IM products of the 2nd, 3rd and 4th mixers are irrelevant due to the up- and down conversation and the associated LP/bandpass filtering at microwave, VHF and Hf frequencies inside the spectrum analyzer. However, it does not mean that the IM products from the first mixer of spectrum analyzer can get away scot-free.

The use of attenuators at the spectrum analyzer input will reduce the level of the spectrum anlayzer's IM products so they are small compared with the IM products from the DUT. The gain slope of the 3rd order IM is 3, so for a 1db rise in mixer level, a 3db rise in 3rd order IM?

The DANL is important in TOI measurement. For DUT with high TOI, the measurement accuracy is severely limited by the low SNR at the noise floor of the spectrum analyzer, so it is really good when the RBW can go down to 1Hz. I read somewhere a spectrum analyzer can have zero a span, how is it even possible?


Quote:
Originally Posted by Radio Wrangler View Post
OK, in the 3324A the 30MHz PLL is the frequency standard/heterodyne mixdown LO

30MHz loop unlocked could simply mean that the external 10 MHz frequency reference has been selected and you haven't connected a 10MHz standard to the unit. If so, the fault should clear if you switch to internal reference.

It also looks like the frac-N loop does self tests at 30MHz and 60MHz frequencies. One of these could fail due to drift in the frac-N loop VCO. Might be a simple tweak to fix, otherwise these frac-N systems scare people off and are full of special and selected parts. The make of one logic gate has around a 20dB influence on spur levels!

David
The 3324A is packed and ready to go back. I wont take the risk to repair myself unless I got it dirt cheap.
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